PHOSPHATE DYNAMICS IN AN ACID SULFATE SOIL UNDER FLOODED CONDITION STUDIED BY A TRACER TECHNIQUE A.K. Alva and S . Larsen

Royal Veterinary and Agricultural University Copenhagen, Denmark

1 Summary

Tracer techniques are efficient for studying P dynamics in f-Doded acid

sulfate soil planted to rice. This study deals with changes in labile P in an acid sulfate soil from the Central Plain of Thailand. Available P was estimated at different times during the rice growing period by de- termining isotopic dilution of P absorbed by plants (L-value) and also by isotopic exchange in the soil (E-value). The influence of varying

levels of applied N, P and K on labile P and the recovery of applied P

were examined in a growth chamber. During the growing period L-values for rice increased the first 80 days

after transplanting and subsequently decreased towards crop maturity, as a result of crop-induced P mobilization followed by immobilization. The influence of rice cropping on labile P was appreciable 50 days after transplanting. AL-values and recovery of applied P, at a uniform rate of P application, increased with increasing N and K enrichment. At a given level of N and K accompanied by a low P application the recovery of ap- plied P remained constant during the major part of the growing period, while at high application of P recovery decreased over the growing per-

iod. The influences of mineral nutrition on P dynamics are discussed. E- values were considerably higher than the L-values at the crop maturity stage. The possible causes for this discrepancy are discussed.

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2 In t roduc t ion

A considerable p o r t i o n of paddy s o i l s i n t h e Cen t ra l P l a i n of Thailand

c o n s i s t s of ac id s u l f a t e s o i l s . These are poorly drained a c i d c l a y s de-

veloped from b rack i sh water depos i t s and t y p i c a l l y have a low productiv-

i t y due t o t h e i r s t r o n g l y a c i d i c r eac t ion . Phosphorus de f i c i ency i s one

of t h e most important growth l i m i t i n g f a c t o r s f o r r ice i n these s o i l s .

P dynamics are extremely complicated i n a c i d s u l f a t e s o i l s p a r t i c u l a r l y

under flooded condi t ions.

Radio-chemical methods and t h e anion r e s i n exchange method are s u i t a b l e

f o r s tudying p l a n t a v a i l a b l e P i n s o i l because they b r i n g about minimal

changes i n the chemistry of t h e s o i l . Alva e t a l . (1980) found a modi-

f i e d r e s i n exchange method (developed by Sibbesen 1977) very u s e f u l i n

s tudying t h e P dynamics i n flooded s o i l s and f o r exp la in ing t h e l a c k of

response of lowland rice t o P f e r t i l i z a t i o n . Radio-chemical methods have

been developed t o determine t h e l a b i l e P f r a c t i o n s by measuring e i t h e r

i s o t o p i c exchange i n t h e s o i l , i .e . , e s t ima t ion of t h e E-value

(McAuliffe e t a l . 1948, Wiklander 1950, Russel e t a l . 1954), o r i s o t o p i c

d i l u t i o n , i . e . , e s t ima t ion of t h e L-value (Larsen 1952). These va lues

are measures f o r t h e q u a n t i t i e s of s o i l phosphate t h a t are exchanged

w i t h added labeled phosphate (E-value) o r t h a t d i l u t e added l abe led

phosphate through uptake and abso rp t ion by p l a n t s (L-value). A s a

measure of p l a n t a v a i l a b l e s o i l phosphate the L-value has cons ide rab le

t h e o r e t i c a l advantage over t h e E-value i n t h a t i t r e f l e c t s no t only t h e

cond i t ions t h a t confront t h e p l a n t when grown i n t h e s o i l b u t a l s o any

e f f e c t t h e p l a n t has on P dynamics i n t h e s o i l (Fr ied 1964).

I n t h i s s tudy t h e L-value technique (Larsen 1951) was app l i ed t o monitor

P dynamics i n flooded acid s u l f a t e s o i l p l an ted t o r i c e a t d i f f e r e n t

l e v e l s of N , P and K. E-values were determined a f t e r ha rves t ing .

3 Materials and methods

3 . 1 S o i l material

The s o i l used f o r t h e experiment was t h e t o p s o i l (0-30 cm) of a S u l f i c

Tropaquept sampled a t Klong Luang Rice Experiment S t a t i o n i n t h e Cen t ra l

P l a i n of Thailand. The s o i l was c o l l e c t e d from a dry paddy f i e l d a f t e r

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,harvest ing. P r i o r t o sampling, r ice had been grown i n t h a t f i e l d under

submerged condi t ions. The d ry s o i l was shipped t o t h e S o i l F e r t i l i t y

Laboratory a t t h e Royal Veter inary and A g r i c u l t u r a l Universi ty , Denmark.

I n Table 1 some physico-chemical p r o p e r t i e s are given of a t y p i c a l

S u l f i c Tropaquept i n t h e same area (according t o Satoru 1973 and Charoen

1974).

Table I . Some p r o p e r t i e s of t h e t o p s o i l sampled

f o r use i n t h e growth chamber experiment

Texture a n a l y s i s (%)

coa r se sand 1.3

f i n e sand 3.2

s i l t 43.7

c l ay 51.8

pH (1:2.5 soi1:water) 4.2

To ta l carbon ( X ) 1.41

CEC mmol/lOO g s o i l 25.8

Base s a t u r a t i o n (X) 62.6

Resin-extractable P mg P/100 g s o i l O . 198

Fe203 (XI 4.4

A 1 2 0 3 ( x > 23.9

3.2 . ' Growth chamber experiment

The s o i l w a s a i r -d r i ed , ground and s ieved through a 5 rmn s i eve . Three

m C i c a r r i e r - f r e e H 3 3 2 P 0 ~ was.mixed with 10 g of f i n e sand. This 32P-sand

was mixed with 30 kg of s o i l i n a r o t a r y mechanical mixer (giving 0.1

m C i 32P/kg s o i l ) . C a C 0 3 w a s added a t t h e ra te of 3 g per kg s o i l . Por-

t i o n s of 8 kg of 32P-treated s o i l w e r e placed i n t o p l a s t i c p o t s (30 cm

diameter and 30 cm h e i g h t ) and varying l e v e l s of N , P and K were app l i ed

as u rea , superphosphate and KC1 (Table 3 ) and thoroughly mixed w i t h

s o i l . Excess water w a s app l i ed and t h e s o i l w a s puddled w i t h i n t h e po t s .

Thirty-aay o l d rice seed l ings of t he v a r i e t y RD-I w e r e t r ansp lan ted i n t o

t h e po t s a t t h e ra te of s i x h i l l s p e r p o t and two seed l ings p e r h i l l .

After t h e establ ishment of t h e seed l ings f i v e cm s t and ing water w a s

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maintained until plant maturity. During the maximum tillering stage and

panicle initiation stage the pots were drained €or a period of three

days. The changes in climatic conditions in the growth chamber during

subsequent growth stages are shown in Table 2.

Table 2. Climatic factors regulation in the growth chamber during dif-

ferent growth stages

Photoperiod

sensitive

Basic phase to Panic 1 e vegetative panicle emergence

Climatic factors stage emergence to maturity

Temperature (OC daylnight) 28/22 32 /26 28/22

Photoperiod (hours per day) 12 10 12

Light intensity (K lux) 14 19 17

20 20 20 Dew point temperature ( C) O

3.3 Plant sampling and estimation of L-values

The aerial parts of the plants were harvested at 30, 4 0 , 50, 80 and 150 (plant maturity stage) days after transplanting, washed in deionized

distilled water and dried at 8OoC for 48 hours. The dry matter was dry-

ashed and collected in 0.2 N HNO3. Ten ml extract was transferred to

clean counting vials and activity of 32P was counted by means of a

liquid scintillation counter. The activity in the standard ( 1 mCi 32P in 1 ml 0.01 N HC1) was counted simultaneously as a measure of activity

applied per pot. Background counts were obtained on 10 ml 100 pmol P/

liter solution and subtracted from the sample counts. The P concentra-

tion was determined colorimetrically (Yoshida et al. 1971) by using a

Technicon Auto Analyser.

L-values were calculated according to the following relationship (Larsen

1969) :

activity applied per pot (cpm/g soil) of in plant activity in the plant (cpm/g dry matter) (pmol dry matter) L =

37 I

3 . 4 Estimation of E-values

After the plants were harvested at maturity, a 5 g soil sample from each

pot was transferred into a milk sample bottle. 90 ml 0.01 M CaCl2

was added and the suspension was shaken for half an hour in an end-over-

end shaker. Next 10 ml of 5 !AM KHzP04 labeled with carrier-free H332P0~ was added (activity applied = 1 UCi/g soil), and further shaken for 18

hours. The suspension was filtered. A bottle without s o i l was used as blank. The activity of 32P in the filtrate and the blank and the con- centration of P in the filtrate were determined. Correction for color quenching was done by using an external standard. E-values were calcu- lated from the following relationship:

activity of 32P added (cpm/g soil) Conc. of ,P in the filtrate (pmol P/g soil)

E = activity of 32P in the filtrate (cpm/g soil)

4 Re sult s

Dry matter weights, L-values at various times during the growing period,

and E-values after the final harvest are shown in Table 3 . Increasing N

and P application depressed dry matter yield during the former half of

the growing period and increased it during the latter period. In terms of grain yield the response to N application exceeded that to P applica-

tion. K application had little or no effect on dry matter production and on grain yield. L-values at various samplings increased with increasing

N levels up to 1.25 g N/pot. E-values increased for every increment in N levels. With increasing P levels L-values increased at all sampling occasions, whereas E-values increased markedly only for the first in-

crement (0.28 g P/pot). There was no clear effect of K application on the L- and E-values. Over the growing period L-values in most of the treatments changed only slightly up to 50 days, increased between 50 and 80 days and decreased thereafter. Table 4 presents the recovery of applied P at various times during the growing period and for each of the N, P and K application. The recovery was calculated as the difference in L-value (AL) between the respective treatments and the standard zero P application (treatment 5 of Table 3,

i.e., 1.25 g N, O P and .O4 g K per pot with 8 kg of soil). The influ-

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ence of d i f f e ren t P leve ls only (0.28 g and 0.56 g P/pot) i s indicated

by ALlp and ALzp. The influence of a s ingle increment of P (0.56 g

P/pot) under varying N levels a r e indicated by AL , N , AL2N and AL3N and

the e f f ec t s of varying K l eve ls i n ALlK and ALzK. With increasing N and

K l eve ls , but uniform P increment, percentage recovery of applied P

increased.

covery of applied P increased up t o 40 days a f t e r t ransplant ing and sub-

sequently decreased. However, a t the crop maturity s tage P recovery was

higher a t highest r a t e of P appl icat ion.

1 A t uniform N and K leve ls but increasing l eve l of P the re-

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Table 3. Dry m a t t e r weight (DM, i n g / h i l l ) and L- and E-values (!Jg P/g s o i l ) a t va ry ing levels of a p p l i e d N , P and*K

Days a f t e r t r a n s p l a n t i n g

150*

30 40 50 80 Straw Gra in

NPK (g /po t ) DM L-value DM L-value DM L-value DM L-value DM L-value DM L-value E-value**

1 0.00:0.56:1.04 1.2 62 2.3 67 3.0 69 11.6 89 12.5 76 6.9 66 333

2 0.62 -"- 0.8 75 1.6 85 2.2 84 14.7 113 21.3 84 13.1 7 8 402

3 1.25 -I'- 0.8 108 1.5 103 1.8 95 1 2 . 2 116 33.0 84 20.8 84 44 I

4 1.88 -I'- 0.7 95 1.0 97 1.9 98 14.8 113 34.8 69 22.7 a 3 485

5 1.25:0.00:1.04 0.9 36 1.6 43 1.8 48 14.7 56 24.4 33 14.1 35 194

6 " 0.28 " 0.5 69 1 . 7 75 2.3 83 9.5 83 29.1 31 19.0 47 500

7 'I 0.56 " 0.6 114 1.0 113 3.6 93 7.8 90 27.9 7 2 21.6 100 448

U 8 1.25:0.56:0.00 0.9 a4 1 . 4 8 2 3.0 80 11.8 119 24.0 61 15.1 86 537

9 - 1 1 - 0.52 1.0 114 1 . 7 109 4.2 106 8 .2 115 28.6 98 18.5 85 410

I O - 1 1 - 1.04 1.0 1 1 1 1.0 106 1 .3 93 14.5 125 22.6 89 1 7 . 2 100 512

* P l a n t m a t u r i t y

** E-values a f t e r f i n a l h a r v e s t

w c.

Table 4. AL-values" (ug P/g soil) and recovery of applied P as

influenced by varying levels of N , P and K enrichment (numbers

in parenthesis indicate recovery of applied P in X )

~~~ _ _ _ _ ~ _ _ _ _ ~ ~ ~ ~

Days after transplanting 150**

AL 30 40 50 80 Straw Grain

A L I N 26 25 21 33 43 31

(37) (34) (30) (47) (61) (44)

78 66 58 59 65 50

( I 12) (94) (83) (84) (93) (71) AL2K

* ALlN = (LI - L5), subscribed numbers are treatment numbers

ALzN = (L - L ) For treatment details see Table 3 2 5 AL3N = (L4 - L5) A L l P = (L6 - L5) AL^^ = ( L ~ - ~ 5 )

AL2K = (Lg - L ) ALzp = (L7 - L5> 5 ** Final harvest at plant maturity

5 Discussion and conclusions 5.1 Changes in L-values during the growing period

Studies on P dynamics in soils generally assume that plant species can increase the labile P only by depletion of the labile pool resulting in

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a mobilization of non-labile P . But L-values have been shown to increase

during the experimental cropping of soils containing residual fertilizer P (Larsen 1971). The results of this study indicate that lowland rice

plants have a dual influence on P dynamics in flooded soil, i.e., a mobilization of P (up to 80 days after transplanting) followed by its immobilization (after 80 days until crop maturity). Our previous study (Alva et al. 1980), using a resin exchange method (Sibbesen 1977) for extraction of plant available P, also showed a considerable influence of rice plants on the dynamics of P in flooded soils. Crop-induced P mobilization was due to increased activity of soil microflora stimulated by a physiologically active rhizosphere, resulting in reduction of ferric phosphate into more soluble ferrous phosphate, a predominant source of P for lowland rice plants. Crop-induced P immobilization was attributed to the reoxidation of ferrous to ferric iron in the rhizo-

sphere, due to the excretion of oxygen (Mitsui et al. 1962), diffusing

from the aerial parts to the roots (Barber et al. 1962), into the soil.

The possible existence of a physiological mechanism within rice plants capable of maintaining a proper balance between these contrasting pro-

cesses has been discussed elsewhere (Alva et al. 1980).

5.2 The influence of varying N, P and K levels on AL-values and recovery of applied P

The nutrition status of plants is one of the factors affecting the oxi- dation-reduction potential in the rice rhizosphere, which in turn has an influence on P dynamics in flooded soils. Results of this study (Table 4) show that at a given application of P, recovery of applied P in- creases with increasing N application. Increasing application of N has been found to lower the redox potential markedly (Chiang and Yang 1969),

probably by increasing bacterial microflora and respiration (Trolldenier 1971). This would help to mobilize P and may explain the observed posi- tive effect of application of N on the availability of P. According to Cunningham (1964) several crop species tend to maintain a definite ratio between the uptake of cations and of anions. If lowland rice has a simi- lar tendency, then N application (N uptake by lowland rice is generally in the NH4 form) would further increase the anion uptake, thus -I-

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i nc reas ing P recovery.

The in f luence of P a p p l i c a t i o n on t h e recovery of P during t h e growing

per iod depended on the l e v e l of app l i ed P. A t t he lower ra te of P appl i -

c a t i o n (0.28 g/pot) t h e recovery of P w a s constant during t h e major p a r t

of t h e growing per iod, while a t a h ighe r rate (0.56 g /po t ) t he recovery

of P decreased with t i m e . A probable explanat ion f o r t h i s d i f f e r e n c e i s

t h a t a t a higher a p p l i c a t i o n of P crop induced P immobilization sets i n

earlier due t o t h e presence of high l e v e l s of l a b i l e P i n t h e s o i l com-

pared t o t h e amount of P taken up. A t a given rate of P a p p l i c a t i o n t h e

recovery of P increased with inc reas ing K l e v e l s . Chiang and Yang (1969)

and Tro l lden ie r (1977) have shown t h a t i nc reas ing K a p p l i c a t i o n favored

ox ida t ion of t h e rhizosphere, a condi t ion conducive f o r P immobilization

r e s u l t i n g i n lower P recovery. Tadano and Tanaka (1970) observed t h a t

t h i s i n f luence of K a p p l i c a t i o n i s apprec i ab le i n s o i l s low i n K. How-

e v e r , K i s no t l i m i t i n g i n t h i s s o i l (Alva and B i l l e 1980; Nielsen

e t a l . 1979). Accordingly ox ida t ion p o t e n t i a l would no t have been af-

f e c t e d by K app l i ca t ion . The reason f o r t h e observed inc reased recovery

of P upon K a p p l i c a t i o n i s n o t c l e a r , bu t may be explained i n the l i g h t

of t h e cation-anion balance concept (Cunningham 1964).

The E-value, t h e widely adapted n o t a t i o n ( introduced by Russel e t a l .

1954) f o r i s o t o p i c a l l y exchangeable P, i s t h e amount of P on t h e s u r f a c e

of t h e s o i l and i n s o i l s o l u t i o n t h a t i s exchangeable wi th t h e ortho-

phosphate ion added i n s o l u t i o n (Fried 1964). The L-values measured i n

t h i s experiment were between 35-125 vg P/g s o i l , o r 1.1-4.0 umol P/g

s o i l . These va lues are c l o s e t o those r epor t ed by Larsen (1969) f o r d i f -

f e r e n t paddy s o i l s . That s tudy showed t h a t i s o t o p i c equ i l ib r ium w a s at-

t a ined by 24 days (measurements were terminated a f t e r 50 days) . I n t h e

p re sen t s tudy t h e f l u c t u a t i o n s i n L-values were s l i g h t up t o 50 days

a f t e r t r a n s p l a n t i n g , i n d i c a t i n g t h a t t h e crop has no apprec i ab le i n f l u -

ence on l a b i l e P during t h e f i r s t 50 days.

The E-values i n t h i s study are considerably higher than those r epor t ed

by Larsen (1967) f o r s e v e r a l paddy s o i l s under anaerobic condi t ions. E-

values were est imated 150 days a f t e r f l ood ing i n t h i s s tudy as compared

l

I

I

377

5.3 Comparison between L- and E-values

to a 5 days period for Larsen (1967) , which may account for the above

discrepancy. This would imply that no value of E obtained at one single time can be regarded as a parameter of the exchangeable P in soils (Russell et al. 1957). Therefore, further investigations are necessary

with more frequent measurement of E-values during the growing period so

that the integrated effect of flooding and rice cropping on the pool of labile P can be studied. This would also allow a better comparison be- tween L- and E-values.

E-values were considerably higher than L-values at plant maturity stage

(Table 3 ) . Russel et al. (1957) regarded E- and L-values as alternative measures for the labile pool of P in the soil. However, they also pointed to the possibility that significant differences may exist be- tween these two measures, depending on soil characteristics. Added P can migrate between sites which have different characteristics. Some soils contain a large number of sites which have sufficient affinity for

labile P ions to prevent its absorption by plants. That L-values were lower than E-values indicates that certain P fractions were retained in a form inaccessible to plants although they remained readily exchange-

able. According to Russel et al. (1957) the relative magnitude of L- and E-values may change with time because of slow migration of labile P between sites with different characteristics. Another possibility is that 32P is 'sandwiched' by adsorption of 3 2 P 0 ~

followed by dissolution and reprecipitation of 31P04 on top of 32P0~. This may happen when a reduced soil is oxidized during the E-value

determination. Therefore, it would be useful to determine E-values while keeping the soil anaerobic (using N2 gas) and varying the period of

shaking.

Acknowledgements

The financial support from the Danish International Development Agency (DANIDA) is gratefully acknowledged.

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