Plant and Soil 143: 289-297, 1992. © 1992 KluwerAcademic Publishers. Printed in the Netherlands.

PLSO 9209

Effect of growth and subsequent decomposition of cyanobacteria on the transformation of phosphorus in submerged soils BISWAPATI MANDAL, S.C. DAS and L.N. MANDAL Micronutrient Research Laboratory, Department of Agricultural Chemistry and Soil Science, Bidhan Chandra Krishi Viswavidyalaya, Kalyani 741235, West Bengal, India Received 7 May 1991. Revised March 1992

Key words:

cyanobacteria, P transformations, submerged soils

Abstract

The effect of growth and subsequent decomposition of cyanobacteria (inoculated and indigenous) on changes in P fractions was studied in four soils under submerged condition. The growth of cyanobacteria in soils caused an increase in organic P with concomitant decreases in Olsen-P, AI-P, Fe-P, and Ca-P, but little change in reductant-soluble F e - P and occluded AI-P. Such changes have been attributed to the solubilization of different inorganic P fractions and subsequent assimilation of the released P by cyanobacteria. The decomposition of cyanobacterial biomass in soils caused an increase in Olsen-P with a simultaneous decrease in other P fractions, except the Ca-bound P. Development of intense reducing condition and formation of organic acids with chelating properties have been suggested as the cause of the above changes. Implications of such changes in P fractions due to the growth of cyanobacteria, and of the decomposition of the cyanobacterial biomass for the P nutrition of rice plants are discussed.

Introduction

Since the discovery of De (1939) that the maintenance of fertility of Indian rice fields should be attributed to N-fixing blue-green algae (cyanobacteria), a number of studies have highlighted the importance of their use as biofertilizer in tropical rice fields. Addition of combined nitrogen and biomass are recognized to be highly important contributions to maintaining and improving the fertility of rice field soils (Roger and Watanabe, 1986). Nitrogen accretion to the extent of 30 kg ha -I crop -1 and biomass addition from a few kg to 16 t ha -1 (fresh weight) due to incorporation of cyanobacteria are considered to be reasonable estimates under field conditions (Roger et al., 1987). Besides enriching the soils with organic C and N, cyanobacteria liberate abundant quantities of

photosynthetic oxygen which modifies the oxidation-reduction status (Eh) of rice field soils. They also release extracellularly a multitude of organic compounds including various acids (Kerby et al., 1987; Vaidya, 1970) some of which are known to form chelates with metal elements. In addition, they cause a diurnal fluctuation in soil and water pH (Katyal and Carter, 1989; Mikkelsen et al., 1978) by photosynthetic utilization and respiratory release of CO 2 during day- and night time, respectively. At the end of the growth period, when the biomass undergoes anaerobic decomposition it results in a lowering of the soil redox potential (Saha et al., 1982) and in formation of various organic acids. The above changes in chemical and physico-chemical soil environment brought about by cyanobacteria during their growth and also the subsequent decay are likely

290

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to influence the transformation and availability of various nutrients in submerged rice soils. Das et al. (1991) have recently shown that the transformations of Fe and Mn in submerged rice soils are greatly influenced by cyanobacteria during their growth and subsequent decomposition. Saha and Mandal (1979) showed that the OlsenP content of submerged soils decreased during the first 90 days of growth of cyanobacteria and then began to increase. However, information relating to the effect of cyanobacteria on transformation and availability of P in submerged rice soils is meagre. The present investigation was, therefore, undertaken to study the influence of growth of cyanobacteria, inoculated and indigenous, and of their subsequent decomposition, on P transformation in submerged rice soils, with a view to ascertain if the algalization of rice fields under the biofertilization programme could in any way modify the P availability to rice.

Materials and methods

Soil samples (0-15 cm) were collected from rice fields situated in four different physiographic regions of West Bengal, a major rice-growing state of India. Soil pH ranged from 5.0 to 7.1, organic carbon from 0.59 to 1.12%, CEC from 4.0 to 24.0cmol(p+)kg -1, total N from 0.063 to 0.130% and clay from 14.4 to 41.7%. Fifty-g portions of soil were transferred to beakers (dia. 9 cm), to each of which was added 75 mL of double-distilled water and 2 mL of the inoculum (a mixed culture of cyanobacteria containing Anabaena, Nostoc, Cylindrospermum and Tolypothrix species, collected at their exponential growth stage, washed several times with de-ionised water and then macerated to a homogeneous suspension). The outside walls of half of the number of beakers were blackened and their mouths covered with glazed black paper (with small perforations to allow exchange of gases) with a view to check the growth of cyanobacteria both inoculated and indigenous as well as other photosynthetic organisms. These served as the control (darkened) series. The rest of the beakers had only their mouths covered with watch glasses so as to allow the entry of sufficient solar radiation inside, thus encouraging

the growth of cyanobacteria and other photosynthetic organisms, including eukaryotic algae, if any. The latter served as the treated (lightexposed) series. All beakers were then inserted into a flooded soil bed in such a way that the levels of soil and standing water in the tray and inside the beakers were the same. The tray containing the beakers was kept for two months in a greenhouse in open sunlight, the ambient temperature ranging between 25°C to 30°C, each treatment being replicated thrice. The temperature of the standing water inside the beakers in the two series (darkened and light-exposed) was occasionally measured and found to vary by about 0.5°C amongst the two series. The greater absorption of heat energy by the darkened wall of the beakers seemed to have been offset/ balanced by the greater entry of solar radiation through the watch glass. After a period of 2 months the biomass produced in the lightexposed beakers was mixed thoroughly with the soil mass. Portions of the soil samples from each of the series (light-exposed and darkened) were analysed (in triplicate) for pH (1:2.5), organic C, total N (Kjeldahl), crystalline oxides of iron, Olsen-P, organic P and different inorganic P fractions following standard methods as described by Jackson (1967), while the remaining bulk portions of the samples after partial drying were distributed over a number of small specimen tubes (5- to 10-g portions in each tube), submerged and incubated in the dark in the laboratory to allow decomposition of the biomass. These samples were periodically analysed for NH 4 + N O ~ - N (Bremner and Keeney, 1966), Olsen-P, organic P and the different inorganic P fractions mentioned before. Phosphorus in the extracts was determined colorimetrically by the chlorostannous-reduced molybdophosphoric-blue color method as described by Jackson (1967) using a Klett Summerson photoelectric colorimeter, while iron was determined by atomic absorption spectrophotometry (Pye Unicam, Model SP9 800).

Results

Results (Table 1) show that there were increases in organic C and total N in all soils of the

Cyanobacterial effects on P in submerged soils

291

Table 1. Effect of growth of cyanobacteria on the changes in pH, organic carbon, total nitrogen and crystalline iron oxides in soils

pH ( 1 : 2.5)

Laterite Alluvial Acidic Saline

Organic carbon (%)

Total nitrogen ( % )

Crystalline iron oxides ( % )

A -~'

A+

A-

A+

A-

A+

A-

A+

6.8 7.5 6.5 7.4

7. I 7.2 6.9 7.2

0.54a h 1.01c (1.56a (1.56a

0.71b 1.35d 0.73b (1.72b

(I.I)64a 0.120c 0.062a 0.060a

0.082b 0.165d 0.080b 0.076b

1.454a 1.223c 11.788e 0.784eg

1,488b 1.244d 0.818f tl,757h

~' A - = without cyanobacteria: A + = with cyanobacteria. ~' Mean separation by Duncan's multiple range test. 5% level.

light-exposed compared with the darkened series after two months of incubation. The increase in organic carbon was due to the growth of cyanobacteria (inoculated as well as indigenous) and also of the eukaryotic algae and hydrophytes, if any, present in the soil, while the increase in total N may be attributed to the growth of cyanobacteria alone. However, a highly significant positive correlation (r=0.978"*) between the increase in organic carbon and that in total N suggests that the increase in the former in the light-exposed series was primarily due to the growth of cyanobacteria and not much to the growth of eukaryotic algae or other photosynthetic organisms. Hence, the observed P transformations in soils of the light-exposed series may reasonably be considered to be the effect of the growth of cyanobacteria (inoculated and indigenous). Since the broad objective of the experiment was to study the effect of algalization on P transformations in soil, it was not considered necessary to eliminate the growth of indigenous cyanobacteria or eukaryotic algae by sterilizing the soils before inoculating them with mixed cultures of cyanobacteria. There was an increase in crystalline Fe oxides in three of the four soils of the light-exposed series, This may be due to the oxidation of ferrous iron formed in the reduced layer, following its diffusion to the oxidised layer, where there was an abundant quantity of photosynthetic oxygen. The pH of the soils in the light-exposed series recorded slightly higher values than those in the darkened series owing to a shift in the equilibrium CO, ~ HCO~ ~-~CO 3 as a result of the photosynthetic activity of the organisms. Waterlogging alone (darkened series) caused a marked increase in Olsen-P in all soils, the mean increase being 76.4% over the initial content (Table 2). Olsen-P in the soils of the lightexposed series, although always higher than the

initial content, was lower than that in the darkened series. This shows that the growth of cyanobacteria and other photosynthetic organisms in soil caused a decrease in Olsen-P, the mean decrease being 12.9% over that in the controls (darkened series). A similar decrease in available soil P during the initial period of growth of cyanobacteria was also observed by Saha and Mandal (1979). Soil organic P, which varied from 68.7 mg kg l (acidic soil) to 133.0rag kg -~ (saline soils), remained almost unchanged in the control (darkened) series while in the light-exposed series it showed a marked increase in all soils, the mean increase being 20.5%. This may be attributed to the assimilation of available P by the cyanobacterial cells. A similar increase in organic soil P due to growth of cyanobacteria was also observed by Singh (1961). The initial AI-P content, which varied from 19.2 mg kg ~~ (laterite soil) to 37.8 mg kg ~ (alluvial soil) showed only a very small decrease in the control (darkened) series while in the lightexposed series a marked decrease was noticeable in all soils, such decreases ranging from 8.9 (saline soil) to 17.7% (alluvial soil) with a mean of 15.0%. The initial Fe-P content, which varied from 27.5 mg kg ~ (saline soil) to 61.7 mg kg-L (laterite soil) showed a marked decrease in all soils of both series, the decrease being 17% higher in the light-exposed than in the darkened series. Ca-bound P, which was high in all soils except the laterite one, showed a decrease in both series, the decrease being slightly higher in the light-exposed series. The amount of reductant-soluble Fe-P in the initial soils was highest in the laterite (38.5 mg kg- L)~ followed by the alluvial (18.0 mg kg-1) soil, but was only a trace in both the acidic and saline soils. The content decreased

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Mandal et al.

Table 2. Effect of growth of cyanobacteria on changes in soil P fractions (mg kg-' ) Soil

Olsen P A~

Laterite 18.0 Alluvial 27.2 Acidic 15.2 Saline 16.0 Mean 19.1 C.D. 5% Cyanobacterial treatment

Organic P

AI-P

B

C

D(-) b

A

B

C

D(+)

A

B

C

D(-)

32.8 44.6 28.8 28.8 33.7

29.6 39.2 23.2 25.8 29.4

9.8 12.1 19.4 10.4 12.9

104.2 124.3 68.7 133.0 107.6

100.0 120.0 64.3 127.0 102.8

116.0 145.3 86.6 140.0 121.9

16.0 21.1 34.7 10.2 20.5

19.2 37.8 24.8 26.0 26.9

17.6 34.5 24.5 25.9 25.6

14.6 28.4 20.5 23.6 21.8

17.0 17.7 16.3 8.9 15.0

0.57

1.38 Ca-P

Fe-P Laterite Alluvial Acidic Saline C.D. 5% cyanobacterial treatment

A

B

C

D(-)

A

B

C

61.7 50.0 49.0 27.5

50.5 42.0 43.0 22.5

42.5 34.0 33.0 20.2

15.8 19.0 23.2 10.2

46.7 122.5 130.0 195.0

42.5 110.0 115.0 172.5

40.0 104.0 110.0 165.2

D(-)

5.9 5.4 4.3 4.2

0.57

0.91 Occluded A1-P

Reductant-soluble Fe-P Laterite Alluvial Acidic Saline C.D. 5% Cyanobacterial treatment

0.45

A

B

C

D(-)

A

B

C

D(-)

38.5 18.0 Tr. Tr.

34.0 15.8 Tr. Tr.

32.5 14.4 Tr. Tr.

4.4 8.9

36.0 31.8 7.8 6.8

24.6 23.8 5.2 4.4

22.4 21.4 5.0 4.0

8.9 10.0 3.8 9.0

N.S.

0.58

A = Initial; B = without cyanobacterial growth; C = with cyanobacterial growth. , D = % increase(+) or decrease(-) due to cyanobacterial growth. slightly in soils o f b o t h series, the decrease being slightly larger in the light-exposed series. T h e c o n t e n t of occluded A I - P which initially r a n g e d f r o m 6.8 p p m (saline soil) to 36.0 mg k g - , (laterite soil) decreased slightly in all soils of b o t h series, the decrease being slightly higher in the light-exposed series. T h e overall results, therefore, show that g r o w t h of c y a n o b a c t e r i a in s u b m e r g e d soils caused an increase in organic P while it resulted in a m a r k e d decrease in Olsen-P, A I - P and F e - P , and a small decrease in C a - P , reductantsoluble F e - P and occluded A1-P.

Effect of decomposition of biomass The

decomposition

of biomass resulted

in a

gradual increase in total inorganic soil nitrogen ( N H 4 + N O 3 ) during the initial period, while it s h o w e d a declining trend later on (data not p r e s e n t e d ) . T h e initial increase m a y be attributed to the mineralisation of organic nitrogen in the biomass and the subsequent decline to the loss of mineral nitrogen t h r o u g h various channels. D e c o m p o s i t i o n o f biomass b r o u g h t a b o u t a m a r k e d increase in Olsen-P (Table 3), which attained p e a k values 20 days after biomass incorporation. This was followed by a decline, but the values r e m a i n e d always higher than those in the c o r r e s p o n d i n g control series, the increase being a b o u t 21 to 24%. Soil organic P, which was m u c h higher in the light-exposed samples, decreased continuously with increasing length of the incubation period,

Cyanobacterial effects on P in submerged soils

293

Table 3. Effect of decomposition of cyanobacterial biomass on changes in soil P fractions (rag kg ~) during varying periods of decomposition Soil

Days of decomposition 0 B"

Olsen P Laterite 32.8 Alluvial 44.6 Acidic 28.8 Saline 28.8 Mean 33.7 C.D. 5% Cyanobacterial treatment

Organic P katerite 99.8 Alluvial 119.99 Acidic 64.32 Saline 126.98 Mean 1(12.8 C.D. 5% Cyanobacterial treatment

AI-P Laterite 17.6 Alluvial 34,5 Acidic 24.5 Saline 25.9 Mean 25.6 C.D. 5e~ Cyanobacterial treatment

Fe-P Laterite 50.5 Alluvial 42,0 Acidic 43.0 Saline 22,5 Mean 39.5 C.D. 5% Cyanobacterial treatment

Ca-P Laterite 42.5 Alluvial 110.0 Acidic 115.0 Saline 172,5 Mean 110.0 C.D. 5% Cyanobacterial treatment

20 C

Db

29.6 39.2 23.2 25.8 29.4

-9.8 -12.1 - 19.4 - 10.4 -12.9

B

38.0 42.4 28.8 27.2 34.1

0.57

115.98 145.32 86.66 139.98 121.9

16.0 21.1 34.7 10.2 20.5

98.8 122.2 67.5 133.6 105.5

- 17.0 -17.7 -16.3 -8.8 - 14.9

9.4 34,0 22.3 24.4 22.5

0.57

22.1 33.(I 18.7 19.1 23.2

96.4 118.7 61. l 121.0 99.3

8.4 26.2 18.6 20.4 18.4

- 15.8 - 19.0 -23.2 - 10,2 - 17.0

42.0 35.5 39.5 19.0 34.0

36.8 37.5 32.5 16.0 30.7

36.8 35.2 23.2 24.0 29.8

44.6 102.5 106.2 164.3 104.4

41.2 108.5 113.0 170.7 108.4

1.03

D

42.0 50.(I 28.(I 28.8 37.2

B

14.1 42.0 20.7 20.0 24.2

C

32.7 33.3 20.0 19.2 26.3

(I.58

-2.4 -2.9 -9.5 -9.4 -6.1

95.3 123.5 63.1 121.7 100.9

90.(/ 1 l 1.0 56.0 116.4 93.3

- 10.6 -22.9 -16.6 - 16.4 16.6

10,0 32.6 19.5 22.4 21.1

5.4 20.2 17.2 18.0 15.2

-5.6 - 10.1 - 11.2 -4.3 -7.8

93.6 122.0 58.8 120.(I 98.6

52.5 38.5 40.0 24.0 38.7

44,0 44,5 35,(I 22,2 36.4

-46.0 -38.0 -11.8 19.6 28.8

10.2 21.2 15.7 19.7 16.7

40.6 96.2 98.7 157,7 98.3

44.0 106,2 105.0 164.7 105.0

1.46

17.7 27.3 20,0 20,8 21.4

92.0 105.0 55.(/ 118.0 92.5

- 1.7 - 13.9 -6.5 - 1.7 -5.9

6.0 17.6 15.2 14.0 13.2

-41.2 -17.0 -3.2 -28.9 -22.6

(/.38

- 16.2 5.6 12.5 -7.5 12.1

58,7 47.0 47.5 34.0 46.8

0.85

7.6 5,8 6.4 3.9 5.4

39.2 42.4 24./) 23.2 32.2

1.57

0.36

- 12~4 5.6 - 17.7 - 15.8 - 15.3

D

(1.38

1.90

0.66

-5.9 -5.4 -4.3 -4.2 -4.9

60 C

0.43

0.91

40.0 104,0 110.0 165.2 104.8

46.4 56.4 34.2 32.4 42.3

B

2,12

0.45

42.5 34.0 33.0 20.2 32.4

D

0.68

1.38

14.6 28.4 20,5 23.6 21.8

40 C

55.0 52.0 44.0 32.6 45.9

-6.3 10.6 -7.4 -4.1 -5.9

(I.83

8.3 10.4 6.4 4.4 7.4

38.0 97.5 87.5 157.5 94.1

40.5 101.2 103,5 162,7 101.9

1.06

6.5 3,8 18.3 3.3 8.(1

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M a n d a l et al.

Table 3. (Continued) Soil

Days of decomposition 0 B~

20 C

Dh

B

40 C

D

B

60 C

D

B

C

D

Reductant soluble F e - P

Laterite 34.0 Alluvial 15.8 Acidic Tr Saline Tr Mean 28.2 C.D. 5% Cyanobacterial treatment

32.5 14.4 Tr Tr 24.9

-4.4 -8.9 ---6.6

30.0 13.5 Tr Tr 21.7

0.58

24.0 11.2 Tr Tr 17.6

-20.0 -17.0 ---18.5

28.0 10.5 Tr Tr 19.2

0.61

20.0 10.0 Tr Tr 15.0

-28.6 -4.8 ---16.7

32.0 12.1 Tr Tr 22.0

1.15

28.0 11.5 Tr Tr 19.7

-12.5 -4.9 ---8.7

NS

Occluded A I - P

Laterite 24.6 Alluvial 23.8 Acidic 5.2 Saline 4.4 Mean 14.5 C.D. 5% Cyanobacterial treatment

22.4 21.4 5.0 4.0 13.2 N.S.

-8.9 -10.0 -3.8 -9.0 -7.9

20.0 18.5 4.0 3.8 11.6

18.0 15.2 3.6 2.2 9.7 0.73

-10.0 -17.8 -10.0 -42.1 -20.0

17.0 18.0 3.5 3.0 10.4

14.8 13.8 3.0 2.0 8.4

-12.9 -23.3 -14.3 -33.3 -20.9

0.46

15.0 15.0 3.2 2.8 9.0

14.0 12.2 2.8 2.0 7.7

-6.7 -18.7 -12.5 -28.6 -16.6

N.S.

B = without decomposing cyanobacterial biomass; C = with decomposing cyanobacterial biomass; b D = % increase or decrease (-) due to decomposing cyanobacterial biomass.

the values being always lower than in the control samples. Decomposition of biomass caused a marked decrease in the A I - P contents of the soils, the mean decreases being 16.6 to 28.8%. The effect was most prominent in the laterite and alluvial soils at 40 days after biomass incorporation. Decomposition of biomass also caused a marked decrease in the F e - P contents of all soils except the alluvial one, where it caused an increase. The rate of decrease, however, declined with increasing length of the incubation period, and at the end of the 60-day period the contents in soils of both series were almost identical for all soils, except the alluvial one. Decomposition of biomass, on the other hand, caused a small increase in the C a - P contents of the soils, the mean increases ranging from 5.4 to 8.0% during the 20- to 60-day incubation period. Both reductant-soluble F e - P and occluded A I - P in the control (darkened) soils decreased gradually during the period of incubation, except for the F e - P after 60 days of incubation. Decomposition of biomass caused a further decrease,

this being more prominent for the F e - than for the A I - P . The overall results, therefore, show that decomposition of cyanobacterial biomass caused an increase in Olsen-P and C a - P , but a decrease in A I - P , F e - P , reductant F e - P , occluded A I - P and organic P in the soils. Statistical correlation The highly significant correlation (Table 4) between the increases in organic C and total N contents of soils and the changes in some of the fractions of P (Olsen-P, organic P and A I - P ) provides evidence that the observed P transformations were the result of cyanobacterial growth. The increase in soil organic P may be attributed to assimilation of P from different forms of inorganic P by photosynthetic microorganisms, including cyanobacteria which were allowed to grow in the soils of the light-exposed series. This assumption receives support from the significant correlation between the increase in soil organic P and the decreases in Olsen-P

(~vanobacterial effects on P in submerged soils

295

Table 4. Coefficients of correlation between the increases in organic carbon and total nitrogen and the changes in different fractions of soil P due to the growth of cyanobacteria

/X, Organic C /X Total N /X Organic P

/6x:' Total-N

/X, Organic P

z~ Olsen-P

/ ~ AI-P

A Ve-P

A Ca-P

0.978**

(I.869" 0.860*

0.652 0.658 0.923**

(1.91)8" 0.928** 0.960**

0.237 0.250 (1.892"

0.483 0.573 0.824*

~'z~X and A = increase and decrease, respectively: * and ** = significant at 0.05 and (1.(11 probability levels, respectively.

(r = 0.92"*), AI-P (r = 0.96"*), Fe-P (r = 0.89*) and Ca-P (r = 0.82*). The data were also subjected to multiple regression analysis, which resulted in the development of the equation: Y=93.7+5.49X~ + 18.43X~ + 3.00X 3 (R 2 =0.978"*), where Y = increase in organic P, X~, X 2 and X 3 = decrease in AI-P, Fe-P and Ca-P, respectively. The above equation could explain as much as 97.8% of the variability in increases in organic P which was due to cyanobacteria assimilating AI-P, FeP and Ca-P in the soils. Since the change in Ca-P due to cyanobacterial growth was comparatively small, the following equation was obtained by excluding Ca-P as a variable: Y = 24.5 + 12.3X~ + 9.7X 2 (R 2 = 0.953**) which shows that 95.3% of the variability in organic P was due to cyanobacteria assimilating AI-P and Fe-P in soils. This suggests that for their nutrition cyanobacteria derive P primarily from these two fractions.

Discussion

The results of the present investigation show that waterlogging alone caused a marked increase in the Olsen-P fractions in the soils, with concomitant decreases in the other inorganic P fractions. Such increases can be attributed to release of P from A1 compounds due to hydrolysis, from Fem compounds due to reduction to more soluble Fe u compounds under the anaerobic conditions, and from Ca compounds due to solubilization by H 2 C O ) and other organic acids (Mahapatra and Patrick, 1969; Mandal, 1964; Mandal and Das, 1970; Mandal, 1979; Patrick and Mahapatra, 1968). The decrease in Olsen-P in the presence of cyanobacterial growth, both indigenous and inoculated (light-exposed series), may be attributed to the assimilation of soluble and labile forms

of P by the living cells, which is evidenced from the increase in soil organic P. Cyanobacteria are considered to be an efficient sink for P, thus continuously causing P transformations in soil (Porcella et al., 1970). As mentioned before, cyanobacteria are known to release extracellularly a variety of organic compounds including organic acids, some of them having chelating properties (Kerby et al., 1987; Vaidya, 1970). These compounds might have formed chelates with AI and Fe, which might have led to the release of P originally bound by these metals. The released P was subsequently assimilated by the living cells. This explains the decreases in AI-P and Fe-P in the presence of growing cyanobacteria population. This view receives support from the presence of a significant correlation between the decreases in AI-P and Fe-P and the increase in soil organic P. Dorich et al. (1980) observed a significant correlation between the (1.5 M NH4F-extractable P (considered to be AI-P) and P assimilated by cyanobacteria and concluded that these organisms could utilize on an average as much as 42% of AI-P present in soil or sediment. A similar observation was made in respect of Fe-P by a number of workers (Dorich et al., 1985; Sager, 1976; Sonzogni et al., 1982; Wolf et al., 1985) who suggested that 0.1 N NaOH-extractable P (considered to be Fe-P) represented the potentially available P for growth of cyanobacteria, Cameron and Julian (1988) observed that cyanobacteria could utilize hydroxyapatite (HA) as a source of P for their growth and suggested that Ca-chelation by chelators synthesised by them were responsible for release of P from insoluble Ca compounds. Others (Bose et al., 1971; Chiou and Boyd, 1974; Golterman et al., 1969) were, however, of the opinion that H 2 C O 3 and other organic acids released by cyanobacteria could solubilize P from Ca sources and thus make them available for their own growth. The

296

Mandal et al.

view that cyanobacteria could cause release of P from A1, Fe and Ca sources and utilize it for their cell synthesis receives support from the multiple regression equation. One important point to be observed is that the degree of mobilization of P was relatively higher from Al and Fe sources than from Ca sources. This may be due to the higher susceptibility of A1 and Fe for chelation because of their higher charges. This is supported by the second multiple regression equation which showed that excluding Ca-P from the equation left the R 2 value almost unchanged. There was a decrease both in reductant-soluhie F e - P and occluded A1-P due to waterlogging alone, which may be attributed to the dissolution of Fe20 3 coatings under anaerobiosis and the subsequent exposure of the phosphates to solubilization. Growth of cyanobacteria caused a small decrease, which is controversial in view of the fact that growth of cyanobacteria provides a more aerobic environment in soil due to release of photosynthetic 0 2. Wildung et al. (1977) observed a decrease in citrate-bicarbonate-dithionite (CBD)-extractable P (considered to be reductant-soluble Fe-P) in lake sediments and suggested such phosphates to be an important source of P for algae, while Lee et al. (1980) were of the opinion that P adsorbed onto hydrated oxides of Fe was unavailable for supporting algal growth. A critical examination of the results revealed that the total increase in organic P (20.2 mg kg- ~) in the presence of cyanobacterial growth was higher (except in laterite soil) than the cumulative decreases (18.4mgkg -1) in the inorganic P fractions. In a similar study with lake sediment, Nalewajko and Lean (1978) observed that during early exponential growth of algae the influx of P from different sources might be less than total P uptake. Wildung et al. (1977) also observed such a change in different forms of sediment-P which could be related to P uptake by expanding algal communities. The results, therefore, indicate that cyanobacteria might be able to solubilize P from these difficulty available forms in soils (Brady, 1990) and assimilate it during their growth. Phosphorus which was assimilated by cyanobacteria and incorporated into their cells during growth was found to be released upon bacterial decomposition in the form of soluble organic

phosphorus compounds or condensed polyphosphates (Bortoletti et al., 1978). These compounds were later mineralised or hydrolysed to orthophosphates resulting in an increase in available (Olsen) P in soils. Decomposition of biomass intensifies the reducing conditions in soil (Saha et al., 1982) and thereby stimulates the reduction of F e m - P to more soluble FeII-p. This also resulted in the formation of various organic compounds having chelating properties, which might have promoted the release of P from Al and Fe sources by chelating A1 and Fe. These findings explain the increase in available (Olsen) P with simultaneous decreases in organic P, AI-P and F e - P as a result of decomposition of biomass, as experienced in the present investigation. Decreases in reductant-soluble F e - P and occluded AI-P seem to be the result of dissolution under intensely reduced conditions of Fe20 3 coating the phosphates. However, the increase in F e - P in alluvial soils in the presence of decomposing biomass is difficult to explain. With increasing length of incubation period, F e - P in soils showed an increasing trend, but more so in the absence than in the presence of decomposing biomass. The hydrated oxides of Fe formed during the period of anaerobic incubation, as explained by Das et al. (1991), might have adsorbed some P released from other forms, resulting in an increase in Fe-P. The comparatively lower values in the presence of decomposing biomass may be due to a more intensely reductive environment in soil in this treatment. The H2CO 3 or organic acids formed during the process of decomposition might have converted some of the dilute (0.025M) HESO4-insoluble calcium phosphate in soil into soluble forms, which may explain the small increase in C a - P in the presence of decomposing biomass.

Practical implications During their growth cyanobacteria can utilize P from various insoluble inorganic P fractions in soil, viz. AI-P, F e - P and Ca-P, and incorporate it into their body cells. At the end of their life cycles, when the cells undergo lysis or decomposition, P is released in plant-available form. Hence, these organisms, serving as potential biofertilizer in rice fields, besides enriching soils

Cyanobacterial effects on P in submerged soils with nitrogen and organic matter, may also help improve the P nutrition of the crop by mobilizing unavailable soil P. Such a role of cyanobacteria is of particular significance in laterite soils, which not only have high P-fixing capacities but also have most of their inorganic P in A1- and Febound forms.

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