Title: J. R. San Diego, CA, 1986. · Title: 9.2.1 5 R. J. Haynes and R. R. Sherlock, "Gaseous...

AP42 Section:

Reference:

Title:

9.2.1

5

R. J. Haynes and R. R. Sherlock, "Gaseous Losses Of Nitrogen", In Mineral Nitrogen And The Plant-Soil System, Academic Press, San Diego, CA, 1986.

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Chapter 5

Gaseous Losses of Nitrogen

R. J. MYNES AND R. R. SHERLOCK

1. INTRODUCITON

An upsurge of interest in gaseous losses of nitrogen from the soil h occurred during the last decade. Much of this research was stimulated b evidence from agronomic nitrogen-balance studies that generally sho an unexplained IO to 30% loss of applied fertilizer nitrogen (Allison, I Legg and Meisinger. 1982). Experiments involving the usc of "N-labele fertilizers confirmed that a significant proportion of applied nit unaccountably lost from soils during cropping (Hauck. 1971; Ha Bremner. 1976). A number of processes contribute to gaseous loss soil nitrogen; these include ammonia volatilization. bacterial denit lion, nitrification. and reactions of N O T Z t h soil components.

the soil surface. The quantities of NHl lost are highly variable de on such factors as rate, type and method of fertilizer nitrogen app soil pH, and environmental factors including temperature, moisture wind (Black et ai.. 1985a. 1985bX Plants can both absorb and evolve from their leaf canopies (Freney et 01.. 1981) but the major factors influence the relative magnitude of the two processes are, as yet. uncl

Denitrificationjs a major biological process through which N from soil is returned to the atmosphere (Payne. 1981; Firestone. 1982). process, which is mediated principally by aerobic bacteria which are pable of anaerobic growth only in the presence of nitrogen oxides, yie nitrous oxide (NzO) and dinitrogen (N?) gases. The role of N?O in stra spheric chemical reactions has generated great interest in the denitrifi tion process. This is because the photochemical breakdown of N20 in t

Ammonia volatilizationjmay occur whenever free NHl is present n

11. An

51rato compc ti a/.. harmf

Nit! tion p: ductio Ihroug neousl

Und denitri and Sn denitri. gases (

In th plant-: losses : also co

Ama procesr atmosp (Terma: son, 19:

A net rnonia (i usually such as matter, the decc are disc:

A wid 10 soils aqua. an ("I)~S soil urea drolysis

.?$ ',& . . ..,.,.

' $. .:.

, .

from the soil has vas stimulated by generally showed en (Allison. 1955; ise of 'sN-labsled

:terial denitrifica- onents.

ria which are cay.

243

siratosphere yields NO, which has a principal role in catalyzing the decomposition of stratospheric ozone (Crutzen and Ehhalt, 1977; McElroy e , 0 1 . . 1977). The stratospheric ozone layer shields the biosphere from harmful exposures to UV radiation.

Nitrous oxide is also released from soil as a by-product OF the nitrification pathway (see Chapter 3) although the exaci mechanism of NIO production is unclear (Schmidt. 1982). Under field conditions losses of NzO through dentrification and nitrification are thought to often occur simultaneously.

Under conditions that favor the accumulation of NO; in soils, chemodenitrification may contribute to gaseous losses of N (Nelson, 1982; Chalk and Smith. 1983). The presence of NO; provides a mechanism for chemodenitrification since NO; tends to react with soil components to form gases (e-g.. NI, NzO. NO, and NO?).

In this chapter, the processes involved in gaseous losses of N from the plant-soil system are reviewed and the major factors influencing such losses are discussed. The magnitude and significance of these losses are also considered.

11. Ammonia Volnrilhlion

11. AMMONIA VOLATILIZATION

Ammonia volatilization is the term commonly used to describe the process by which gaseous NHI is released from the'soil surface to the atmosphere. The subject has been reviewed in depth by several workers (Terman. 1979; Freney er nl.. 1981, 1983; Vlek and Craswell, 1981; Nel- son, 1982).

monia (Le., NH](.,, and NHl(,,) near the soil surface.)The source of NH, is usually soil NH;. The supply can be from organic .nitrogenous sources such as urine or feces of animals, plant residues, or native soil organic matter, all of which decompose to release NH:-N. The factors affecting the decomposition of organic residues and the subsequent release of NH: are discussed in detail in Chapter 2.

A wide variety of NH;- and ":-forming compounds are also applied to soils as fertilizers (e.g., (NHI)ISO~, NHINO,, (NH4)zHPO~, NH,CI, aqua ammonia. and urea). Ammonium-containing fertilizers such as (NH4)zSOd dissolve in soil solution and NH: ions are produced. In the soil urea, from either animal urine or applied fertilizers, undergoes hydrolysis catalyzed by the enzyme urease to form (NH4)2C0,:

(1)

A necessary prerequisite for NH, volatilization is a supply of free am- 1

(NHI)?CO + 2H:O- (NH,)]CO,

5. Gaseous Losses of Nibogcm

This reaction causes localized areas of high pH close to the site of hydro-

C0:- + H20 = HCO; t OH-

resulting in electrostatic binding of NH: ions to clay and organic colloids. Some of this NH: may become "fixed" in clay lattices (see Chapter 4). However, NH: ions in soil solution also enter into equilibrium reactions with NHl.

A. Processes

1. Well Aerated Soils

surface may be represented as

F = k ("11g17oi1 - "~ig iarrn)

where NHj(,,,,il is the NH,,,, concentration in equilibrium with the s . solution at the soil surface, N H I , ~ , ~ , ~ is the NHI1,, concentration o f t

(1 1 adsorbed NH: e NH,+(aq) in soil S o I u I ~ o n

N H s ( = ~ ) in soil solution

NH3(g) gas in soil

N H X ( ~ ) ga9 in atmosphere

Fig. 1. The various equilibria that govern ammonia loss from soils.

; - ,

ous Losses or Nitrogen .

I the sile of hydro- . :"

(2) .:_ J.'

omplex of the soil ... d organic colloids. '?!. 's (see Chapter 4). ,." iilibrium reactions , '_

s NH; rather than lally regulates the > '

:ss. N can also be liquid but changes

shown in Fig. 1. , '

, 2 :

:I ;IS both a source ) or out of the soil

(3) ,,.

rium with the soil ,;p ncentration of the ' .,$

.9

. ,

ion ,.,,J ,$ ',.;I. L.

11. Ammonis Volalilialian 245

bulk atmosphere, and k is an exchange coefficient whose value may vary with windspeed (Vlek aqd Craswell, 1981; Freney et o l . . 1983). Whether ",(,I is absorbed or volatilized is therefore largely determined by the difference in NHII,, concentration between the soil surface and the atmosphere.

Atmospheric NH, concentrations, although variable. are usually very low, e.g., 2-6 pg NH,-N m-I (National Research Council, 1979). and there is no evidence that they seriously limit volatilization rates in the field (Vlek and Craswell. 1981; Freney er ol., 1983). No direct measurements of equilibrium NHl(Blroil concentrations have been reported but cal- culhions by Vlek and Craswell (1981) show that for NH,I,,,,, concentrations of 2-6 ppb. NH,,.,, concentrations of 0 ~ 5 ppm or greater are sufficient to promote volatilization. These workers maintained that where NH, volatilization is a problem, such levels of NHII,,, are easily reached. Therefore, under these conditions NH,rglroil is likely to greatly exceed NH,,,,,,,. whereupon equation (3) can be simplified to

F = k (NH,i8,>oi~) (4)

Thus, ammoniacal N added to the soil from whatever source may be subject to loss as NHII,,. Also, the actual magnitude of any loss is likely to depend primarily on the concentration of NHl~g,~oi , , which in turn depends on the total concentration of ammoniacal N species, the values of the individual equilibrium constants (Fig. I ) . and the rate of attainment of equilibrium at each stage. Factors such as pH. temperature. etc., which can influence any or all of these separate equilibria, can influence the magnitude of NHI loss. Likewise, all strategies detigned to limit volatilization losses attempt to manipulate these equilibria either directly or indirectly to reduce the NH1(,, concentration at the soil-air interface.

2. Flooded Soils

The fact that NH, is the most soluble gas known makes it tempting to suggest that a soil flooded with water would serve as an almost infinite sink for the gas and that any volatilization to the atmosphere would be negligible. This may be so for unfertilized flooded soil but can be demon- strably incorrect for fertilized systems. For example, Vlek and Craswelf (1979) showed that up to 50% of urea surface applied to floodwater was'i lost as NH, within 2-3 weeks. 1

It i s now clear that equation (3) and the equilibria described in Fig. I apply equally well to flooded and nonflooded soils (Freney ef ol. , 1983). Indeed, since it is relatively easy to measure the ammoniacal N concentration, pH. and temperature within the water overlying a flooded soil and also to obtain values for the exchange coefficient k for the transport of

NH, away from the floodwater surface, i t has been suggested that the,, direct application of equation (2) may provide a simple way of predicting NHI losses from Hooded rice paddies (Leuning ef u / . , 1984). However, in; testing this hyporhesis Leuning er u / . (1984) found [hat because of temper: ature gradients within Ihe water. the NHI concentration at the water-air.: interface was no1 generally characteristic of that in equilibrium with the ' bulk of the floodwater. This implies that for flooded soils transport pro;., cesses in both the air and water are important in determining the rate of ~:. NH, volatilizalion.

Loss of NHI from flooded soils is also strongly influenced by wind through a mechanical mixing of the surface water. This and several other factors that influence NHl losses from flooded soils are discussed in following sections.

3. Calcareous Soils

taken from various pans of the world. a strong correlation between N loss and CaCO] content has been reported (Lehr and Van Wesema 1961; Fenn and Kissel. 1975).

Apart from its effects on the alkalinity and buffering effect on so CaCOl also appears to have a more specific effect since when NH:

other salts having more soluble reaction products with Ca (e.g., NO CI-. I-) gave lower losses.

It has been suggested that applications of NH,' compounds to calc ous soils result in the formation of (NH4)2COl (Fenn and Kissel, 1

(1982) follows: CaCO, + (NH.),SO. - Ca" + 2 0 H - + CaSO, + ZNH.HCO,

2NHIHC0, -- ZNH, + ZCOl + 2H.O

The Ca2+ and OH- ions produced during hydrolysis of CaCOl may th

Ca" + ZOH- + (NH4)J0. - CaSO, + ZNH, + 2H10

react with (N&),SO, as follows:

[I. Ammonia Volnlili

The overall reacti shown:

CaCC

Thus. the form of CaCO, theret deprotonate NH;

B. Factc

1. pH

The equilibriur

Thus, the concei [he soil solution concentration) di NHI. The propc present as NH,,,, 0.0004, 0.004, 0.

Many worker: strated that NH, d.. 1956; Volk. Lyster ef ul., 19t Lo interpret sinc sumed to charac

Such assumpr also be represen

Hence, volatil (Avnimelech an1 ization. The orig ling the extent 01 is high,

That factors c the fact that sub: Ernst and Mass, [he solution imrr considerably mc

,- ; E . :? . . '

' L 'y i Losses 01 Nilrogrn

, ,

, _ i . r

Egested that the ;i.. ray of predicting .' 14). However. in '' i causeoftemper- . ,, at the water-air , $ ilibrium with the .~ '!$ . I s transpor( pro- ' :t lining the rate of ?'>

uenced by wind '

ind several other 1, - discussed in fol- .

.~

, ,

: volatilization of ., .j

: 1981). For soils ion between NH] :

. ~

Van Wesemael, :'

effect on soil pH,

ed with the fertil-

HPOa-) whereas h Ca (e&. NO,,

roundS to calcare- and Kissel, 1973).

11. Ammonia Volaliliintion 241

The overall reaction (Le., summation of equations (5). (6). and (7)) can be shown:

CaCO, + (NHI)!SO, - ZNH, + CO: + H?O t CaSOi (8)

Thus, the formation of an insoluble Ca salt encourages the dissolution of CaCO, thereby generating the bases, HCO; and OH-. that act to deprotonate NH; to NHl and sustain volatilization.

B. Factors AkTecting Volatilization

1. pH

The equilibrium between NH: and NHJ can be represented as NH; + OH- = NHI + H:0 (9)

Thus, the concentrations of NH: and NHl are determined by the pH of the soil solution. An increase in pH @e.. an increase in hydroxyl ion concentration) drives the equilibrium to the right thereby producing more NH,. The proportion of aqueous ammoniacal N (NHG,,, plus NHl,.,,) present as NH3(,q, at pH 6, 7. 8. and 9 can be calculated as approximately 0.0004. 0.004. 0.04. and 0.3, respectively (Hales and Drewes, 1979).

Many workers in both laboratory and field experiments have demonstrated that NHJ losses increase as the soil pH increases (e.g.. Wahhab et 01.. 1956; Volk, 1959; Ernst and Massey, 1960; Watkins er 0 1 . . 1972; Lyster et ol.. 1980). Nonetheless, the direct effects of soil pH are difficult to interpret since more often than not the original soil pH has been assumed to characterize the soil pH throughout the duration of NH, loss.

Such assumptions are not necessarily correct since equation (9) can also be represented as

NH:- NH,+ H' . (10)

(11 ) H' t OH- - H1O

Hence, volatilization is accompanied by net acidification of the system (Avnimelech and Laher, 1977). which tends to decrease the rate of volatilization. The original soil pH is. therefore, of prime importance in control- ling the extent of volatilization only when the buffering capacity of the soil is high.

That factors other than soil pH can influence NHI loss is indicated by the fact that substantial NH, volatilization can occur from acid soils (e&. Ernst and Massey, 1960; Blasco and Cornfield, 1966). Indeed. the pH in the solution immediately surrounding a urea or NH; salt granule may be considerably more important in determining NH, losses than the soil pH

5. Geseolu Larseo 01 Ni

For NH] volatilization to occur from flooded soils buffering substanc

major proton acceptor normally present at typical pH values of floodw ter (Vlek and Stumpe, 1978; Vlek and Craswell, 1981) and therefore vol tilization in flooded systems can be represented as

NH;,,, + HCO& - "I,,, + COX,, + H?O

respiratory balance of algal growth in the floodwater (Mikkelsen er ol.;

rise to 9 or above resulting in large losses of NH,.

2. Temperature

rate and ultimate extent of NHl volatilization increase with incre

Watkins el a/ . , 1972; Lyster er 0 1 . . 1980). The effect of temperature on NH, volatilization can be expla

leas1 in pan by the temperature dependence of the equilibrium co

'- temperature (Wahhab er 0 1 . . 1956; Volk, 1959; Ernst and Massey,

The higher the temperature the greater the proportion of NH],,,, (e

1985a). The diurnal pattern appears to be predominantly related to t perature fluctuations as illustrated clearly in Fig. 2 although the effect evaporation of water and windspeed cannot be ignored (Denmead er

The extent of losses can also follow a seasonal patteny Ball- Keeney (1983), for example, found losses of NHl from urine patches averaged 5 . 16. and 66% of added urine N under cool moist ( ter), warm moist (spring), and warm dry (summer) conditions, respe

Fig. 2. Di w i n e manur p. 93 by per!

3.

The am1 NH, evob constant. and total Kroontje, percentagt a/. . 1956; Black et a plied urea urea hydn

Factors influence i

mechanisn affect the more obvi, immobiliz; and nonex crease the

us Loses of Nilrqtn

al.. 1984. 1985a,

'Cering substances ; iter caused by the le (HCO;) is the ialues of floodwa- .''.

nd therefore vola- ',

0 (12) . . '

'hotosynthetic and (Mikkelsen er a!. . ,ion of CO2 in the l e floodwater may

..

the instantaneous se with increasing and Massey. 1960;

in be explained at Jilibrium constants e. 1978: Hales and Goh. 1984. 1985a). of NH,,,,, (equilib- , '

(equilibrium (3)) . , loss.

follows a marked :ion (McCarity and :hamp er al . . 1978; 1982; Black e l 01.. :

ntly related to tern- . '

hough the effects of .,-' :d (Denmead er a/. ,

I pattern. Ball and -: ,;: n urine patches that .. :.' r cool moist (win- ,; ;?. conditions, respec- .:'

, . .

4.I

, p ,$f .. . , 9,, , , ,.

249 - Temperature - Ammonia loss - Y u

e a

0 8 16 2 4 32 4 0 4 8

TIME (hours)

Fig. 2. Diurnal Ruc~uatiuns in air lempernlures 2nd rale of ammonia loss from liquid swine manure. [Redrawn from Holl e , 01. (19811. Reproduced Cram J . Environ. Q W I . IO. p. 93 by permission o l American Sociely of Agron0my.l

3. Ammonium Concenmarion

The amount of NH: added to the soil must have a direct effect on the NH, evolved as predicted by equation ( I O ) if all other factors are held constant. A linear relationship between the rate of fertilizer application and total NHJ loss has been shown in a number-.of studies (Chao and Kroontje. 1964; Hargrove et ul. . 1977; Hoff er al . . 1981). In other studies percentage losses increased as rates of application increased (Wahhab er ul., 1956; Volk, 1959; Kresge and Satchell. 1960; Lyster e l a / . , 1980; Black er a / . . 1985b). Such nonlinear relationships occur mainly from applied urea and are the result of the increasing soil-surface p H induced by urea hydrolysis.

Factors that influence the NH: concentration in soil solution wil l also influence the potential for losses o f NH3 through volatilization. Many mechanisms can induce changes in the NH: concentration and thereby affect the chain of equilibria that determine the extent of NH, loss., The! more obvious include plant uptake. nitrification, denitrification. leaching. immobilization, and the fixation of NH: by clay minerals in exchangeablei. and nonexchangeable forms.) All these mechanisms would tend to decrease the NH: concentration in soil solution and so reduce NH, losses.

Of major importanceJin determining the soil solution NH: concentrd. tion is the cation exchange capacity (CEC) of the soil; The adsorption of- the positively charged NH: ion onto the exchange complex of soils reduces the amount of NH: and therefore NHJ in soil solution at a given pH. Hence, many workers have observed a negative relationship between soil CEC and NH, volatilization (Wahhab e! 01.. 1956; Gasser, 1964; Ryan and Keeney, 1975; Fenn and Kissel, 1976; Lyster el o l . . 1980; Ryan erol. , 1981). The effect of increasing CEC in reducing NH, volatilization is illustrated in Fig. 3. When other soluble cations are applied along with' ammoniacal fertilizer. comDetition for the exchange sites can result. For'

L . ~. change sites leading to enhanced NHJ losses (Fenn ef 01. . 1 9 8 3 examde. soluble Ca'+ ma; deoress normal a d s i d i o n of NH: on ex: '"'

6 0

5 z 0 E E a

Fig. 3. Rate of ammonia volatilizalion as influenced by the calion exchange capacily sail-sand mixluren. [Data from Daitardar and Shinde (1980). by counesy of Marcel Dekk

0 0 30 60 90 120

INCUBATION P E R I O D ( h o u r s )

11. Ammo1

AS no1 capacily

XHJ- Th, SIaIUs WI

other for given pH c w n t su above si] fering ca

The prf 1967; Ra, Sarkar. I altogethe be miner immobili. compose solution

5

Soil m, volatiliza in soil so lure cont leading t( in a numi Fenn ani

Howel losses of mally obi are allow and Esca ing. or ai Over timt Drying al Port diss Some w( increasin and Mas: promote> concurre

In the ,

tion bec:

r Nitrogen

ncentra. .< rption of . soils re-

~ a given , '

between .I,'.

an et ul.. .. .,;

d o n is ong with sult. For ; on ex- ,

I.

6J:Ryan ;

11. Ammonia Volmlilizntion 251

As noted earlier when discussing the effect o f soil pH, the buffering capacity o f the soil can be an important factor influencing NH, volatilization because the dissociation o f NH: ions releases H+ ions as well as NH,. Thus, volatilization i s normally more prolonged in soils of high base status where the acidity produced can be neutralized by carbonate or other forms ofalkalinity. Avnimelech and Laher (1977) showed that at a given pH. NHI losses increase with increasing buffer capacity. To some extent such an effect may confuse the negative effect o f CEC described above since, in general, the higher the CEC of a soil the greater i t s buffering capacity.

The presence of organic residues has been reported to accelerate (Moe. 1967; Rashid, 1977), decrease (Tripathi. 1958), or not affect (Verma and Sarkar. 1974) NH, volaLilization from added urea, Such results are not altogether surprising, since while organic matter wilh a low C : N ratio wil l be mineralized with the release of NH:. that with a high C : N ratio wil l immobilize NH:-N from the surrounding soil (Chapter 2). Partially de- composed organic matter wil l also have a CEC that wil l influence soil solution NH; levels.

5. Soil Moisture Content and Moisrure Loss

Soil moisture content has an important influence on lhe rate of NH, volatilization since i t affects the concentration of NH; and therefore NH, in soil solution. Ammoniacal N concentrations in solution at high moisture contents are likely to be lower than those at low moisture contents leading to lower net losses of NH, from wetter soils. This has-been shown in a number of studies (Martin and Chapman, 1951: Wahhab et ul., 1956; Fenn and Escarzaga, 1976).

However, interpretation o f results can be confused by simultaneous losses of water. Indeed, the largest amounts of volatilized NH, are normally obtained from soils o f high moisture content (below saturation) that are allowed to dry (Martin and Chapman, 1951; Wahhab ef a / . . 1956: Fenn and Escarzaga, 1977). Loss of water promotes NHI evolution by increasing, or at least maintaining, ammoniacal concentrations in soil solution over time, which leads to greater losses than if no soil drying occurred. Drying also results in the upward movement o f water, which helps transport dissolved NH: and NHI to [he soil surface (Freney er o l . , 1981). Some workers have therefore reported that NHI losses increase with increasing initial moisture content up to field capacity (Volk, 1959; Ernst and Massey. 1960; Kresge and Satchell, 1960). Although moisture loss promotes NHl evolution, the volatilization of NHj can occur without concurrent loss o f water (Ernst'and Massey, 1960; Terry e l ol., 1978).

In the case of dry fertilizer materials (e.&, urea prills) a low initial soil

proceed through macropores thus transporting more NH;(,,, lo the surface of initially wet soils.

Time of rainfall or water application canJalso be an important factor; influencing losses of NH, _I For example, before its hydrolysis urea moves, rapidly into the soil when water is applied and losses from surface-applied urea decrease with increasing amounts of applied water (Fenn and Miy:.

of urea can significantly reduce losses of NH, (Carrier and Bernier. 1971; Morrison and Foster, 1977; Black and Sherlock. 1985). However, as illustrated in Table I, once hydrolysis has proceeded (by 24 hr). the effective- ness of water applications is markedly reduced.

6. Windspeed

Table I

Percenlmge 01 Applied Urea N Lasl PI Ammonia M e r 7 Dsys 89 AIIecled by Ihe Amaunl of Waler Applied and I t s Time OI Application"

(hours after urea

(mml 0-3 6-10 14-26 48-50 applied

0.5 n . d ~

D a b from Black and Sherlock (19851. Urea granules were applied 10 a pasure sur-

lace al a rate of 100 kg N ha-'. n ~ d . . no1 deler. m i n d

11. Ammonia '

Under such aidow (Wal have reporl crease in th 1967: T e w

Increasin: promoting n However. u cedure lo n liquid dairy rela~ion~hip gested that limited by ( was possibi process.

I t has, hc over a flooc meester an( Denmead e lance to lr volatilizatic surface wa floodwater ization rate

7.

Since an tendency fi however. 1: following i i

quate depl physical pi (Ernst and 1966; Nels, due to its I

As alrea from c a b (e&. (NH surface ap ally resuit lions of n, (Terman e:

., m e 3 o l Nitrogen' ,

Ition, can slow ' " . result in small . ''

imilarly. when 1' ses of NH, are ' ': 1983: Vallis et . '

ito soil colloids Hiar, may tend would lend to . ~

,q, to the surface 'I

mporiant factor ysis urea moves ' '

surface-applied ; (Fenn and Miy- "

:Ice applications ,.,

id Bernier. 1971; :owever. as illus- i r ) . the eFfective- :

. ,

lizer applications ., x: inrealistically low 3

?3 s (Nelson. 1982). ,g . , I .~

I]. Ammonia Volalilization 253

Under such conditions NH, volatilization can be directly proportional to aiflow (Watkins er a / . , 1972; Kissel e r n / . . 1977) and a number of workers have reported that the rate of NH, volatilization increases with an increase in the rate of airflow over the samples (e.g., Overrein and Moe, 1967; Terry er a/ . , 1978; Vlek and Stumpe. 1978).

Increasing windspeed should tend lo increase the volatilization rate by promoting more rapid transport of NH, away from the air-soil interface. However. when Beauchamp er nl. (1978, 1982) used an aerodynamic pro- cedure to measure NH, losses from surface-applied sewage sludge and liquid dairy manure under field conditions, they found no discernable relationship between windspeed and NH,(,, flux. These workers suggested that volatilization from the soil was diffusion controlled and was limited by depletion of ammoniacal N at sites from which volatilization was possible. Windspeed presumably had little effect on this diffusion process.

It has, however, been clearly demonstrated that increasing windspeed over a flooded soil surface increases the NH, volatilization rate (Bouw- meester and Vlek, 1981; Denmead er d., 1982: Moeller and Vlek, 1982). Denmead et a/. (1982) suggested rhal there may be considerable resis- tance to transport of NH, in the liquid phase and that the enhanced volatilization in high winds is due to better mechanical mixing of the surface water. Such mixing would also avoid the development at the floodwater surface of a region depleted of NHI that might limit the volatilization rate. - --

7 . Form and Placement of Applied Fertilizer

Since anhydrous NH, is applied to soils as a gas one might expect 3 tendency for i t to escape to the atmosphere. I t has generally been shown, however, that regardless of soil type, losses of N during and immediately following injection of anhydrous NH, are small if it is applied a[ an ade- quate depth (5 to 13 cm) and provided soil moisture conditions and soil physical properties are s G h that the injection channel is rapidly sealed (Emst and Massey. 1260; Khan and Haque, 1965; Parr and Papendick. 1966; Nelson, 1982). Thus, anhydrous NH, is normally retained in the soil- due to its reactions with soil components (see Chapter '!).

As already noted, NH: salts that produce the largest losses of NH, from calcareous soils are those that Form insoluble precipitates with Ca (e.g., (NH,),SO, or (NH4):HP04). In acidic and moderately acidic soils surface applications oFalkaline fertilizers such as "#OH or urea generally result in larger losses of NH, through volatilization than do applications of neutral or acidic fertilizers such as NHIHzPOa or (NH,),SOa (Terman e! a[ . . 1968; Matocha, 1976). The effect of N form when applied

5. Gaseous Losses or Nilmgm

to a pasture on pH around the fertilizer granules and on NHI volatilization, from a pasture surface is shown in Fig. 4 . The localized rise in pH. due to urea hydrolysis, and the consequently larger loss of NHI from urea are obvious.

Several methods have been examined to minimize NHI losses from. applied urea. Mixing neutral ammonium salts O r acidifying agents (e&, N H ~ C I , N H ~ H ~ P O ~ , HNO], or ~ $ 0 ~ ) with urea prior to application can

Total losses

urea= 1 1 ~ 2 %

DAP = 3 ~ 1 %

A M s = 0.6%

Mikkelsen el

animals can k has been est:

N solution ( effective rat,

in localized,

TIME FROM APPLICATION ( d a y s )

lilrogeo , ., i

zation .: due to

i (e.g., ' '

on can

!'

4 I '

. ,

.: *)

markedly reduce losses from surface applications (Terman, 1979). Other techniques include the use of slow-release forms of urea (e.&, sulfur- coated urea) (Matocha. 1976; Presad, 1976; Vlek and Craswell. 1979) or urease inhibitors (Moe. 1967; Bremner and Douglas, 1973). Such techniques work by attempting to retard the rate of urea hydrolysis and so prevent the rapid buildup of ammoniacal N.

Another method of reducing losses of NH, from applications of NH: salts or urea is by placement of fertilizers below the soil surface or by thoroughly incorporating them into the topsoil (Ernst and Massey, 1960; Overrein and Moe, 1967; Fenn and Kissel, 1976; Vlek and Craswell, 1979; Hoff el ol., 1981). This technique effectively reduces the ammoniacal N concentration of the soil solution at the soil surface thereby reducing losses of NH,. However, in some circumstances it is common practice to broadcast all fertilizers on the soil surface (e-g.. on pastures or under minimum tillage) in which case the opportunity for NH, volatilization is enhanced (Black ef ul., 1984).

As indicated previously, applications of nitrogenous fertilizers to flooded soils can result in considerable losses of", (Vlek and Craswell. 1979). Placement of fertilizers into soils before flooding can markedly reduce volatilization in comparison with broadcasting the fertilizer over the floodwater (Macrae and Ancajas, 1970; Craswell rl a/.. 1981; Mikkelsen el ol.. 1978).

8. Presence of Animals

a. Grazing onimals. In grazed grassland ecosystems the presence of animals can greatly influence the cycling of N within the system,J.ndeed i t has been estimated that 85 to 95% of N ingested by grazing herbivores is excreted (Henzell and Ross, 1973) and most of this is voided as urine in localized patches on the soil surface (Doak, 1952). Urine is a concentrated N solution (approx. 10 gm N liter-' of which 80-90% is urea) and the effective rate of application within urine patches is often greater than the equivalent of 500 kg N ha-'. Such application rates are considered to be much too high for efficient plant utilization (Ball and Keeney, 1983; Car- ran e l ol . . 1982). The urea is rapidly hydrolyzed to NHi-N. which results in localized areas with both high pH and high ammoniacal concentrations. It is evident that urine patches provide concentrated focal points within a pasture from which significant NH, volatilization will occur. Such losses have been reported to be in the region of 20 to 60% of the urine N (Denmead et a!., 1974; Ball e l ol., 1979; Ball and Keeney. 1983; Carran el al., 1982; Sherlock and Goh, 1984).

b. Feedlots. Modem animal-feeding practices in which large numbers of animals are concentrated in small areas have led to problems in the

5. Gaseous Losses of Nilrogem

the materials since losses on the order of 30 to 80% of the N originally present are not uncommon (Vanderholm, 1975). Ammonia volatilization from feedlot areas can be 10 to 30 times that from surrounding areas (Hutchinson and Viets, 1969; Elliott et a/., 1971; Luebs e t a/.. 1973).

Volatile aliphatic amines of different molecular weights are also emitted from animal manures. Mosier el a/ . (1973) identified seven basic aliphatic

Significant losses of NH, occur when animal manures are surface-ap

cation cawmarkedly reduce losses (Hoff er a/., 1981).

9. Presence ojPIants

creased losses of applied N through NH, volatilization (Sherlock and; Goh, 1985b).

11. Ammonia V'

la1 conductar voring CO2 2

levels of nuti Factors th.

cavity are un ter 6. Excep dehydrogena occurs throur tamate synth parlicularly i nia producec plant tissue : the substom' NH: supplie surrounding photorespira creases. Los senescence (

A number canopies (ME 1979; Weilar a/., 1980; Le cing plants (1 tial evidence plants can IC 1980). Some

The rnagnj estimates va cent N detec sec-l but lo! less (Farquh

Volatile ar lady during

When the that in the (Farquhar er phase throuE in water film metabolized the uptake ( Hutchinson Cowling and

Gaseous LOWS Nilrogen

980). Appreciable NHI h heaps of manure and :nsive odors produced, .ion in fertilizer value of . 10% of the N originally Ammonia volatilization rom surrounding areas Luebs t‘f a / . . 1973).

veights are also emitted ed seven basic aliphatic dimethyl, ethyl, n-pro-

nating from a high-den- ies constituted only 2 10 ance released as NHI. nanures are surface-ap- mrner months (Lauer el I of the added N can be 1975) although incorpo- ediately following aPPli- 1981).

be important factors in lution or, in the case of a reaches the soil surface. .ansformations including af surfaces also possess zation of the hydrolysis as been demonstrated by ison and Melsted, 1962; Since leaf surfaces have

y it seems possible that cover may result in in-

itilization (Sherlock and

of NH, from plants have d., 1980. 1983). A finite

stomatal cavities of plant .t of the atmosphere. net PI evolution include high i of NH,, and high stoma-

11. Ammanin Volalilizstion 251

tal conductances. Stomatal conductance is greatest under conditions fa- voring C 0 2 assimilation: high light intensity, ample moisture. and high levels of nutrition.

Factors that influence the partial pressure of NHI in the substornatal cavity are unclear. Ammonia assimilation in plants is discussed in Chap- ter 6. Except at high tissue ammonia concentrations, where glutamate dehydrogenase may play a role, the majority of ammonia assimilarion occurs through the combined action of the glutamine synthetase and glutamate synthase enzymes. The combined action of these two enzymes is particularly important i n the refixation of the massive amounts of ammonia produced during photorespiration. Thus levels of NH: and NH, in plant tissue are normally extremely low^ The partial pressure of NH, in the substomatal cavities is thought to be maintained by small amounts of NH; supplied in the transpiration stream plus small amounts present in surrounding leaf cells (Farquhar ef a / . , 1983). During leaf senescence, photorespiration declines but proteolysis (with the release of NH:) increases. Losses of NH, from plants may, therefore. be greater during senescence (Farquhar et d, 1983).

A number of workers have measured losses of NH, from healthy plant canopies (Martin and Ross, 1968; Stutte and Weiland. 1978; Stutre el a / . . 1979; Weiland and Stutte. 1979, 1980; Weiland et a/.. 1979; Farquhar er a / . . 1980; Lemon and van Houtte, 1980; Hooker ef a / . . 1980) and senes- cing plants (Farquhar ef al. , 1979; Hooker ef a / . . 1980). Some circumstan- tial evidence from N-balance studies also indicates that annual cereal crop plants can lose N when approaching maturity (Wetselaar and Farquhar. 1980). Some of this could be lost as NH,.

The magnitude of gaseous losses of NH, from plants is uncertain and estimates vary greatly. Stutte and co-workers, using pyrochemilurnines- cent N detection. suggested losses averaging 9 nmol (m’ leaf surface)-‘ sec-’ but losses estimated by other methods are an order of magnitude less (Farquhar ef a / . . 1979; Hooker et a/ . , 198.0).

Volatile amines also appear to be liberated from growing plants particularly during flowering (Richardson, 1966; Farquhar er a[., 1983).

When the partial pressure of NHI in the ambient atmosphere exceeds that in the substomatal cavity then net absorption of NH, will occur (Farquhar er a / . , 1980, 1983). Indeed, NH, can be absorbed in the vapor phase through open leaf stomata of leaf canopies and it may also dissolve in water films on the plant leaf surfaces and be subsequently absorbed and metabolized (Denmead e l a / . . 1976). Many workers have demonstrated the uptake of NHI by leaves placed in “]-enriched atmospheres (Hutchinson et a/ . . 1972; Porter ef a / . . 1972: Rogers and Aneja, 1980; Cowling and Lockyer. 1981). Faller (1972) showed in a long-term experi-

5. Gaseow Losses ot N i r m e

It is evident that plants can both absorb and release NHI from their canopies. Indeed, field measurements of NH, Rux within and above the' canopy of several crops have clearly indicated that NHI released at the soil surface can be absorbed within the leaf canopy while some NHI may also be simultaneously released from the top of the canopy (Denmead er u/., 1976. 1978; Lemon and van Houtte, 1980). The major factors that determine whether net absorption or evolution of NHI occurs are. as yet, unclear. Nonetheless. an increase in the height and density of the crop canopy appear to be important factors that tend to reduce NH, losses (Denmead er a/.. 1982).

111. BIOLOGICAL GENERATION OF GASEOUS NITROGENOUS PRODUCTS

Gaseous nitrogenous products can be produced by three groups o organisms: dissimilatory denitrifying bacteria, nondenitrifying ferment& tive bacteria and fungi, .and autotrophic nitrifying bacteria. Denitrifying bacteria are thought to be the most important organisms contributing to losses of nitrogenous gases from soils under anaerobic conditions (Letey et a/.. 1981; Payne, 1981; Firestone. 1982) but under oxidized conditions nitrifying bacteria could well be important agents (Freney el ol . , 1979; Bremner el ol., 1981).

The organisms and processes involved in the biological production gaseous N in soils are discussed below and the major factors that a thought to affect such production are reviewed.

A. Processes

1. Dissimilatory Denitrification - Dissimilatory denitrification is a respiratory process that is present

of O2 while reducing NO; or NO; to N2 andlor N20. The majority bacteria are heterotrophs and obtain their energy and cellular

serve as terminal electron acceptors for respiratory electron trans during the oxidation of the organic substrate and a more reduced N o or N2 is produced. The Drocess is described as a dissimilatory reducti

111, Biolu

a . Dc accept0 raporlel 2nd Ve: T;lble l~ ubiquit; lion pe: cuncenl

The sfudies dmilrifi known ally mc- cnumer delrime

Denii verse. I 3s eleci energy Paroco Thauer trrxcun. One grt !oh; 19'

A f e ~ under strains the roo Neyra ~

Several

)sed at the 4; tenmead el f actors that ~:,,

are, as yet, 5.1 I f the crop j;: NH, losses e:.

: NH, may .%

c i

111. Biolo@cnl Generation of Gaseous Nilrogenaus Produclr 259

a . Denitrifying bacteria. The capacity to use N oxides as electron acceptors in place of Oz with the evolution of NIO andlor N2 has been reported in approximately 20 genera of bacteria (Payne. 1973. 1981; Focht and Verstraete. 1977; Firestone, 1982; Knowles, 1982). These are listed in Table 11. The presence of denilriiiers in surface soils may be regarded as ubiquitous (Payne, 1981) since their density frequently exceeds one mil- lion per gram of soil (e.&!., Jacobsen and Alexander, 1980) and higher concentrations are present in the rhizosphere (Alexander. 1977).

The most common bacteria used for physiological and biochemical studies of denitrification are Pnrococcrrs denirrificans, Psrirdomotrns denitrificans. and Pseudomonas perfectomarinus. Nevertheless, i t is not known which denitrifying organisms are either numerically or function- ally most important in soils since laboratory methods of isolating and enumerating these bacteria are likely lo favor some organisms to the detriment of others (Firestone. 1982).

Denitrifying bacteria are biochemically as well as faxonomically diverse. Most are chemoheterotrophs: they use carbonaceous compounds as electron donors (reductants) and as sources of cellular C and chemical energy sources. Some grow as chemolithotrophs, oxidizing H2 (e.g., Paracoccus denirrificans and Alcaligenes spp.) (John and Whatley. 1975; Thauer el a [ . . 1977) or reduced sulfur compounds (e.g., Thiobacillus denitri/;cans) (Ishaque and Alaem, 1973; Baldensperger and Garcia, 1975). One group is photosynthetic (e.g., Rhodopseirdomonas sphaeroides) (Sa- toh, 1977; Sawada e t a / . , 1978).

A few N2-fixing organisms are known to have the ability-to denitrify under anaerobic conditions. These include a considerable number of strains of Azospirillum bmsilense. which are commonly associated with the roots of many tropical grain and forage grasses (Eskew e l al., 1977; Neyra and van Berkum, 1977; Neyra e r a / . . 1977; Scott and Scott. 1978). Several strains of Rhizobium, including R . japonicrrm, R. melilori, and

Table I1

The Reported Genera of Denitrifying Bacteria

Acinerobvcrer Halobacrerium Rhizobium Alcaligenes Hyphomicrobium Rhodopseirdornunos Azospirillum Micrococcus Spirillum Bocillvs Morarello Thiobocillus Cyrophoga PWOCOCCUS Vibrio Flmobocrerium Pmpionobocrcrium Xonrhomonas Cluconobocrer Preudomonos

01.. 1980, 1982). Possession of a denitrifying pathway may help free-livi bacteria survive anaerobic conditions in the soil (Daniel et al., 1980) a

generally represented as

NO; - NO; - NO - N,O - N?

plexes necessary to reduce NO; to N1, some lack NO; reductase and ar

those lacking NO? reductase and NzO reductase and can therefore redu NO; to NO; and NO to N:O and organisms that lack NO;, NO, and N reductases and are capable of only limited reduction of NO; to NO?.

Characteristics of enzymes involved in the reductions vary dependin

Knowles. 1982). Nitrite reductase catalyzes the reduction of NO; to ga eous products although there i s still considerable controversy as whether NO is the in vivo product. There appear to be two main types nitrite reductase; copper-containing metallollavoproteins and the app

1982). Nitrite reductase appears to be membrane-associated but readil solubilized (Knowles. 1982). The reactive nature of NO makes isolatio and characterization of NO reductase difficult and unequivocal evidenc

111, Biologid

for i t s role in &O reducta: contains Cu. types b and

2. F Nitrous 0)

]any of “no1 1970; Bollag denitrifying < NO;. but gi NH: (Soren Nitrous oxic

The physi isms is not c related to gr( [he case for 1980).

The signif evolution frc 1981) sugge: denilrifiers. by nondenit: minor agron

3. i

The NH:- lobus have t intermediate under most Nicholas. 11 1980; Goreal sible for th, Chapter 3.

Laboratol road and IC heitenbeck shown that : ever, less ce with those f

I t has alsc cation (Vers

L o w s 01 Nilrogen

ling capabilities n produce N1 or 1979: Daniel er '

1 help free-living C I a / . . 1980) and i by maintaining 1973).

,nd biochemistry iewhere (Payne. 83) and only the ,: side reduction i s ~

0) N:

reductase, NO; galory participa- ons i s s t i l l debat- I

(NOH) has been ollocher. 1982). : ,ase enzyme Corn- reductase and are yield N1O as the lack the ability to groups are some- I). These include 1 therefore reduce ?i 0;. NO, and NzO f NO; to NO;. IS vary depending purification used. tory nitrate reduc- Insists of multiple s (Firestone. 1982; ion of NO; to gas- controversy as 10 two main types of ins and the appar- ype cd (Knowles, ociated but readily IO makes isolation equivocal evidence

111. Biologid Generation or Gaseous Ninogenovs Producb 261

for i t s role in denitrification has yet to be provided. Litt le i s known about N20 reductase although i t i s thought to be membrane-associated, possibly contains Cu, and i s linked to electron transport through cytochromes of types b and c (Knowles, 1982).

2. Fermentative Nitrite Dissimilation

Nitrous oxide can also be produced in soils by the actions of a miscel- lany o f "nondenitrifying" fungi and bacteria (Yoshida and Alexander. 1970; Bollag and Tung, 1972; Smith and Zimmerman. 1981). These nondenitrifying organisms are only able to respire NO; anaerobically as far as NO;. but growing fermentatively they can further dissimilate NO; to NH: (Sorenson, 1978; Caskey and Tiedje, 1979: Cole and Brown, 1980). Nitrous oxide i s produced as a minor product.

The physiological function, if any, of NIO production by these organisms i s not clear. Nitrous oxide production does not appear 10 be directly related togrowth or energygeneralion (Smith and Zimmerman. 1981) as i s the case for the fermentative reduction of NO; to NH; (Cole and Brown, 1980).

The significance o f nondenitrifying NO; reducers as a source of NzO evolution from soils i s unknown. Some research (Smith and Zimmerman, 1981) suggests that these organisms are more numerous in soils than denitrifies. However, since only a small proportion of the NO; reduced by nondenitrifiers i s released as NzO they are generally thought to be of minor agronomic importance.

3. Aurotrophic Nirrijicorion _- The ":-oxidizing bacteria /Vitrosomonus, Nilrosospiro, and Nirroso-

lobus have the capacity to produce N?O from NH; or hydroxylamine (an intermediate in the oxidation of NH: to NO; by these microorganisms) under most conditions (Yoshida and Alexander, 1970, 1971; Ritchie and Nicholas, 1972, 1974; Bremner and Blackmer. 1980; Blackmer et a / . . 1980; Goreau et a / . , 1980). The mechanisms that are thought to be responsible for the production of NIO by these organisms were outlined in Chapter 3.

Laboratory (Bremner and Blackmer. 1978; Freney et u l . . 1978; Good- road and Keeney. 1984) and field evidence (Denmead PI a/.. 1979; Breitenbeck el a / . , 1980; Mosier e1 a / . , 1981. 1982; Smith el a / . , 1982) has shown that losses of Nz0 can occur from soils during nitrification. How- ever, less certain i s the significance of these N:O emissions in comparison with those from denitrification.

It has also been suggested that NO., emissions may result during nitrification (Verstraele, 1981). Lipschultz e / a / . (1981) showed that cultures of

262 5. Gaseous Lasses or Nihoge

Nilrosomonos eirropaea liberated both NO and NzO during the oxidatio of NH:. The ratio of NO produced relative to NzO rose as the O1 conten within the medium decreased and a mean value of 7.5 mol NO per mole NrO produced was observed. Indeed, emissions of NO, are often served during nitrification of NH; fertilizers when applied at high ra (e.g., band applications) although they are thought to occur principally a result of NO; accumulation and subsequent chemodenitrification (s Section IV).

B. Factors AlTecting Biological Gaseous Losses

Most research dealing with factors influencing gaseous losses of N2 and N2 from soils has been centered on the process of dissimilatory deni trification. An unknown proportion of N10 emitted during such experiments is likely to have originated from nitrification of native soil NH;. detailed discussion of the factors influencing nitrification was presented Chapter 3. Conditions that favor nitrification will obviously tend to pr mote N20 losses from native or applied fertilizer NH; since the ratio N 2 0 evolved to NO, produced during nitrification appears lo be reason ably constant (Goodroad and Keeney, 1984). The factors that are gener ally known to influence denitrification are discussed below and. wher appropriate. factors known to influence losses of N1O through nitnfica- tion are also noted.

_- .- I . Aeration and Moisture

The activity and synthesis of all the N oxide reductase enzyme systems involved in denitrification are repressed by O1 (Firestone, 1982; Knowles, 1982). The presence of oxygen also inhibits the activity of preformed reductases (Payne, 1973; Stouthamer, 1976). Indeed, it appears that under aerobic soil conditions the N oxide reductase enzymes are present in repressed form and when 01 is removed from the soil there are rapid increases in the absolute and relative activities of these enzymes and denitrification commences almost immediately (Smith and Tiedje. 1979a).

Denitrification can probably occur even in well structured aerobic soils due to the occurrence of anaerobic microsites (Firestone. 1982). Anaere bic pockets in soils may often be localized areas of intense respiratory activity where Oldemand exceeds the supply (Ciaswell and Martin, 1975; Smith, 1980) rather than areas of passive anaerobiosis. Clearly, factors such as the rate of 0 2 consumption and 0 2 diffusion rate and structural considerations such as pore geometry and degrce of soil compaction are important (Smith, 1977; Ryden and Lund, 1980). In well aerated soils

111. Biologic

emissionS cation (Frl

Soil moi since with water. He! ients the r 1958; Pilot Manin, 19: lion or rair: for brief pc tion occur moisture c. nitrificatior (Goodroad

.

Fig. 5. Rate c moisture tensior. The Williams an

'P . x 3 .+!

;ses of N1O ; latory deni- '

,uch expen- ioil NH;. A ' ';

. .

, -

1 be reason- ~ I 11 are gener- and. where $

',! (mesystems ! ,2: Knowles. f preformed :. rs that under I e present in ;: re are rapid :

nzymes and edje, 1979a). aerobic soils 82). Anaero- ,: ; respiratory '!

Hartin, 1975; , :arly. factors . : nd s t ruc tud 1 mpaction are aerated soils

j

111. Biologicml Generation of Geseaus Nilragenous Products 263

emissions of NzO are likely to originate. at least partially, through nitrification (Freney et ai., 1978).

Soil moisture is obviously an important factor that influences aeration since with increasing moisture content, air in soil pores is displaced with water. Hence, as illustrated in Fig. 5, with increasing soil moisture contents the rate of denitrification generally increases (Bremner and Shaw. 1958; Pilot and Patrick, 1972; Bailey and Beauchamp, 1973; Craswell and Martin. 1974; Ryden and Lund. 1980). Following periods of intense imga- lion or rainfall, soils may become saturated with water at the soil surface for brief periods. During such periods short bursts of intense denitrification occur (Ryden et ai., 1979; Ryden and Lund, 1980). Increases in soil moisture content up to about -33 to -10 kPa also increase the rate of nitrification (Chapter 3) and thus the release of N:O from applied NH; (Goodroad and Keeney, 1984).

-10 -

m n - m z Ln

" ? -20

z

+ 4

+ z Lv -10 0 z 0 0

+ 4

2 a

Y

a t o

z z

"d 0 z 4 I 0 r10 I I I

0 - 2 - 4 - 6

SOIL MOISTURE CONTENT ( k P a )

Fig. 5. h i e a ini tnte reduclion in LWO soils (Crevasse and Mhwn) as influenced by roil moisture lension. [Rednwn from Pilol and Palrick (1971). Reprinted wilh permission from The Williams and Wilkins Co.. Ballimore.1

264

Although drainage should reduce denitrification losses by improvi soil aeration, it also transfers more dissolved inorganic N from the soil water courses, ditches. and rivers (Dowdell. 1982). Drainage water can, fact, contain measurable amounts of dissolved N?O (Dowdell er al. 1979b. Dowdell. 1984) while NO, transported in water is subject to deni trification during its journey in field drains and within stream course (e.g., Swank and Caskey. 1982).

The later reductase enzymes in the denilrification sequence appear t be more sensitive to oxygen than are the earlier reductases (Krul an Veeningen. 1977; Betlach and Tiedje, 1981) so that with increasing oxy gen concentrations there is an increase in the mole fraction of N?O emit ted (Focht, 1974).

Fluctuating moisture contents can influence the ratio of N?O:N evolved. The mole fraction of N 2 0 produced is generally high immedi ately following the onset of anaerobiosis and the proportion of N! p duced generally increases with time (Rolston e! ul.. 1976. 1978; Letey 01. . 1979; Ryden e / o l . , 1979). This effect has been attributed to hi concentralions of NO; being initially present in the soil (Rolston. 198 As noted later, high NO; concentrations lend to inhibit the reduction N 2 0 to N2.

2. Organic Carbon

The most abundant denitrifiers are heterotrophs, which require or compounds as electron donors and as a source of cellular material. the availability of organic matter is an important factor moderating the rate and total extent of denitrification. High levels of readily de posable organic matter can also indirectly enhance the potential for trification through a general stimulation of microbial respiration, causin rapid O2 consumption and an acceleration of the onset of anaerobiosis.

A general relationship between total soil organic C or N and denitrifica tion has been observed by several workers (Bremner and Shaw, McGarity, 1961; Reddy er a/ . , 1982). Howevei. it is the quantity of re available soil organic carbon that is of particular importance. Thus, rat of denitrification are highly correlated with “available” soil C as evalu ated by extractable reducing sugars (Stanford el 01. . 1975), by water soluble organic C (Bremner and Shaw. 1958; Burford and Bremner. 1975 Reddy et a[.. 1982). or by readily mineralizable C (Burford and Brem 1975; Reddy e / al.. 1982). The linear relationship between water-solub and the denitrification capacity of 17 soils is shown in Fig. 6. Factors increase the levels of available C in soils (e&, drying and rewetting freezing and thawing) have been shown to increase the capacity of soils denitrify added NO; (McCarity, 1962; Patten er 0 1 . . 1980). Denitrificati

Fig. 6. Relation 17 soils. [Redraw Pergarnon Press.]

is generally stin soils (Nommik Reddy e / al., I! relationship be! Interest since re Source for deni

The depende: tials being gene and Moore, 19t generally decre: 01.. 1978; Cho down to 60 to .

IS Loses of Nitrogen

es by improving r[ from the soil to luge waler can. in (Dowdell ef a / . , s subject to deni- I stream courses

quence appear to c m e s (Krul and 1,:.

h increasing oxy- '.'$ :ion of N ~ O emit-

alio of N10 : N2 ! illy high immedi- onion of N? pro- 6. 1978: Letey el tttributed to high I (Rolston. 1981). I ! the reduction of

:h require organic 1~ ar material. Thus moderating both

i f readily decom- ioienlial for deni- ipiration, causing of anaerobiosis. N and denitrifica- and Shaw. 1958; luantity of readily ance. Thus, rates ' soil C as evalu- 1975). by water-

,d Bremner, 1975;. 3rd and Bremner, n water-soluble c ig. 6. Factors that and rewetting Or apacity of soils to 0). Denitrification

111. Biological Generation of C~crous Nilrogenovs PraducLr 265

WATER-SOLUBLE ORGANIC CARBON (agC--g-' ) Fig. 6. Relationship between denitrification capacity and water-soluble organic carbon in

17 soils~ [Redrawn from Burford and Bremner (1975). Reprinted wirh permission from Pergamon Press.]

is generally stimulated by additions of organic compounds and residues to soils (Nommik, 1956; Bowman and Focht. 1974; Guenzi ef a / . . 1978; Reddy et a / . . 1978; Rolston et a/.. 1982). Indeed, in many situations, the relationship between total soil C and denitrification is only of academic interest since readily available C sources are likely to provide the major C source for denitrification.

The dependence on C availability results in higher denitrification potentials being generally found in surface soils rather than in subsoils (Khan and Moore, 1968). Denitrification activity and populations of denitrifiers generally decrease with soil depth (Bailey and Beauchamp, 1973; Brar ef 01.. 1978; Cho ef a/ . , 1979) although significant denitrification can occur down to 60 to 70 cm (Myers and McGarity, 1972; Rolston er al. . 1976;

266 5. Gaseous Losses a1 Nitrogen'

Gilliam et 01.. 1978). Organic soils generally have high potentials for deniii trification (Bartlett et al . , 1979; Reddy e / al., 1980; Terry el al., 1981).

Carbon availability can also influence the proportion of N20 and N2 produced. With increasing available C there is generally a more complete' reduction of NO; and therefore less N 2 0 production in relation to that of N2 (Delwiche, 1959; Focht and Verstraete, 1977; Rolston er a l . , 1978; Smith and Tiedje. 1979b).

3. Nilrate Supply

The apparent K, values (which indicate the NO; concentration re- quired to give half the maximum velocity ofdenitrification) for the dissim-' ilatory reduction of nitrogen oxides, determined in uiuo or with purified enzymes, are normally in the range of 5 to 290 pM (see Knowles, 1982).' Firestone (1982) noted that a K , value of 15 pM NO; for denitrifying I

bacteria would be equivalent to a concentration of 0.04 p g NO;-N per' gram of soil that had a moisture content of 20%.

Thus it is not surprising that at relatively high concentrations of NO,- (greater than 40 to 100 p g N g n - ' ) the rate of denitrification in soils has been shown to be independent of NO; concentration, Le., denitrification follows zero-order kinetics (Kohl et a / . , 1976; Focht and Verstraete, 1977; Blackmer.and Bremner, 1978). However, in soils the diffusion of N the sites of denitrification can become an important limiting factor (Phil- lips et al . . 1978; Reddy e t a!. , 1978) so that denitrification reactions ar frequently reported to be first order up to 40 to 100 p g N gm-l in soils (Stanford et al., 1975; S t a n and Parlange, 1975; Ryzhova. 1979). Indeed presumably because of the limiting effecl-of the diffusion of NO;, values for NO; reduction in soil are much higher than those obtaine cultures and range from 130 to 12,000 p M NO; (Bowman and Foc 1974; Yoshinari el al., 1977).

in the ratio of N 2 0 to N2 in the product gases (Nommik. 1956; Blac and Brernner, 1978: Firestone e t al., 1980; Letey et a(., 1980; Terr Tate, 1980; Gaskell et al.. 1981). The effect of NO; concentration I acts with soil pH such that the inhibitory effect of NO; on N20 reduct increases markedly with a decrease in soil pH (Blackmer and Bremner

High levels of NO7 can inhibit the reduction of N20 causing an increas

4. Nihifioble N

the amino acid alanine) yielded more NzO than similar nonamended

NO;-trea. In these e able N ar nitrapyrin tion) (Bre expected frequentl! 1982; Smi

5.

In pure positively (Nommik Generally of PH (Bi values bel Ol . , 1978; AI and h. workers I below 5.0 and Keen (0 some e solubility

lasses or Nitrogen

more comdete 7

ation to that of '4 n e t a / . , 1978; ;

'.

ncentration re- . ,

for the dissim- !';$ )r with purified lvt

howles. 1982). \ $ lor denitrifying ,;%

i@ pg NO;-N per "II

in:

,A,

rations of NO; ion in soils has , denitrification erstraete, 1977; ision of NO; to ing factor (Phil- In reactions are N gm-I in soils , 1979). Indeed. sn of NO;, Km lose obtained in nan and Focht.

sing an increase 1956; Blackmer 1980; Terry and :entration inter- n N20 reduction :r and Bremner,

,O have accrued atory incubation

111. Biological Generation or Gaseous Nitrogenous Produds

Table 111

Quenlilies of Nilrous Oxide Released horn Well Aermled Soils Trealed wilh Dillerenl Forms or Nilrogen"

N?O.N released (ng em-l soill

Form Rate 8 days JOdays

None 0 2 5

(NH.)?SOd so 52 58 Urea 50 58 61 KNO, 50 4 5

(NH,j?SO, IW 146 I53

KNO, IW 4 7 Urea 1W 118 124

e Dam from Bremner and Blackmer (1978).

267

NO;-treated soils (Table 111) (Bremner and Blackmcr, 1978, 1980, 1981). In these experiments. N 2 0 production often increased linearly with nitrifi- able N and. furthermore. losses were markedly reduced by addition of nitrapyrin (a compound that selectively inhibits autotrophic NH: oxidation) (Bremner and Blackmer. 1978). Under field conditions, however, the expected relationship between soil NH: concentrations a id N1O fluxes is frequently complicated by simultaneous denitrification (Mosier et al.. 1982; Smith e t 01. . 1982).

5. pH

In pure cultures and in soils, the overall rate of denitrification is often positively related to pH and has an optimum in the range of pH 7.0 to 8.0 (Nommik, 1956; Van Cleemput and Patrick, 1974; Muller er al.. 1980). Generally, in the neutral pH range of soils (pH 6 to 8) there is little effect of pH (Burford and Bremner. 1975; Stanford et 0 1 . . 1975) but at soil pH values below 6.0 denitrification can be strongly inhibited (Klemedtsson et a/. , 1978; Muller et a! . . 1980). At pH levels below 5.5, toxic levels of soil AI and Mn could well limit microbial activity. Nevertheless. several workers have reported significant denitrifier activity at soil pH values below 5.0 (e.&, Gilliam and Gambrell, 1978; Muller e t a / . , 1980; Koskinen and Keeney. 1982). In short-term laboratory studies, increasing pH may, to some extent, increase denitrifier activity by temporarily increasing the solubility of soil organic matter (Fillery. 1983).

o r because of denitrilier populations with a low pH optimum or wide pH

tant N1O emission, from a limed soil (pH 6.7) than from an unlimed: control (pH 4.7).

It appears that the NIO reductase enzyme system is more sensitive than' the otherreductases to low pH such that, as noted in the previous section; the mole fraction of N?O produced increases as the pH falls (Blackmer,'

major product (Nommik. 1956). Such an effect appears to occur only in the presence of added NO; (Firestone et a).. 1980).

6. Temperafure

cation.increases for a 10°C rise in temperature) o i about 1.0 (Dawson and

The unusually high optimum temperature reported for denitrification may. in part. be the result of the presence of,thermophilic Bacillus spp::, (Focht and Verstraete. 1977). However, above 50°C chemical decomposi- i tion reactions of NO; increase in importance (Keeney et ai.. 1979) so that: the high optimum may not be wholly of biological origin. To some extent,'

was raised from 10 to 3 0 T .

and Chalamet. 1982). Maximum rates generally occur in the afternoon an

perature dependence of both denirrification and nitrification. Denitrification appears to follow a seasonal trend with losses of N

plus N2 being markedly higher in summer than in winter (Rolston e / a

111. Biologid General

Table I V

ERed a1 Temp Rmlia Evolved

Temperalure ( T I

7 I5 25 40 50 60 65 67 70 75

Data from 43, p. 1126 by

Initial NO;

1978). Bremner er N10 fluxes from UI

In general, incre N2 to N20 in the pr Keeney el a/., 1975 little effecr (Bailey

7. Plants

Plant roots have ence the potential materials (organic tion of soluble corn and production 01 1979). Thus, large I sphere (Woldendor numerous than in I carbonaceous mate [he soil of Oa. as,

It is evident that Presence of plant rc

i Losses or Nitrogcn::~~.:

wnding soil and/$ num or wide pH #, for nitrification : i:

oad and Keeney : :.' .;. and concomi- a

:om an unlimed'..j

re sensitive than irevious section,, .:> falls (Blackmer

N:O may be the to occur only in . ;:

. ,

8 L . . ,!?

. ? re. Below lo°C denitrification is

To some extent

111, Biological Generslion of Gaseous Nilrogenour Producls 269

Table IV

ERrcl or Tempemlure on Gaseous N Lo- and Ihr NIO/(N,O + N, J Ratio Evolved'

Total Gaseous N Temperamre incubarion (% of iniiial N?O/(NIO + N?)

("C) lime (days) NO;-N in sys~ern)~ (%I

I5 7

40 25

50 60 65 67 70

75

16 16 16 4 4 4 4 4 4 4

I I I2 44 63

125 134 127 143 I09 0

44 49 19 69 0 0 0 0

87 -

a Daia from Keeney et 01. (1979). Reproduced lrorn Soil Sci. 50c. A,". J . 41, p. I126 by permission of rhe Soil Science Sacieiy of America.

lnilial NO;-N in rhe soil syrlern = I ? ? wg N gm-'.

1978). Bremner el a/. (1980b) observed similar seasonal Ruciuations in N?O fluxes from unfertilized agriculturaS soils.

In general, increasing temperature tends to increase the proportion of N: to N!O in the products of denitrification (Nommik, 1956; Bailey, 1976; Keeney el a[., 1979) although in some cases temperature appears to have little effect (Bailey and Beauchamp. 1973).

7. Plants

Plant roots have several effects on the rhizosphere soil that may influence the potential for denitrification. First, roots release carbonaceous materials (organic substrate for denitrifiers) into the rhizosphere by excre- tion of soluble compounds, sloughing off roo1 surface and root cap cells, and production of mucigel polysaccharide (Warembourg and Billies, 1979). Thus, large populations of denitrifiers frequently exist in the rhizosphere (Woldendorp, 1963), where they may be 10 to 100 times more numerous than in the root-free soil (Netti, 1955). The metabolism of the carbonaceous material by the rhizosphere microflora will tend to deplete the soil of 0 2 . as, indeed, will root respiration.

It i s evident that if denitrification is limited by O2 or C supply then the prcsence of plant roots will tend to stimulate denitrification. Many studies

I I I

IV, ChomodenitriliC.

(Catchpoole ef 1.

release of N20, y content (Sheri and release Of b ,uggested (Sherl lion due to r a w readily available

9. Tilla

In compariso and the presenc bulk density, a aerobic aggrega soil (Dowdell er C can also be h 1984). Such soil ers and denitrif under zero rathg Doran, 1984; BI N>O (Burford e P I 01.. 1984a,b) tilled fields alth ably. Aulakh ei conventionally respectively.

The generall! and the smalle Smith, 1983; BI zero as compar cation rather i t

“1. . 1979). How

Iv. C€

Chemodenit! chemical reacti variety of nitrc Such gases an sterilized soil I tion of added P and Bremner,

seam Losses of Nitrogen ,i 'ii T nces the denitrifica-

efanson, 1972a,b,c; '1; 976) and sometimes I oduced (Stefanson, .:.

while wheat roots 1 er content they had -:$ ' p NO; and so deplete 'i: y of NO; is limiting , . . '

e of denitrification . Tiedje (l979b) con- .':, lenitrification rates concentrations are .:

L:

of roots.

b y grazing animals NH, volatilization

al amount of N (N: + 4

IV. Chernodenihifieetion 271

(Catchpoole et a/.. 1983; Sherlock and Goh. 1984) and leaching (Ball et ul. , 1979). However, deposition of urine also results in an immediate release of N10. which does not occur from additions of urea of equivalent S content (Sherlock and Goh, 1983). The reason for this rapid production and release of NzO is unknown although several mechanisms have been suggested (Sherlock and Goh, 1983), including stimulation of denitrification due to rapid onset of anaerobiosis caused by concomitant inputs of readily available C and rapid urea hydrolysis.

9. Tillage Merhod

In comparison with conventional tillage, the lack of soil disturbance and the presence of a surface mulch under zero tillage result in increased bulk density. a reduction in large pores, reduced aeration. larger but less aerobic aggregates, and a generally higher moisture content in the surface soil (Dowdell er a/., 1979a: Lin and Doran. 1984). Levelsof water-soluble C can also be higher in surface soils under zero tillage (Lin and Doran. 1984). Such soil conditions obviously tend to favor the activity of denitrifiers and denitrifier populations are generally greater in the surface soil under zero rather than conventional tillage (Aulakh er a/ . , 1984a; Lin and Doran. 1984; Broder er 01. . 1984). Studies have also shown that losses of NzO (Burford et a/., 1981) or N?O plus N? (Rice and Smith. 1982: Aulakh er d.. 1984a.b) are greater from zero tilled rather than conventionally tilled fields although the ratio of N20 : N? emitted is not changed measur- ably. Aulakh er d. (1984a) estimated N20 plus N? losses from cropped, conventionally tilled and zero tilled fields as 3-7 and 12-16 kg N ha-' yr-I respectively.

The generally lower rate of mineralization, and therefore nitrification, and the smaller populations of nitrifiers under zero tillage (Rice and Smith. 1983; Broder ef a / . , 1984) suggest that greater losses of NZO under zero as compared to conventional tillage are the result of greater denitriii- cation rather than nitrification. -

N. CHEMODEMTRIF'ICATION

Chemodenitrification is the term commonly used to describe various chemical reactions of NO: ions within soils that result in the emission of a variety of nitrogenous gases (e.g., N?, NO, NOZ, and sometimes NzO). Such gases are of nonbiological origin since they are also evolved from sterilized soil to which NO; has been added. Normally, a higher proportion of added NO;-N is converted to (NO plus NO&N than to NI (Nelson and Bremner. 1970b; Bollag er d.. 1973). Also the [(NO + NO&N : Nz]

5. Gaseous Losses 01 Nitq

ratio usually varies from 2 : I at soil pH values of 5.0 to 5.8 to about I : 1

sometimes evolved from soils following treatment with NO; (Reuss Smith, 1965; Smith and Clark, 1980a.b).

A. Nitrile Accumulation

(Nelson, 1982; Chalk and Smith, 1983) and are outlined below.

proceeds at a faster rate than the conversion of NH; to NO; (see Ch 3). Consequently, NO; is not normally present in amounts greater t pg gm-'. High concentrations may, however, accumulate when N

reach 10 and the N concentration 'may be several thousand pg N (Parr and Papendick. 1966; Chalk er a/ . , 1975). The activity of the oxidizer Nitrobocrer is more greatly inhibited by high pH and high levels than is that of the NH: oxidizers. Thus, in fertilizer bands, can accumulate up to several hundred pg N gm-' (chalk er d., 19

Nitrite has been shown to accumulate during nihhcation of urea, salts, and anhydrous and aqua ammonia (Hauck and Stephenson, Wetselaar er a / . . 1972; Pang eta/ . , 1973. 1975; Chalk era / . . 1975). N is particularly reactive under acidic conditions. which may occur ar the periphery of fertilizer granules o r bands where nitrification of fertilizer is complete. Nitrite that accumulates in the alkaline fertili

son. 1965). Nitrite can also accumulate in alkaline soils treated with hydrolyzing NH: fertilizers such as (NH,),SO, (Bezdicek er 01.. and in urine patches on grazed pastures (Vallis et d., 1982) where amounts of urea N are deposited over a small surface area. Accumul

fication (Cady and Bartholomew. 1960; Doner ef a/.. 1975; Cooper

In laboratory studies, a number of workers have observed large defi

1". Chrmodenilrilicalial

such losses to cherr Steen and Stojanov N as N20, and app: of NHt-N (see Sec

B. Mechan

A variety of rea losses of N by che: below.

I . Decomp

Nitrous acid is p soils:

At pH 5.4, and 3, th nitrous acid are 1.9 Nitrous acid can UT

In closed incubat- lions in the laboratc ber of additional fac NO1 and both gase (Nelson. 1982). The

In an anaerobic : appears in the atmc is

The proportion c 'related to soil orgal because of the large Nelson. 1969; N e k cant amounts of NC PH greater than 5.5 indicate that self-d where the pH is con (Nelson and Bremn

Losses 01 Nitrogen .,'$ it,

to about 1 : I at - $ f N 2 0 are also ? IO; (Reuss and $

iere NO; accu- of NO; in soils letail elsewhere JSlOW.

of NO; to NO; 0; (see Chapter !s greater than I e when N fertil- '$ band-applied to .$i ite, urea ammo- $I

,lkaline fertilizer

:ek el d.. 1971)

IV. ChemodenihiScmtian 273

such losses to chemical reactions of NO; (Hauck and Stephenson, 1965; Steen and Stojanovic, 1971). It is. however, noted that gaseous losses of N as N20. and apparently NO, can occur during autotrophic nitrification of NH:-N (see Section 111).

B. Mechanisms

A variety of reactions have been proposed to account for gaseous losses of N by chemodenitrification. The major reactions are discussed below.

I . Decomposition of Ninous Acid

Nitrous acid is produced when NO? is added to, or formed in. acid

NO; + H * - HNO? (13)

At pH 5 , 4 . and 3. the proportions of the nitrite N present as undissociated nitrous acid are 1.9, 16, and 74%, respectively (Chalk and Smith, 1983). Nitrous acid can undergo spontaneous decomposition as shown below:

soils:

?HNO, - NO + NO? + H?O (14)

In closed incubation vessels, which are often used to study these reactions in the laboratory, the products actually obtained depend on a number of additional factors. In an aerobic system, NO is usually oxidized to NO2 and both gases may then be absorbed by the moist soil as HNO, (Nelson, 1982). The overall reaction then becomes

ZHNO? + O1 - IHNO, (15)

In an anaerobic system, NO? is normally adsorbed as before but NO appears in the atmosphere. Under these conditions. the overall equation is

3HN0, - 2N0 + HNO, t H,O -. (16)

The proportion of added NO; evolved as (NO plus NO?)-N is not related to soil organic matter content but increases with decreasing pH because of the large propoflion of HNO2 present at low pH (Bremner and Nelson, 1969; Nelson and Bremner, 1970b; Bollag ef a/.. 1973). Signifi- cant amounts of NO and NO2 are produced when NO; is added to soils of PH greater than 5.5 (Porter, 1969; Nelson and Bremner, 1970a). This may indicate that self-decomposition o l HNO? occurs at colloid surfaces, where the pH is considerably lower than that of the measured bulk soil pH (Nelson and Bremner, 1970a).

214 5. Gaseous Lossn of Nilrogc

Under field conditions, the extent to which any of these decomposition reactions takes place is not well documented. Several workers have ques-' tioned the importance of HNOl decomposition in relation to gaseous. losses of N from soils (e.g., Broadbent and Clark, 1965; Allison, 1966: Broadbent and Stevenson. 1966). Despite this. a number of workers hav? recorded emissions of (NO plus N02)-N from untreated soils and from soils treated with urea and NH: fertilizers (Steen and Stojanovic, 1971: Kim. 1973; Galbally and Roy, 1978; Smith and Chalk, 1980a; Johansson' and Granat, 1984).

2. Reactions of Nirrous Acid with Organic Matter

Several researchers have shown a positive relationship between so organic matter content and the rate of NO? decomposition, with the concomitant emission of N2 and N?O, when NO; is added to soils (e.g., Reuss and Smith, 1965; Nelson and Bremner. 1970b). It constituents of soil organic matter that are largely, if not entirely, respo sible for such formation of N2 and N2O (Bremner and Nelson. 196 Stevenson el a/. , 1970).

reaction of H N O ? ~ with phenolic constituents are only stood. The reactions are known as nitrosation reactions addition of the nitroso group (-N=O) to an organic molecule by react- with nitrous acid. Two possible mechanisms thought to be responsible the reactions of "02 with phenols are sh0w.n in-Fig. 8.

Nitrosation reactions also result in fixation of NO; by soil orga matter through the formation of nitroso groups on phenolic rings (Bre ner. 1957; Bremner and Fuhr, 1966; Smith and Chalk, 1980b). Bremne and Fuhr (1966) found that when NO; was added to soils with pH values ranging from 3 to 7, part of the N was fixed by organic matter (lO-ZS%) and part was convened to gaseous forms (33-79%). The NO; that is fixed to organic matter is resistant to biological decomposition (mineralization) (Bremner and Fuhr, 1966; Smith and Chalk, 1979).

As well as Nz and NzO. NO and nitromethane (CH,ONO) have detected as reaction products of the reactions of NO; with organic mat (Stevenson and Swaby, 1964; Edwards and Bremner. 1966; Stevenson al . . 1970; Steen and Stojanovic. 1971). Several reaction m been suggested to explain such emissions (see Chalk and Smith, 1983

The mechanisms involved in the formation of gaseous products

3. Reactions of Nihous Acid with Compounds Containing

The reaction between "01 and compounds containing free amino

Amino Groups

- groups (e-g.. amino acids, urea, and amines) has long been suggested as'

IV. Chemodenilrilica.

I OH

k Fit. 8 . Two pos i t

lomation of p-niuoro lamaLion of NI and : romaLion of an o - n i r ~ group in the diazoniun (Arcer Nelson (1982). i ed.1 Agronomy Mono. Madison. Wirconsin~l

possible mechani: reaction only take equal quantities fr

Although this re lance as a mechan several workers h: produced when fi Stevenson et al . , (1980b). for instan sterilized soils w a the added NOT-N Such results suggt nitrosation reactic evolved N2.

4. Reacth

Solid ammonium Produce N, gas (V slowly from concc (Smith and Clark,

jy reaction onsible for

i i l organic Igs (Brem- I. Bremner pH values

r (10-28%) hat is fixed :ralization)

have been anic matter evenson el

IV. Chemodenilrifieation 275

2 OH OH OH OH

k A R R

Fig. 8 . Two possible reactions of phenols wilh n i l m u acid. Mechanism ( I ) involves formaiian of p-niirasophenol. Iaulorne%?alion of !his producl lo a quinone monoxime. and formation of N 1 and N1O by reaction of Ihe oxime wilh "0 , . Mechanism (2) involves formalion of an o-nilrosophenal and production of N, through decomposilion of the diazo group in lhe diazonium compound formed by reaction of lhir o-nitrosophenol wilh "0: . [After Nelson (19821. Reproduced from "Nilrogeo in Agrjcullural Soils" (F. J. Stevenson, ed.) Agronomy Mono. 22, p. I53 by permission of the American Society of Agronomy. Madison. Wisconrin.1

possible mechanism for gaseous N loss from soil. This "Van Slyke" reaction only takes place at low pH and the N2 gas evolved is derived in equal quantities from the two reactants:

R-NH, + "0; - R.OH + H20 + N1 (17)

Although this reaction is generally considered to be of limited impor- lance as a mechanism for gaseous losses of N from soils (Nelson, 1983, several workers have suggested it as responsible for a! least part of the N2 produced when NO; is added to acid soils (Reuss and Smith, 1965; Stevenson et ol., 1970; Smith and Chalk, 1980b). Smith and Chalk (l980b), for instance, found that the 'IN enrichment of evolved N2 from sterilized soils was approximately one-half that of the ISN enrichment of the added NO;-N. Christianson er d. (1979) observed similar results. Such results suggest that Van Slyke-type reactions were involved since nitrosation reactions alone would result in no isotopic dilution of the evolved N1.

4. Reaction of Nitrite with Ammonium

Solid ammonium nitrite ("do2) explodes on heating to 60 to 70°C to produce Nr gas (Weast, 1977). The same reaction proceeds much more Slowly from concentrated solutions of NH4NOz at low pH (pH < 5.2) (Smith and Clark, 1960). Since applications of NH: or NH:-forming fer-

v. ~ r i e o l . Sigoficmnc

that Fez+ may pro1 lion of NO; in search since signi soils. Indeed. Mol position of NO;. cally in a high Fez- found that soil cor NO; decompositi.

tilizers can result in the accumulation of both NH: and NO; in s some workers have suggested that significant gaseous loss of N mi occur by chemical decomposition of ",NO? (e~g., Allison, 1963; E and Bauet. 1966):

NH; + NO; - ",NO: - NI + H?O In general, ",NO? decomposition does not occur during incubation

air drying of acidic soils containing NH: and NO? but some decompo tion of ",NOz can occur when light-textured. neutral, and alkaline soils. treated with NH; and NO? are air dried (Wahhab and Uddin, 1954; Bre ner and Nelson, 1969; Jones and Hedlin. 1970)~

Thus, it is thought that the reaction is not of general significance i regard to gaseous losses of N from soils (Nelson, 1982) except perhaps when neutral or alkaline soils containing high concentrations of NH; and NO; are subjected to drying conditions.

5. Renction of Nitrous Acid with Hydrorylamine

A number of workers (e&, Arnold, 1954; Wijler and Delwiche, 195 Vine, 1962) have speculated that the chemical reaction of hydroxylami (NHzOH) with HNO? might generate NzO:

":OH + HNO: - N1O + ?H1O Although i t has been shown that "?OH can be quantitalively dec

posed by HNOZ (Nelson, 1978). Bremner el o/.~(-1980a) found that NHzOH was added to soils, large amounts of NzO were formed in absence of HNO?. It was suggesred by Bremner P I ol. (198Oa) that Nz formed in soils through other nonbiological transformations of NH and very little is generated by the reaction of "?OH with HNOz. S nonbiological transformations were postulated to involve oxidized fo of Mn and Fe (e.g.. Mn02 and Fe?O,).

The fact that NH20H has not been detectedin soils makes the cance of the above reactions questionable (Nelson, 1982). Hydroxy is, however, a postulated intermediate of both the biological re NO; to NH; (Alexander, 1977; Yordy and Ruoff, 1981) and the oxidati of NH: to NO; (see Chapter 3). The fact that it is not present in soils m be a consequence of its rapid decomposition.

6. Other Reactions

Several other reactions have been suggested as possible pathw chemodenitrification including reactions of HNO? with clay min transition metal cations. Such reactions seem unlikely to be si sources of gaseous N loss from soils although Nelson (1982) sugge

V. EXTE:

Global estimate were presented ir, ments of gaseous such emissions ar

A. Amm, 1. Extenr

The amounts o yielding fertilizer: such factors as t) and environment; fenilizer N applie, be in the range of Craig and Wollur Significant, but v; plications to f l o o ~ applied in the irri

Volatilization o or sewage sludge; 01.. 1981; Beauch in !he range of 10- Pastures can simi Plied urea N (Har Coh. 1984).

2. Fore o

Ammonia is pr ammonium in wai

..

3 Loses ol Nilrogen

d NO; in soils,

j in . 1954; Brem-

lalively decom- nmd lhat when : formed in the 3Oa) that N 2 0 is ons of NHlOH :h "0:. Such oxidized forms

ikes the signifi. Hydroxylamine :al reduction Of

,d the oxidation tnt in soils may

le pathways Of .y minerals and ) be significant 982) suggested

v. ExIeol. Slgnihcnace. mnd Fnle ol Loses 217

that Fe2+ may promote decomposition of NO; formed by microbial reduction of NO, in waterlogged soils. This suggestion deserves further research since significant amounts of Fez+ are often present in anaerobic soils. Indeed, Moraghan and Buresh (1977) showed that chemical decomposition of NOT, with the evolution of Nz and N1O. occurred anaerobically in a high Fez+ ion environment while Van Cleemput and Baert (1984) found that soil conditions promoting the formation of Fe'+ also promoted NO; decomposition and NO emissions.

V. EXTENT, SIGMFICANCE, AND FATE OF LOSSES

Global estimates of gaseous losses of N from the plant-soil system were presented in Chapter I . In this section some recent field measurements of gaseous losses of N are reported and the fate and significance of such emissions are outlined.

A. Ammonia Volatilization

I . Extent of Losses

The amounts of NH, volatilized from applications of NH:- or NH:- yielding fertilizers are extremely variable (Terman. 1979) and depend on such factors as type, rate, and method of fertilizer appl&tion, soil p H , and environmental factors such as temperature and moisture. Losses of fertilizer N applied to the surface of grassland or bare soil often appear to be in the range of 0 to 25% (Hargrove and Kissel. 1979; Hoff et a/.. 1981; Craig and Wollum. 1982; Catchpoole el d.. 1983; Black el al., 1985b). Significant. but variable, losses of NH3 can occur following fertilizer applications to flooded soils (Vlek and Craswell. 1981) or when fertilizer is applied in the irrigation water (Denmead et d.. 1982).

Volatilization of NH3 from organic amendments (e.g., animal manures or sewage sludge) applied lo soils can be large but also variable (Hoff e/ 01.. 1981; Beauchamp e / a/.. 1982; Beauchamp. 1983). Losses are often in the range of IO-60% of applied N. Losses from urine patches on grazed pastures can similarly be relatively high, ranging from IO to 60% of applied urea N (Harper et a / . . 1983; Simpson and Steele. 1983; Sherlock and Goh, 1984).

2 . Fate of Ammonia Emissions

Ammonia is present in the atmosphere as a gas and in the form of ammonium in water droplets and solid particles. Concentrations of NH,

278

and NH: vary widely in the atmosphere due to inhomogeneity of th'e sources and sinks (e.g., biological sources and precipitation scavengi The major removal mechanisms for atmospheric ammonia are by wet dry deposition. These processes are very rapid and the mean atmosph lifetime of NH, is in the region of 7 to 14 days (Hahn and Crutzen. 1982 Galbally and Roy, 1983).

Gaseous deposition of NH, is an imporrant mechanism for the return o volatilized NH, to the biosphere since plants, land surfaces. lakes, and' oceans can all act as sinks as well as sources for atmospheric NH, (Calder. 1972; Dawson. 1977; Georgii and Gravenhorst. 1977; Farquhar e;.

5. Gmseour Lasses or Nitmg

a / . , 1983). Ammonia volatilized at a particular sile can therefore be reab sorbed by direct gaseous uptake nearby. However, the fraction of NH that is converted to an atmospheric aerosol can travel to more distanf locations.

Ammonia is extremely soluble in water and once dissolved it ionizes to NH; :

NH, + H:O ==== NH; + OH-

Thus, in the troposphere, NHl quickly dissolves in water droplets clouds with the formation of NH:-containing aerosols. Atmospheric ae sols of HzSOq (often from industrial sources) can be quickly ammon to NHqHS04 and (NH4)zS04 under most tropospheric conditions (H zicker et 01.. 1980). Much of the emitted NH, is therefore present in atmosphere as aerosols in the form of NH: salts (Taylor ef a/. . 1983) as N H ~ N O I or (NH4)zSOd. It is removed from the atmosphere pred nantly by wet deposition. Ammonium found in rainwater can orig from sources hundreds or thousands of kilometers away (Lenhar Gravenhorst, 1980). Upon evaporation of water, aerosol particles also be returned by dry deposition.

B. Denitrification and Nitrificalion

1. Exrenr of Losses

Few studies have measured directly total losses of applied I5N plus "N20) through denitrification (and nitrification) occurring unde conditions (Rolston, 1978; Rolston and Broadbent, 1977; Rolston e 1978, 1982). Rolston et al . (1978) measured Nz plus NzO losses from at two tempentures, two water contents. cropped and uncropped

v. g t e n l . Sign

added). Rok an entire grc which repre$

Several wc 1984) have i! y20 fluxes i i

of N10 to b inhibits nitrii thus NIO em lion losses o and 500 kg 01 Dowdell. 19: y r - ' from a from winter

From the I

can vary COJ ized that din nnge from C grassland so

While the ingly variabl 1981; Aulakb denitrificatic merits have den er al., I

The exter known althc NIO are emi son and Mc Cochran e l , Losses of P ringed from a/.. 1981) wI from less t h (Brcmner r , may have o by denitrific

Although sphere. dur Water. Mea: EOilS (Dowd with gaseou

iomogeneity of the i 'itation scavenging). ~,

lonia are by wet and e mean atmospheric : and Crutzen. 1982; '

ism for the return of ; surfaces, lakes, and #r atmospheric NHI ,,:,, st . 1977: Farquharer !j in therefore be reab- the fraction of NHI

ivel to more distant

lissolved i t ionizes to

in water droplets in s . Atmospheric aero- quickly ammoniated

ric conditions (Hunt- :refore present in the ylor er ul.. 1983) such atmosphere predomi- inwater can originate j away (Lenhard and aerosol particles may

s of applied "N ("Nz ) occurring under field

io (for the uncr

v. Exleal. Significance. and Fsle 01 Loves 279

added). Rolston and Broadbent (1977) measured N? plus NzO losses over an entire growing season and calculated a loss of about 13 kg N ha-', which represented approximately 9% o f applied fertilizer N.

Several workers (e&. Ryden el al. , 1979; Ryden. 1981; Colbourn eta/ . , 1984) have indirectly measured total denitrification losses by measuring N?O fluxes in the presence of acetylene (which inhibits funher reduction of N20 to N2 in the soil). The injection o f acetylene into the soil also inhibits nitrification (Hynes and Knowles, 1978; Walter er ul.. 1979) and thus NiO emissions from that source. Ryden (1981) estimated denitrification losses of I 1 and 29 kg N ha-' yr-' from grassed plots receiving 250 and 500 kg o f fertilizer N ha-I. respectively. Other workers (Colbourn and Dowdell. 1984; Colbourn r r ul.. 1984) estimated losses of 18-38 kg N ha-' yr- I from a grassland receiving 210 kg N ha-' and 7-13 kg N ha-' yr-' from winter wheat receiving a fertilizer addition of 70 kg N ha-'.

From the small amount ofdata available i t is evident that gaseous losses can vary considerably. Colbourn and Dowdell (1984). however, general- ized that direct and indirect estimares o f losses of N: plus N?O from soils range from 0 to 20% o f fertilizer N applied to arable soils and 0 to 7% on grassland soils.

While the mole ratio of N1O produced during denitrification i s exceed- ingly variable (Rolston er a/., 1978, 1982; Ryden er 0 1 . . 1979: Rolston, 1981; Aulakh er ul.. 1984a). in general. the quantity o f N? produced during denitrification i s much greater than that o f N10. For example, field experiments have yielded time-averaged N20 mole fractionsol 0.12-0.18 (Ry- den er ul., 1979) and 0.20-0.30 (Rolston er 01.. 1982).

The extent of losses of N 2 0 through the nitrification pathway is not known although field studies have confirmed that significant amounts of NIO are emitted during nitrification of applied NH: fertilizers (Hutchin- son and Mosier. 1979; Breitenbeck et a/., 1980; Bremner er ul.. 1981: Cochran era/., 1981; Mosier and Hutchinson. 1981; Mosier er ul.. 1981). Losses o f NIO following applications of urea or- NH: fertilizers have ranged from 0.2 to 0.6% of applied N (Breitenbeck era/.. 1980; Mosier cr [ I / . . 1981) while losses following injection of anhydrous NH, have ranged from less than 0.1% (Cochran el a/ . . 1981) to 4.0 to 6.8% o f applied N (Bremner et al . . 1981). Although losses of N?O in [he above experiments may have originated predominantly from nitrification. emissions caused by denitrification in anoxic microsites cannol be ruled out.

Although N 2 0 is usually assumed to be lost from soil only to the atmosphere. during winter it can also leave the soil dissolved in drainage water. Measured losses range from 0.25 to 4.4 kg N ha-I from agricultural soils (Dowdell er a / . . 1979b; Harris rr ul., 1984). which were comparable with gaseous losses o f N 2 0 over the same period.

280

2. Nitrous Oxide and Snarospheric Ozone

Much research on denitrification, and more recently nitrification, h& been prompted by a concern that NzO released into the atmosphere b; these processes may increase the rate of reactions in the stratosphere that' lead to the destruction of the ozone (0,) layer (Crutzen and Ehhalt. 1977; McElroy er a/. . 1977; National Research Council, 1978). The strat$ spheric ozone layer shields the biosphere from harmful UV radiation and also influences the vertical temperature profile and thus earth surfaci; temperatures (Ramanathan el a/., 1976; Wang et d. 1976; Hahn, 1979):

Atmospheric photochemistry in relation to the role of nitrogen oxides has been reviewed in detail elsewhere (Crutzen. 1981. 1983; Hahn a id Crutzen, 1982). The low solubility of NzO in water means that there is ne significant removal of atmospheric NIO from the troposphere by preci tation and it penerrates. almost unimpeded, into the stratosphere. At spheric destruction of N 2 0 occurs through photochemical reactions in stratosphere:

N1O + h v - N, + 0

N1O + O('D) - N? + 0:

N,O + O('D) - ?NO The electronically excited O('D) atom is produced by photolysis

ozone in the stratosphere. Approximately IO%--of stratospheric Nz0 thought to be convened to NO by reaction (23). Direct transport into the stratosphere from the earth's surface is unlikely becaus short atmospheric residence time of NO,, which is quickly conv HNOl aerosols and thermally unstable organic nitrates (e-g.. peroxya tyl nitrates-PAN) and removed by wet and dry deposition (Crutz 1981).

One of the major sinks of 0, is reaction with NO,, which catalyzes destruction of Ol above 25 km in the stratosphere (Crutzen, 1981, 19 However. below 25 km, NO, protects 0, from destruction (Logan et 1978; Zahniser and Howard, 1979). Thus, the major effect of inc production of NO, in the stratosphere is likely to be a lowerin center of gravity of the stratospheric ozone by a transfer of altitudes below about 25 km (Crutzen. 1981). Nonetheless, Crutzen ( calculated that increasing N 2 0 emissions will tend to enhance 03 1 through net catalysis of its destruction in the entire stratosphere; a d bling of atmospheric NzO abundance might yield a 12% decreas total O1.

It is interesting to note that, overall. the global source of NlO fro fertilizer applications is probably smaller than a few Tg N yr-' (Crulz

v. Exlent, Sigdcance.

1983). Thus, the ir, ozone is un

[hat the global UPH o.?% per year, wt increase in global :

3. Fore oj

The quantities 01 lion are extremely rrogen molecule is of millions of year

Upon release of removed by gaseoi c / d . . 1975). Howl when potentially h N?O to Nz is usu: seems unlikely tha 1978).

Thc approximar and Roy, 1983). ~

removal of N 2 0 fi (211, (22). and (23: NO. More than 91 stratosphere and mans, 1972). As (

Table

E E m . slld thi -

Con

NO;.: 0

-

" D 5ci. S, S o c k

E 01 Nihogea

:ation. has )sphere by iphere that halt. 1977; 'he strato- liation and l h surface ihn. 1979). gen oxides Hahn and rhere is no by precipi- :re. Atmo- .ions in the

(21)

(22) (23)

otolysis of ric N:O is ort of NO, iuse of the inverted to peroxyace- 1 (Crutzen,

talyzes the 981. 1983). )gan er al., f increased ring of the 31 mass to tzen (1983) Ice 01 loss ere; a dou- lecrease in

I?O from N I (Crutzen,

V. Erlenl, Sipili-ce, and Fsle 01 Losses 281

1983). Thus, the impact of the increasing use of N fertilizers on stratospheric ozone is unlikely to be great. Weiss (1981), for example, observed that the global upward trend in atmospheric N20 concentrations is about 0.2% per year, which can be explained in terms of the 3.5% per year increase in global N 2 0 emissions caused by fossil fuel combustion.

3. Fare of N2 and N20

The quantities of N2 evolved from the earth's surface during denitrification are extremely small in relation to the atmospheric content. The dinitrogen molecule is very stable and its atmospheric lifetime is of the order of millions of years. N2 constitutes 79% of the atmospheric mass.

Upon release of N1O from soils. an unknown portion is believed to be removed by gaseous deposition to vegetation. soil, and water (Rasmussen e / al.. 1975). However, under conditions of high soil NO; concentrations, when potentially high rates of denitrification may occur, the reduction of N20 to N 1 is usually inhibited (Firestone er 01. . 1980) (Table V) and i t seems unlikely that the soil will act as a major sink for N?O (Freney el ol . . 1978).

The approximale stratospheric lifetime of N?O is 100 to 150 yr(Galbal1y and Roy, 1983). The only known photochemical reactions that lead to removal of N?O from the stratosphere were presented earlier (equations (21). (22), and (23)). Thus, N 2 0 in the stratosphere is converted to N2 and NO. More than 90% of the NzO is thought to be transformed to N2 in the stratosphere and the remainder is converted to NO (Nicolet and Peeter- mans, 1972). As discussed in the next section, the NO produced can be

Table V

EEed 01 Nihale N Concenhtion on Ihe Denitrification Role and lhe Proportion 01 GPI Evolved SJ Nz snd N2O0

Percentage of total "N gas

evolved Concentration Denitfication of added rare

NO;-N (WR am-') (WR N nm-' hr-I) N2 NIO

0 - 95.2 4.8 0.5 0.54 93.9 6~ I 2.0 0.73 89.8 10.2

20.0 1.15 85.4 14.6

0 Dam from Fireslone el 01. (1979). Reproduced from Soil Sci. SOC. Am. J . 43, p. I143 by permission o l lhe Soil Science Societv of America.

rapidly transformed to NO? and thence to H N 0 3 . These substances eventually returned to the troposphere and then to the earth's surface wet and dry deposition.

C. Chemodenitrification

1. Extent of Losses

and NO, and identify the factors affecting such losses. Emission of N and possibly NO during the nitrification process would tend to confou such results.

Few attempts have been made to directly measure losses of NO, fr

Johansson and Granat (1984) measured annual emissions of NO fr

respectively.

2. Fare of NO. Emissions

As discussed in Chapter I . the two major sources of atmospheric NO' are emissions from soils through chemodenitrificalion and combusti sources (automobiles, furnaces, forest fires, etc.). The quantities p duced by the two sources are thought to be comparable on a global sc

by 01 in the lower atmosphere: NO + 0, - NOI + O1

This NO? can be absorbed by the local plant- and^ soil surface (Gal 1974; Rogers el a/.. 1979; Elkiey and Ormrod, 1981; Galbally and 1978) or transported long distances in the atmosphere (Galbally and 1983). Nitric oxide can also be absorbed by plants but much more SI

of soluble species in cloud and rain droplets with subsequent removal precipitation. I n the stratosphere NO? reacts with OH radicals to yi

VI. canclvriono

wiihln a few day5

form since gasel asrosol particles ccnsous condens blned with NH: qheric aerosols I 2nd Lazrus, 198C

The NO;-cont: and rainout procL [hen the NO, sal

be about 1.5 day

VI. COh

There are seve: plant-soil system rrification, nitrific

Volatilization c by a combination sary prerequisite NHN,,, and N H I is thus a m: soils. Sources'of residues. animal c

taining or -yieldir. affected by pH, tc and CEC of the : organic matter.

Most of the N. quickly returned lifetime of NH3 ir relurned to the b faces, and water Iroposphere, the Clouds with the t deposition as NH: may evaporate ar

Growing plants Absorption of N1 above can greatl)

There are seve

ubstances are .$ h's surface by , ~5

. .

:s of N:, N20, ' - nission of N20 ..'i id to confound . '

:s of NO., from ;

vi, Conclusions 283

within a few days and the mean atmospheric lifetime of NO, is thought to be about 1.5 days (Hahn and Crutzen. 1982). Aerosol NO; can readily form since gaseous H N 0 3 is attached to, or dissolved in, preexisting aerosol particles (e.&, HISOJ, (",),SO,, o r NHaHSOJ) through hetero- - geneous condensation (Taylor et ai., 1983). Indeed, SO:- and NO; combined with NH; are usually dominant inorganic species found in atmospheric aerosols (Stevens er 0 1 . . 1978; Scott and Laulainen. 1979; Huebert and Lazrus. 1980).

The NO,-containing aerosols are then removed principally by washout and rainout processes (Fowler, 1978). If vaporization of droplets occurs then the NO; salts may be removed by dry deposition.

VI. CONCLUSIONS

There are several mechanisms that lead to gaseous losses of N from the plant-soil system. These include ammonia volatilization. biological denitrification, nitrification. and chemodenitrification.

Volatilization of NH, to the atmosphere is a complex process affected by a combination of physical, chemical, and biological factors. A neces. sary prerequisite for NH, volatilization is a supply of free ammonia (i.e.. NHJlaq, and NH?,,,) near the soil surface. The conversion of NH: ions lo NH, is thus a major process regulating the potential loss of NHj from soils. Sources of NH; in the soil include native soil organicmatter. plant residues, animal excretions, added organic materials. or added ";-containing or -yielding fertilizers. The equilibrium between NH; and NH, is affected by pH. temperature, water loss from the soil, buffering capacity and CEC of the soil, and fixation of NH, and NH; by clay minerals or organic matter.

Most of the NH, emitted to the atmosphere from the soil surface is quickly returned to the earth's surface via wet and dry-deposition. The lifetime of NH, in the atmosphere is only I to 2 weeks.-Ammonia can be returned to the biosphere via gaseous deposition since plants. land surfaces. and water bodies can all act as sinks for atmospheric "I. In the troposphere, the emitted NH, quickly dissolves in water droplets in clouds with the formation of NH: ions. The NH: is returned via wet deposition as NH; salts dissolved in rainwater, or the water in the aerosol may evaporate and particles are returned via dry deposition.

Growing plants can act as either sources or sinks for atmospheric N H j , Absorption of NH,, emitted from the soil surface, by the leaf canopy above can greatly reduce losses of NH, from the plant-soil system.

There are several pathways for the biological generation of gaseous

284 6 5. Gereous Lases 01 Nib%&

nitrogenous products. Dissimilatory denitrification is carried out b limited number of aerobic bacteria that can grow in the absence of mol ular oxygen while reducing NO; or NO; to gaseous products (NzO N2). These bacteria are biochemically and taxonomically diverse altho most are chemoheterotrophs and use carbonaceous compounds as e tron donors and sources of cellular C and chemical compounds as ene sources. The denitrification process is promoted by anaerobic conditions high levels of soil NO;, and a readily available source of carbon and, general, is positively related to soil pH and temperalure. The quantity NzO emitted during denitrification is normally considerably less than tha of NZ although the mole ratio of NzO produced is influenced by ma factors and can vary from 0 to 1.0. The ratio is generally raised un conditions of high NO; levels and low pH and lowered by high tempera, lures and increasing anoxia.

There is a group of “nondenitrifying” bacteria and fungi that is able respire NO; anaerobically as far as NO; and when growing ferme tatively they can further reduce NO; to NH: with N 2 0 being produced a minor product. The magnitude of the contribution that these organ make to NzO evolution from soils is unclear although i t is gene thought to be small.

The autotrophic NH:-oxidizing bacteria Nitrosomonos, Nitrosospi and Nitrosolobus have the capacity to produce NzO. and apparently N during the oxidation of NH: to NO;. The exact mechanisms throu which these gases are produced are unknown. The potential for losses N20 through nitrification is greatest when NH:-containing or -yieldi fertilizers are applied to aerobic soils. _-

The atmospheric lifetime of Nz is millions of years and that for N2O ’ about 150 yr. A major sink of N2O from the troposphere is diffusion int the stratosphere, where the major part forms N2 and a small fractio forms NO,. The nitrogen oxides (NO,) catalyze destruction of ozone (0

~ ~ r c r c n c e 5

soils as an 1

evolved from Several me

These includc NO?, reactioi with the form pounds conta with NH: an lively, and re: Although res( significance a lished.

Nitric oxidc NO? by OJ in by the local F Nilrogen diox; yield “0,. 1 is in the rang formed in the formed aeroso and dry depos

REFEl

in the upper stratosphere but protect i t from deStNCtiOn in the I stratosphere. Hence, the net result of increased emissions of NzO the earth’s surface is likely to be a transfer of OJ mass to lower alti rather than destruction of the ozone layer. The NO, formed in the s

Chemodenitrification is a term that encompasses the processes sible for gaseous loss of N from soils through chemical reactions Accumulation of NO? does nor normally occur except when nit fertilizers that form alkaline solutions upon hydrolysis (urea, N and anhydrous and aqua NH,) are band-applied to soils or in NO,-

I

Alexander, M. ( I S Allison, F. E. (19: Allison. F. E. (196

ing nitro Allison. F. E. (19t Arnold. P. W. (19: Aulakh. M. S ~ . Re

under zc ron. Qu,

Aulakh. M. S.. RC denilrifir 190-794~

Consider: Barley. L. D~ (1976

Sci. 56, . Bailey. L. D.. and

ated roo1 soils. Co,

Avnimelech. y.,

m is carried out by a n the absence of rnolec- DUS products (N20 and nically diverse although :.

'us compounds as elec- 11 compounds as energy y anaerobic conditions, mrce of carbon and, in :ramre. The quantity of isiderably less than that is influenced by many ;"

generally raised under . wered by high tempera-

and fungi that is able to ,"

when growing fermen- N20 being produced as

on that these organisms ilthough it is generally

somonos. Nitrosospira, !O. and apparently NO, c t mechanisms through le potential for losses of -containing or -yielding

ears and that for N?O is Dsphere is diffusion into q2 and a small fraction estruction of Ozone ( 0 3 ) estruction in the lower emissions of N20 from mass to lower altiiudes

10, formed in the slrato- low rate and then IO the

ts the processes respon- : m i d reactions of NO;. xcept when nitrogenous 'olysis (urea, NH: salts. D soils or in NOT-treated,

. ..

?

References 285

soils as an intermediate of denitrification. A variety of gases can be evolved from soils treated wilh NO; including Nz, N20. NO, and NOz .

Several mechanisms are thought to be involved in chemodenitrification. These include decomposition of nitrous acid with the emission of NO and NO?, reactions of HNOz with phenolic constituents of soil organic matter with the formation of Nz , N?O, and NO, reactions of HNO: with compounds containing free amino groups to liberate N2, reactions of NO; with NH; and hydroxylamine with lhe release of N? and N20, respectively, and reactions of "02 with metallic cations IO form NO and N?. Although research has established that such reactions can occur, lheir significance and magnitude under field conditions have yet to be established.

Nitric oxide released from the soil surface can be quickly converted to NO2 by O3 in [he lower atmosphere. Atmospheric NOl can be taken up by the local plant and soil surface or transported into the atmosphere- Nitrogen dioxide is highly soluble in water and reacts with OH radicals to yield "0,. The combined atmospheric lifetime of NO, NO?, and HN03 is in the range of I 10 2 weeks. Heterogeneous aerosol particles are formed in the atmosphere by interaction by gaseous HN03 with preformed aerosols and the nitrales are returned to the earth's surface by wet and dry deposition.

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