Factors affecting nitrogen dynamics in a semiarid woodland (Dry Chaco, Argentina)
M.J. MAZZARINO ~, L. OLIVA 2, A. ABRIL 2 and M. ACOSTA 2 National Council of Research and Technology, Argentina. Present addresses: tNitrogen Fixing Trees Project, P.O. Box 90-CATIE, Turrialba, Costa Rica and 2Soil Microbiology, Faculty Agricultural Sciences, National University of Cdrdoba, Argentina
Received 1 November 1990. Revised May 1991
Key words: ammonifier- and nitrifier densities, leguminous trees, N immobilization, N mineralization, semi-arid ecosystems, soil respiration.
Abstract
In an effort to elucidate the factors affecting soil N dynamics in the Dry Chaco ecosystem, soil respiration and microbial biomass N were measured for one year underneath 5 vegetation types: a leguminous tree (Prosopis flexuosa DC), a non-leguminous tree (Aspidosperma quebracho-blanco Schlecht.), a non leguminous shrub (Larrea spp.), the open interspaces, and a pure grassland. Ammonifier and nitrifier densities and N content in litter were also measured in some cases. Results were compared with previously reported N mineralization rates and soil fertility.
During the dry season microbial biomass N and net N mineralization were low, while accretion of easily mineralizable C occurred (estimated through soil respiration rates in lab under controlled temperature and moisture). With the onset of rain, microbial biomass N and N mineralization increased markedly, resulting in a decrease in easily mineralizable C. Throughout the wet season N mineralization varied with soil moisture while microbial biomass N remained consistently high. Mean values of immobilized N in this ecosystem were high (20-140 mg kg-~), of about the same order of magnitude as accumulated net N mineralization (50-150 mg kg -~ yr -~). Microbial decay in the dry season, consid- ered as a source of easily mineralizable N, accounted for only 40% of gross N mineralization increase at the beginning of the wet season. Ammonifier densities correlated significantly with soil moisture and N mineralization, but nitrifiers did not.
The highest values of total N, N mineralization, inorganic N, microbial biomass N, nitrifier densities, N content in litter, total organic C and easily mineralizable C were found under Prosopis and the lowest values under shrubs and the interspaces. The main differences between tree species were in N mineralization at the beginning of the wet season, in total and inorganic N pools, and in nitrifier densities; all of which were significantly lower under Aspidosperma than under Prosopis.
N mineralization in the pure grassland was very low despite high values of total N and C sources. Although N immobilized in microbial biomass was similarly high under Aspidosperma, Prosopis and the pure grassland, net N mineralization rates were quite different.
Introduction
The 'Dry Chaco Plain', in northwestern Argen- tina, encompasses 5 to 8 million ha; the promi-
nent vegetation is a scrub woodland composed of xerophytic trees and shrubs with an understory of perennial herbs (Cabrera, 1976; Morello et al., 1977; Ragonese and Castiglioni, 1969).
86 Mazzarino et al.
Clear-cutting and overgrazing are leading to shrub invasion and loss of grassland productivity (Karlin and Diaz, 1984).
Tree and shrub conservation in arid and semiarid regions has often been considered im- portant to ensure adequate grass production, since they can play a major role in both hydro- logic and nutrient cycles, particularly the nitro- gen cycle (Bernhard-Reversat, 1982; Charley and West, 1977; Cox et al., 1984; Kovda et al., 1979; Skujins, 1981; Tiedemann and Klemmed- son, 1973).
Nitrogen dynamics and efficiencies in semiarid ecosystems are affected by the seasonal and vari- able nature of precipitation and the high spatial herterogeneity (beneath and between plant canopies) (Bolton et al., 1990; Fisher et al., 1987; Gutierrez and Whitford, 1987).
In a previous work (Mazzarino et al., 1991) we compared the effects of 5 vegetation types on soil fertility and N mineralization throughout one year in the Dry Chaco. Vegetation types in- cluded: a leguminous tree (Prosopis flexuosa DC), a non-leguminous tree (Aspidosperma quebracho-blanco Schlecht.), a non-leguminous shrub dominant in overgrazed areas (Larrea spp.), the open interespaces between trees/ shrubs and a pure grassland. In this study we found that: i) trees transfer P from deep in the soil profile and from lateral interspaces and pro- mote the accumulation of organic C and total N in the surface soil; ii) despite high N miner- alization rates under trees during the warm wet season, total N pools are high, implying N ac- cumulation; iii) leguminous trees are more effec- tive, than non-leguminous trees in maintaining N availability; iv) greatest soil impoverishment and lowest N mineralization rates occur under Larrea spp. and in the interspaces; and v) organic C, and total N and P are high throughout the entire profile under pure grasslands, but N miner- alization rates are very low.
In semiarid and arid ecosystems soil carbon is considered a limiting element for biological ac- tivity (West and Skujins, 1978). Microbial N immobilization is also mentioned as a cause of low N mineralization (Vitousek et al., 1982), but reported results are contradictory. Dommergues et al. (1980) consider that N immobilized in microbial biomass is negligible in semiarid re-
gions, whereas Bernhard-Reversat (1982), who found quite high mineralized-N values, suggests that it was derived mainly from microbial biomass. Another factor regulating N miner- alization may be nitrifier population. Some studies (Belser, 1979; Vitousek and Matson, 1982) suggest that nitrification is strongly con- trolled by nitrifier population size, yet Donal- dson and Henderson (1990) found a weak corre- lation between both parameters.
Given such discrepancies, the objective of our study was to identify the biological factors con- trolling N dynamics and efficiencies in the dry Chaco. We studied microbial biomass N and soil respiration (as an index of C source quality) during one year under the five vegetation types mentioned above. Our study also measured am- monifier- and nitrifier densities under and be- tween trees, and litter N content under trees and shrubs.
Material and methods
Study site and sampling
The study area is located in the Forest Reserve "Los Pocitos", west of C6rdoba city, in north- western Argentina, 65 ° 30' W, 31 ° S. The climate is characterized by high summer temperatures and moderate winters, mean annual precipitation of 300-500mm distributed in spring-summer (October-March), and potential evapotranspira- tion index (Thornthwaite) of less than -20 (Kar- lin and Diaz, 1984). Annual precipitation during this study was exceptionally high, 790 mm. The soils are Entisols of alluvial origin, classified as Ustorthents (areas of trees and shrubs) and Ustifl- uvents (pure grassland area). The Ustifluvents have a finer texture than the Orthents which translates to higher CEC and greater water- holding capacity in the pure grassland area (Table 1).
Three sampling areas were chosen: i) a natural forest dominated by Aspidosperma quebracho- blanco Schlecht. and Prosopis flexuosa DC, ii) an area invaded by Larrea spp., approximately 15 yr old, and iii) an area of pure grassland, kept free of shrubs and trees for 15 years, and domi- nated by perennial grasses of the genus
Nitrogen dynamics in a semiarid woodland 87
Table 1. Soil properties at 0-10 cm depth under different vegetation types. Common letters indicate values (means of n = 3) are non significantly different for p < 0.05
Vegetation pH E.C. C N C/N extr.P C.E.C. B.D. types (Sm ') (gkg t) (gkg ~) (mgkg -t) (cmol kg -~) (gcm ~3)
Papophorum. None of the sites had been grazed for a period of at least 10 years.
Samples for microbial biomass N, soil respira- tion and density of ammonifiers and nitrifiers were collected every 28 days in the upper soil ( 0 -10cm) from April 1987 through May 1988, concurrent with sampling for N mineralization (Mazzarino et al., 1991). In each area, 10 trees/ shrubs of similar dimensions were chosen and samples were taken under 3 randomly selected individuals of each species at about 50 cm from tree trunk (10-20 cm from shrub main stem). In the pure grassland, samples were taken from 3 randomly selected points. Areas between shrubs or trees (interspaces) were sampled at about 1--1.5 m from the canopy border of each chosen individual. Each month, two adjacent soil cores were collected at each point. Prior to each sam- piing, the corer was sterilized with sodium hypo- chlorite. One of the cores in each pair was taken with tin cans, 13 cm high and 7 cm in diameter, sealed with a polyethylene film, replaced in the core hole and incubated in situ for 28 days for N mineralization. The other core in each pair was taken to the lab within 3 h of collection; soil was mixed and roots and other inclusion >0.5 cm in diameter were removed. Four subsamples were then removed for determination of initial values of N mineralization, soil water content, micro- bial biomass N and soil respiration. The remain- ing soil from the three original replicates was mixed in a composite sample for microorganism density determinations. Soil respiration under Larrea and the pure grassland was measured from April 1987 through February 1988. Mi- croorganism densities were estimated only under trees and their respective interspaces.
Total litter was sampled in the same areas
beneath 8 randomly selected trees/shrubs with a 35 cm x 35 cm wire frame. Samples were taken seasonally in March, June, September and De- cember. Sampling for chemical and physico- chemical characterization of soils was carried out once at the beginning of the study; methods and results are reported in Mazzarino et al. (1991).
Incubation and chemical methods
Samples for N mineralization were extracted in 2 M KC1 and analyzed for N H 4 - N and NO3-N. Net N mineralization was estimated as the final amount of N H 4 - N + NO3-N after the 28 days incubation minus the initial amount (Mazzarino et al., 1991).
Microbial biomass N was determined in 50-g subsamples using a modification of the chloro- form fumigation-incubation technique (Jenkin- son and Powlson, 1976) proposed by Vitousek and Matson (1985). One mL of chloroform was added directly to each soil sample, which was then incubated for 10 d at about 20-25°C after the chloroform was dissipated. Soil moisture was maintained at field capacity (i.e. approximately 60% of water-holding capacity). Samples were extracted in 100ml 2N KC1 and analyzed col- orimetrically by the indophenol blue method for N H 4 - N (Keeney and Nelson, 1982). Microbial biomass N was calculated as the amount of NH 4- N released divided by a recovery coefficient of 0.33 (Matson et al., 1987).
Microorganism densities were estimated using the dilution technique of Pochon and Tardieux (1962) and the table of 'most probable number ' (Cochran, 1950). For ammonifiers, 15-day incu- bation in a liquid salt medium with asparagin was employed; NH~-N presence was detected with
88 Mazzarino et al.
Nessler's reagent. A 21-day incubation in am- monium sulfate medium was used for nitrifiers; NO3-N and NO2-N presence was determined with diphenylamine in concentrated sulfuric acid.
Soil respiration was estimated through CO 2 evolution at 28°C in samples incubated for 10 days (Dommergues, 1968). Soil moisture was maintained at field capacity. Carbon dioxide was determined by trapping in NaOH and titration with sulfuric acid.
Litter samples were dried at 70°C, sieved through 2-mm mesh (<2 mm fraction was elimi- nated), weighed and ground. Nitrogen concen- tration in litter was determined in 4 replicate 1-g samples with the Kjeldahl method.
Statistical analyses consisted of: i) correlation analysis among variables (soil respiration and microbial biomass N vs. soil moisture; microor- ganism densities vs. soil moisture and N miner- alization) using the Student t-test for significance and, ii) analysis of variance (ANOVA) by treat- ment and time followed by Duncan analysis of significance for p <0.05. The following treat- ments were considered: Prosopis (under and be- tween), Aspidosperma (under and between), Larrea (under and between) and pure grassland. Analyses were carried out using the statistical package SPSS/PC (1984).
Results
Nitrogen mineralization and soil moisture under the five vegetation types are given in Figures 1 and 3A (Mazzarino et al., 1991).
Microbial biomass nitrogen and soil respiration
Progressive microbial dieback occurred during the cool dry season, microbial biomass N reach- ing a minimum in the driest month (Figs. 2B and 3B). At the onset of the warm wet season, microbial biomass N increased markedly and then remained relatively constant through the wettest months of the year. Soil respiration was highly variable, tending to be high in the dry season, low at the beginning of the wet season, but then rising again by the third wet month (Figs. 2A and 3B).
No significant correlation was found between soil moisture and either microbial biomass N or soil respiration. Statistical analyses for all treat- ments averaged over all dates showed significant differences in the following order: Microbial biomass N: Prosopis = Aspido- sperma > grassland > Larrea and interspaces. Soil respiration: Prosopis = Aspidosperma= grassland > Larrea and interspaces.
Ammonifier- and nitrifier densities
Nitrifier values ranged from 0 to 10 3.3 cells ~er g soil and those of ammonifiers between 1 0 to 1012'3 cells g-i (Fig. 4). There were no statistical- ly significant differences between tree and inter- space treatments when ammonifier densities were averaged over all sampling dates (other treatments not analyzed). Nitrifier densities were significantly higher under Prosopis (10 L8 cells g-~), but equal under Aspidosperma and the interspaces (1012 cells g-l) .
Ammonifier densities correlated significantly with soil moisture (p<0 .001) and N miner- alization (p <0.05), but nitrifiers did not.
Nitrogen immobilized in litter
Total litter decreased in winter (June, cool dry season), with no significant differences among species (Table 2). Nitrogen concentration in lit- ter, on the contrary, increased in winter under Prosopis and Aspidosperma and was significantly lower and more variable under Larrea. Inter- spaces and pure grassland were not sampled.
Nitrogen content in litter under trees ranged between 10.7-21.0g m -2 and was higher than under Larrea (5.3-13.2 g m-Z).
Comparison of N-pools and fluxes under the five vegetation types
The highest values of net N mineralization, mi- crobial biomass N, total and inorganic N, N content in litter, and organic and respired C were found under Prosopis and the lowest under Larrea and the interespaces. Differences be- tween tree species were in N mineralization rates at the beginning of the wet season, in total and inorganic N pools, and in nitrifier densities,
Ni t rogen d y n a m i c s in a semiar id w o o d l a n d 89
F/g. 1. N mineralization (A) and soil moisture (B) under trees, shrubs and open interspaces. Interspaces are represented by those near Prosopis plants. ES = end of September. (From Mazzarino et al., 1991).
which were lower under Aspidosperma. In the pure grassland, N mineralization was twice as low as under Prosopis despite similar values of total N and C sources (high organic C and respired C), and even higher soil moisture. Al- though N immobilized in microbial biomass was quite high under the pure grassland, values were lower than under Prosopis (Table 1; Figs. 1, 2, 3 and 4).
When total, mineralized, microbial biomass, plant-available N and N content of litter were transformed from concentration to volume and area basis (ha-~ to a depth of 10 cm), no change in trends was observed (Fig. 5).
Interestingly, N content in litter, immobilized N and mineralized N had values of the same order of magnitude: 100-160 kg ha -~, 80-160 kg ha ~ and 110-220 kg ha -1, respectively. Nitrogen
90 Mazzarino et al.
800
600 ! Ob
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A ) S O I L R E S P I R A T I O N
I" I • o ' l ~ • "~/....,,.. ....:~}. ....o .... .,
ok. ~ . o % ' % ~ O S ' S I p . - - . -o * ' " ~'0 S
0 / ~ . 0 - - / " - ' , . . . "% ¥ I" , .0
• • • | | , , a • . J a | a II
A M d d A S ES , 0 N D d F M , A M MONTH Wet season
Fig. 2. Soil respiration (A) and microbial biomass N (B) under trees, shrubs and open interspaces. Means are based on n = 3.
Table 2. Total litter (n = 6-8) and N in litter (n = 4) at four sampling dates under three vegetation types. Common letters indicate values among treatments for the same date are not significantly different (p < 0.05)
Prosopis Aspidosperma Larrea
Litter N N Litter N N Litter N N (gm -2) (%) (gm 2) (gm 2) (%) (gm 2) (gm-2) (%) (gm-2)
IN litter I00 I LARREA plant uptake + denitrification
~ l ' ~ Jn et N mineraliz\l I
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! N litter n.d. J INTERSPACES plant uptake +
~ ~ , denitrification net N mineraliz\l I
112 / I 104 I
leaching
! N litter n.d. J GRASSLAND plant uptake +
~ ~ Jnet N mineraliz.~J 107 /J denitrifieation 97 IJ leaching
Fig. 5. Nitrogen flows (kg h a -1 yr 1) and N pools (kg ha 5) under five vegetation types in Dry Chaco. Microbial biomass N, plant-available N and litter N were calculated as the mean of all dates; mineralized N and losses by plant uptake, denitrification and leaching correspond to the accumulated values during one year. Nmb = microbial biomass N; Ni = inorganic N or plant-available N.
turnover, as represented by the percentage of total N mineralized in one year, is apparently reduced in the interspaces and the pure grassland (4.7-5.9% vs. 8.4-10.4% under trees and shrubs).
The pool of inorganic N remaining in the soil represented less than 1% of total N and 6-9% of mineralized N during one year. Absolute values were significantly higher under Prosopis whereas no differences were found among the other treat-
ments. The remaining 91-94% of mineralized N was taken up by plants or was lost by denitrifica- tion and leaching.
Discussion
Microbial biomass and N mineralization
Studies in African savannas have shown that N mineralization is high at the beginning of the wet season, but decreases before its end (Bernhard- Reversat, 1982; Bernhard-Reversat and Poupon, 1980; Singh and Balasubramanian, 1980). The decline has been attributed to lack of easily mineralizable N. However, we found that N mineralization in the Dry Chaco increases as a function of soil moisture and temperature, with- out apparent substrate limitation (Mazzarino et al., 1991). Bernhard-Reversat (1982) suggests that the main source of easily mineralizable N between two wet periods could be dry-season mortality of microbial biomass. However, other authors found that: i) soil drying destroyed only 1/3 to 1/4 of microbial biomass and that, at each cycle after rewetting, the biomass was progres- sively restored to approximately the same size as before drying (Bottner, 1985), ii) N derived
Nitrogen dynamics in a semiarid woodland 93
from microbial biomass accounts for only 40% of mineralized N during 12-week lab incubations (Robertson et al., 1988), iii) it is likely that the microbial biomass is not a major source of plant- available N during a growing season (Bonde et al., 1988). In the present study, N released from microbial decay during the dry season was com- pared with the net N mineralization and micro- bial biomass N (gross mineralization) at the be- ginning of the wet season (Table 3). This source accounted for approximately 40% (on average) of the gross mineralization produced at the be- ginning of the wet period, only in one case, between Larrea canopies, N released by micro- bial decay was greater than gross mineralization.
Nitrogen mineralization increased as a func- tion of soil moisture (Fig. 1), but microbial biomass N increased also at the beginning of the rains and then remained high (Fig. 2B). This suggests that in addition to dead microbes other sources of easily mineralizable N (non-biomass active fraction) were available: i) physical disrup- tion of soil aggregates and rearrangement of components during long dry periods could in- crease the exposed surface to subsequent micro- bial attack (Birch, 1960; Bottner, 1985), ii) solu- ble nitrogen compounds could be leached from dead leaves at the onset of rains (Skujins, 1981).
Table 3. Decrease in microbial biomass N (mg kg -j) during the dry season, increase in gross N mineralization (mg kg -~ ) at the onset of rain and percentage of the gross mineralization (at the onset of rain) explained by microbial decay during the dry season, under different vegetation types. Values are means of n = 3
Prosopis Aspidosperma Larrea Grassland
under between under between under between
Microbial biom. N 47.5 decrease during the dry season (DMB-N)
Microbial biom.N 45.8 increase at the onset of rain (MB-N)
N mineralization 35.9 increase at the onset of rain (Nmin)
Gross N mineraliz. 81.7 increase at the onset of rain (GM = MB-N + Nmin) (DMB-N/GM) x 100 58
12.3 7.0 17.7 29.9 51.4 42.0
25.5 44.8 37.4 51.5 22.9 57.1
10.0 16.3 3.5 16.1 2.5 7.3
35.5 61.1 40.9 67.6 25.4 64.4
35 11 43 44 200 65
94 Mazzarino et al.
Bernhard-Reversat (1982) considered that litter accumulated during the dry season could not be a direct source at the beginning of the wet season, since it must be partly humified before mineral N can be released. It has been demon- strated that most of the mineralizable N is pro- duced in organo-mineral fractions (heavy frac- tions) and that light fractions play a minimal role (Bernhard-Reversat, 1982; Sollins et al., 1984). In the late wet season, litter might be partly humified and contribute to N mineralization. Sources of mineralizable N should increase, therefore, with increased intensity and frequency of drying (microbial dieback, disruption of soil aggregates) and wetting (leaching of soluble N compounds, litter humification) cycles. This would explain the previously reported higher N mineralization rates in laboratory incubations of wet-season samples compared to dry-season sam- pies, despite similar moisture and temperature conditions (Mazzarino et al., 1991).
According to Dommergues et al. (1980), N immobilized in microbial biomass is negligible in semiarid soils of West Africa. In contrast, our findings showed relatively high values of micro- bial biomass N in the Dry Chaco, of about the same order of magnitude as mineralized N. Dur- ing the wet season N mineralization was depen- dent on moisture pulses, while microbial biomass N remained relatively constant, even when soil moisture decreased in November. This suggests that microorganisms were capable of surviving dry periods of short duration keeping their biomass constant but markedly decreasing their activity. During the dry season both microbial biomass and activity were reduced. While am- monifier densities correlated positively with soil moisture and N mineralization, nitrifier popula- tions did not. Recent studies in upland forests have also shown little correlation between nit- riflers and N mineralization (Donaldson and Henderson, 1990). Density of nitrifiers in the present work was low (0-103 cells g- l) and similar to values found by Dommergues et al.
2 3 (1980) for sandy soils of Senegal (10 -10 cells g-l) . Nevertheless, these low numbers do not necessarily mean less microbial activity than in temperate conditions, since activity can be very high in localized habitats representing a small volume of the soil such as organo-mineral parti-
cles and plant rhizospheres (Dommergues et al., 1980). The low values found in our study may also have been due to limitations of the method of quantification employed. Insufficient incuba- tion periods and a high inherent statistical error have been mentioned as serious drawbacks of the 'most probable number' technique (Belser and Schmidt, 1978; Schmidt, 1982).
Soil respiration
Since nutrient cycles in soil depend markedly on the energy supply to the soil biota, they are closely linked to carbon metabolism (McGill et al., 1975; Van Veen et al., 1985). According to Dommergues (1968), soil respiration measure- ments in the laboratory reflect soil availability of easily mineralizable organic C and differences in soil environmental factors, other than moisture, temperature and aeration. During the dry period, soil samples were wetted to field capacity before incubation, whereas most samples of the wet season did not need to be wetted, and were incubated at field moisture. Dry soil rewetting causes a rapid increase in the respiration rate depending on the change intensity of the water potential (Kieft et al., 1987; Orchard and Cook, 1983; Sparling and West, 1989). In our case, this occurred in the laboratory during the dry season as a consequence of rewetting, and in the field during the wet season following precipitation. Soil respiration data of the dry season might be, therefore, equivalent to the potential availability of easily mineralizable C, while those of the wet season would correspond to the actual availa- bility.
The distribution of the ratio of respired C to N in microbial biomass (Fig. 6) showed a maximum peak at the end of the dry season when microbial decay was also higher, increasing the content of easily mineralizable C. As for easily miner- alizable N, this pattern could, in part, be attri- buted to an increase in exposed surface due to physical disruption of soil aggregates. The peak was higher in the interspaces and under Larrea than under trees. This was probably due to greater fluctuations in temperature. A striking decrease in the C:N ratio occurred at the begin- ning of the wet season. A considerable portion of energy-rich substrate may have been con-
Nitrogen dynamics in a semiarid woodland 95
Z
z
( , 9
o e , .
I
o
e~
i , I
O~
o p-
re"
6
0 M
Fig. 6. Ratio of C respired
R Prosopis ,/i!
. . . . . . . Aspidosperma - - - Larreo // ' i i . . . . . Interspaces !t . . . . . . G r a s s l a n d ,~
A M d J A S E S , O N D d F M , A M MONTH Wet season
to N in microbial biomass under the five vegetation types. Values are means of n = 3.
sumed at that time to increase microbial activity or generate new biomass. This suggests that, at the beginning of the wet season, N dynamics might have been limited by C supply, as has been mentioned by West and Skujins (1978) for arid zones.
Differences among the five vegetation types
The main factors affecting N mineralization are substrate availability, soil moisture, temperature, 0 2 availability, pH (Focht and Verstraete, 1977; Vlek et al., 1981), phosphorus availability (Pur- chase, 1974; Virginia and Jarrell, 1983) and mi- croorganism populations (Charley and West, 1977; Vitousek and Matson, 1985). In a previous work we found that: i) soil moisture was similar in all treatments, ii) pH and P were in the optimum range for N mineralization, and iii) although soil temperature was probably different between treatments, N mineralization under con- trolled temperature and moisture conditions showed similar trends than in the field (Maz- zarino et al., 1991).
Differences among vegetation types, there- fore, were due mainly to substrate availability and to microorganism population size. The fol- lowing scenarios possibly explain observed dif-
ferences: i) Prosopis, being a leguminous tree, might fix N 2 at some depth and deposit a portion of this N near the soil surface as litter or through the turnover of surface roots (Charley and West, 1977; Virginia, 1986; Virginia and Jarrell, 1983); ii) nutrient exhaustion from the interspaces through tree lateral roots (Tiedemann and Klem- medson, 1973) limited N dynamics, iii) litter and light soil fractions are lost due to eolic and hydric erosion in the interspaces, and iv) rates of litter decomposition are different, depending on C/N ratios, content of soluble compounds, lignin, polyphenols, etc. (Melillo et al., 1976; Palm, 1988; Witkamp, 1966).
Increased soil N mineralization is reported beneath shrub canopies when compared with interspaces in some desert ecosystems (Charley and West, 1977; Fischer et al., 1987). In con- trast, our study shows low mineralization under Larrea. These findings are consistent with other studies of genera Artemisia and Atripex (Bolton et al., 1990, Skujins, 1981).
In the pure grassland the low N mineralization rates could be a consequence of the finer texture of the soils, due to a more stable association between organic matter and fine mineral frac- tions (Ladd et al., 1985). Nevertheless, we think that the observed pattern is better explained by
96 Mazzarino et al.
the vegetation type rather than by the soil tex- ture. Although the idea that grass roots inhibit nitrification has been questioned (Purchase, 1974; Robertson and Vitousek, 1981), recent evidence supports this theory particularly for C-4 grasses of tropical regions (Bernhard-Reversat, 1988; Sylvester-Bradley et al., 1988).
laboratory. We also thank L Nufiez, J Roskoski, J Herrick and L Cooperband for draft correc- tions. This research was supported by the Na- tional Science Foundation (USA) through a grant to the Dept. of Marginal Area Manage- ment (Fac. of Agricultural Sciences, Nat. Univ. of Cordoba) directed by O Karlin.
Conclusions
The main factors affecting N mineralization, and accordingly plant-available N, in Dry Chaco ecosystems are soil moisture, temperature, easily mineralizable sources of C and N, and nitrifier densities. Carbon can be especially limiting at the beginning of the wet season. Nitrogen de- rived from microbial decay during the dry season explained only 40% of gross mineralization at the beginning of the wet season, suggesting that other sources of easily mineralizable N are avail- able (non-biomass active fraction).
As observed in other semiarid ecosystems, leguminous trees appear to be more efficient than non-leguminous trees, shrubs and grasses in maintaining soil N content and availability. The highest rates of N mineralization were found under Prosopis and the lowest under Larrea, interspaces and the pure grassland. Differences between Prosopis and either Larrea or inter- spaces are due mainly to differences in C and N sources (of both, total and easily mineralizable). In the pure grassland, total and easily miner- alizable soil C and total soil N were quite similar to those under Prosopis, suggesting that either i) the non-biomass active fraction is higher under Prosopis, and/or ii) that mineralization may be inhibited under grasses. Beneath both tree species N immobilized in microbial biomass, easily mineralizable C and mineralized N were similar. Plant-available N, total soil N and nit- rifler population size were, however, higher under Prosopis than under Aspidosperma.
Acknowledgement
We would like to thank E Buffa, O Bachmeier, O Karlin, G Nufiez and A Nufiez for their helpful comments and assistance in the field and
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