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Nitrogen mineralization and microbial biomass-N in a subtropical humid forest of Meghalaya, India
Pergamon
Soil Eiol. Biochem. Vol. 29. No. S/IO, pp. 1609-1612, 1997 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-0717/97 $17.00 + 0.00 SOO38-0717(%)002982 0
PII:
SHORT COMMUNICATION NITROGEN MINERALIZATION AND MICROBIAL BIOMASS-N IN A SUBTROPICAL HUMID FOREST OF MEGHALAYA, INDIA A. K. DAS,‘* L. BORAL,* R. S. TRIPATHI’ and H. N. PANDEY2 ‘South Asian Regional Network, School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India and %partment of Botany, School of Life Sciences, North-Eastern Hill University, Shillong 793 014, India (Accepted 5 October 1996 I
The natural nitrogen supply for plants and microorganisms results principally from the mineralization of organic N compounds (Runge, 1983). This and process occurs in two steps: ammonification
nitrification. They play a key role by making N available for plants and microbes. In forest ecosystems most of the N on the forest floor is contained within the litter layer and the soil organic fraction (Rosswall, 1976; Tate, 1987). Less than 1% of the total soil N is in inorganic forms readily available for plant uptake. Therefore, in an unfertilized forest soil N availability is determined by the rate at which organic N pools are mineralized. The mineralization of N in the field is often balanced by microbial immobilization. The microbial biomass N is an important repository of plant nutrients that is more labile than the bulk of soil organic matter (Jenkinson, 1988). It can contribute substantial amounts of nutrients in the soil (Marumoto er al., 1982). The soil microbial biomass is in a constant state of turnover and dead microbial cells are readily mineralized by the remaining microflora. Recognition of the importance of soil microorganisms has led to increased interest in measuring the nutrients held in their biomass (Jenkinson and Powlson, 1976; Martikainen and Palojarvi, 1990; Singh et al., 1989, 1991). We have studied the seasonal variation of the NH4-N and NO,-N pools, N mineralization rates and microbial biomass-N in the soil of a subtropical humid broadleaved forest of northeast India. The study was carried out at Mawphlang, 25 km southwest of Shillong (25” 25’N 91” 41’E at an altitude of 1825 m in Meghalaya, northeast India. The climate has typical monsoonal character. The year can be subdivided into a cold winter (NovemberFebruary), spring (March-April) and rainy season *Author for correspondence.
(May-October). Mean air temperatures in the winter, spring and rainy seasons are 12.4, 16.6 and 19.9”C, respectively. The annual rainfall averages 2500 mm, a major portion of which occurs during the rainy season. The forest is undisturbed subtropical, humid and broad-leaved and represents a relic climax vegetation (Bor, 1942). The dominant tree species are Quercus dealbata L., Q. grzjithii H. and Schima khasiana Dyer. while Symplocos chinensis (Lour.) Druce and Daphne shillong Banerji are the main shrub components (Barik et al., 1992). The soil of the study area is a Ferralsol, the texture loamy sand with a mesic temperature regime. The ranges for different soil properties are: pH, 4.8-5.0 (determined in a 1:2.5 soil-water suspension); organic C, 7.7-10.3% (determined by Walkley and Black’s rapid titration method); total Kjeldahl nitrogen (TKN), 0.76-0.94% (determined by acid digestion of the samples on a block digester followed by distillation and titration in a Tecator kjeltek auto 1030 analyser); available P, 5-7 pg g-’ (determined by the chlorostannous dilute acid flouride extractant (Jackson, 1958). Five paired soil samples were collected randomly from the study site from 0 to 5 and 5 to 10 cm depth. The samples were taken in December 1990, March 1991 and July 1991. The soil moisture content ranged from 35 to 71% (determined gravimetrically at 105C) and soil temperature from 9 to 18°C. One part of each pair was placed in a polythene bag, sealed and inserted back into the soil pit for field exposure. The other part was analysed after air drying. Nitrogen mineralization in the soil was determined by an in situ buried polythene bag technique (Eno, 1960). Inorganic N (NH4 and NOs) was extracted with 1 M KC1 at initial and 30 days of field exposure and then estimated by the distillation method (Allen, 1989). Microbial biomass-N was determined in the field moist sample, using chloro-
1609
Short Communication
1610
form fumigation-extraction method as proposed by Brookes et al. (1985). One-way ANOVA was used to test the statistical significance amongst the various seasons; the Fisher’s LSD at the P < 0.05 level was used to determine further the significance between any two seasons; and a paired Student’s ttest was used to test if depthwise mean values differed significantly (Zar, 1984). The lowest NH4 and NO3 concentrations were observed during the rainy season and the highest during the winter. Extractable NH4 was always higher than the extractable N03. The average annual NH4 concentrations were 39 and 24 pg N g-’ for O-5 and 5-10cm depths, respectively. The corresponding values for NO3 were 16 and 11 pg N g-’ (Fig. 1). Net N-mineralization exhibited pronounced seasonal changes. The rate was highest during the rainy season (peak growing period) when soil moisture content and soil temperature were also high. Ammonification was much faster than nitrification in all the three seasons. NHd-N contributed to more than 70% of N-mineralization during winter and spring. During the rainy season this was reduced to 60%. The highest mineralization, ammonification and nitrification rates were 56, 34 and 22 pg N g-’ 60-
LSD
mineralization
: 8m
40
z ah E + +* g
20
3
__ LSD D
2.57
m 4.32 Ammonification
LSD “”
LSD m ;
2.23
IZ‘J 0.73 g m
40 - Nitrification
z
Winter
Spring
Rainy
Seasons
Fig. 2. Net mineralization, net ammonification and net nitrification at two soil depths (m, O-5cm; ?,?5-10cm) during winter, spring and rainy seasons;* indicates significant (P < 0.05) difference between depths.
0
Winter
spring
30 day-’ in the O-5 cm and 33, 19 and 14 pg N g-’ 30 d-’ in 5-10 cm depths, respectively. The difference in N-mineralization, ammonification and nitrification was significant among the seasons (P < 0.05) (Fig. 2). Rainy
Seasons
Fig. 1. Seasonal patterns of extractable NH4 and NO3 at two soil depths (m, O-5 cm; W, S-10 cm);* indicates significant (P < 0.05) difference between depths.
The microbial biomass-N was high in the winter and low in the rainy season (Table 1). The average values of microbial N, 236 pg N g-’ soil for O-5 soil for 5-10 cm depths, and 164pg Ng-’
I611
Short Communication Table I. Seasonal pattern of microbial biomass-N (ag g-‘) at two soil depths in the subtropical humid forest and other forest types. Values are mean standard error Authors
Soil depth
Forest type O-5 cm
5-IOcm
Subtropicalhumid Winter Spring Rainy LSD (P = 0.05)
271 rt 6.09 232 + 3.75 205 t 0.29 15.67
194 + 2.33 169 + 5.12 129 + 4.58 14.44
Present study
Amazon
(O-IO cm)
27-119
Livingston ef al. (1988)
Finland deciduous
(O-10 cm)
180
Martikainen and Palojarvi (1990)
Dry tropical mixed
(O-IO cm)
53-88
accounted for 2.25 and 2.50% of the total soil N, respectively. The marked seasonality in the rates of net Nmineralization must be related to low soil temperature, since the soil moisture content is seldom limiting in this forest due to the thick litter layer, which does not allow the soil to dry out completely. This is in contrast to the situation reported for the dry deciduous forests of India (Singh et al., 1989), where the litter layer is prominently seen only during the dry season. In our study the net ammonification rate was greater than the net nitrification rate. The low nitrification rates in forest soils were also reported by Nadelhoffer et al. (1983); Richards er al. (1985) and Saxena and Ramakrishnan (1986). The availability of ammonium and phosphorus in soil have been suggested as a general mechanism regulating nitrification in some terrestrial ecosystems (Ellenberg, 1977; Richards ef al., 1985). In our study the low nitrification could be due to low P availability: NH4 was always present in a higher quantity than NO. The microbial biomass-N in our study is higher than the values reported in several evergreen rain and deciduous forests (Table 1). The higher microbial biomass-N in the soil may be considered as nutrient conservation strategy, as suggested by Tate (1987). The proportion of microbial biomass-N to the soil N (2.25-2.50) in this forest is within the range reported in the literature (Stevenson, 1986). The reduced microbial N and increased microbial turnover in the rainy season may result from feeding by microbiovore populations (Singh et al., 1991). It is pertinent to mention here that nutrients released from microbial biomass may not necessarily be taken up by plants. Thus, N released from microbial biomass may be transferred to the next generation, fixed by soil colloids or taken up again by soil organisms (Tate, 1987) leading to the conservation of N in the microbial biomass. Higher N-mineralization rates and microbial biomass-N in the upper soil layer (O5 cm) compared to the sub-soil (5-10 cm) could be related to the availability of resources (more organic matter) as reported by Maggs (1991).
Srivastava
(I 992)
Our results add to knowledge on the mineralization potential of a humid subtropical broad-leaved forest of northeast India that it is characterized by high ammonification and low nitrification rates. Low nitrification rates would be helpful in conserving N loss through leaching and high ammonification rates would favour the growth of heterotrophic microorganisms, which in turn may enhance microbial immobilization of N as a conservation strategy and source of labile pool of nutrients in the soil. Acknowledgements-This research was supported by the University Grants Commission, New Delhi through the DRS Programme sanctioned to the Botany Department of N.E.H.U., Shillong.
REFERENCES S. E. (1989) Chemical Analysis of Ecological Materials, 2nd Edn. Blackwell Scientific Publications,
Allen
Oxford. Barik S. K., Pandey H. N., Tripathi R. S. and Rao P. (1992) Microenvironmental variability and species diversity in tree fall gaps in a subtropical broadleaved forest. Vegefatio 103,31-40. Bor N. L. (1942) The relict of veeetation of Shillone olateau, Assam. indian Forest Recoyd 3, 152-195. _ ’ Brookes P. C., Landman A., Pruden G. and Jenkinson D. S. (1985) Chloroform fumigation and release of soil N: a rapid direct extraction method to measure microbial biomass N in soil. Soil Biology and Biochemistry 17, 837-842.
Ellenberg H. (1977) Stistoff als Standortsfaktor, insbesonderfur mitteleupaische Ptianzen geselleschaffen. Oecolgia Planrarum 12, 1-22.
Eno C. E. (1960) Nitrate production in the field by incubating the soil in polythene bags. Soil Science Society of America Proceedings 24,277-279.
Jackson M. L. (1958) Soil Chemical Analysis. Prentice Hall, Englewood Cliffs. Jenkinson D. S. (1988) Determination of microbial biomass carbon and nitrogen in soil. In Advances in Nitro.gen Cvclinn in Agricultural Ecosvsrems (J. R. Wilson, Ed.) pp. 368-386. C.A.B. . International, Wallingford. Jenkinson D. S. and Powlson D. S. (1976) The effect of biocidal treatments on metabolism in soil, V. A method for measuring soil biomass. Soil Biology and Biochemistry 8, 209-2 13.
1612
Short Communication
Livingston G. P., Vitousek P. M. and Matsoff P. A. (1988) Nitrous oxide flux and nitrogen transformation across a landscape gradient in Amazonia. Journal of Geographical Research 93D, 1593-l 599.
Maggs J. (1991) Nitrogen mineralization and nitrification in rain forests of contrasting nutrient status and physiognomic structure near lake Eaacham, north east Queensland. Australian Journal of Ecology l&47-5 1. Martikainen P. J. and Palojarvi A. (1990) Evaluation of the fumigation-extraction method for the determination of microbial C and N in a range of forest soils. Soil Biology and Biochemistry 22, 797-802.
Marumoto T., Anderson J. P. E. and Domsch K. H. (1982) Mineralization of nutrients from soil microbial biomass. Soil Biology and Biochemistry 14, 469475.
Nadelhoffer K. J., Aber J. D. and Melilleo J. M. (1983) leaf litter production and soil organic matter dynamics along a nitrogen availability gradient in South Wisconsin (U.S.A.). Canadian Journal of Forest Research 13, 12-21.
Richards B. N., Smith J. E. N., White G. J. and Charley J. L. (1985) Mineralization of soil nitrogen in three forest communities from the New England region of New South Wales. Australian Journal of Ecology 10,429-441. Rosswall T. (1976) The internal nitrogen cycle between microorganisms, vegetation and soil. In Nitrogen, Phosphorus and Sulfur-Global Cycles (B.H. Stevensson
and R. Soderlund, Eds), pp. 157-167. Ecological Bulletin, Stockholm. Runge M. (1983) Physiology and ecology of nitrogen nutrition. In Physiological Plant Ecology III. Responses to the Chemical and Biological Environment, Vol. 12C (0. L. Lange, P. S. Noble, C. B. Osmond and H. Ziegler, Eds) pp. 163-200. Springer, Berlin. Saxena K. G. and Ramakrishnan P. S. (1986) Nitrification during slash and bum agriculture (Jhum) in north-eastern India. Acta-Oecologia-Oecologia Plantarum 7, 307319.
Singh J. S., Raghuvanshi A. S., Singh R. S. and Srivastava S. C. (1989) Microbial biomass acts as a source of plant nutrients in dry tropical forest and Savanna. Nature (London) 338,499-500.
Singh R. S., Srivastava S. C., Raghuvanshi A. S., Singh J. S. and Singh S. P. (1991) Microbial C, N and P in dry tropical Savanna: effects of burning and grazing. Journal of Applied Ecology Z&869-878.
Srivastava S. C. (1992) Influence of soil properties on microbial C, N and P in a dry tropical ecosystem. Biology and Fertility of Soils 13, 176180. Stevenson F. J. (1986) Cycles of the Soil. Wiley, New York. Tate R. L. III (1987) Soil Organic Matter. Biolological and Ecological Efsects. Wiley, New York. Zar J. H. (1984) Biostatistical Analysis, 2nd Edn. Prentice Hall, Engelwood Cliffs.
Soil Eiol. Biochem. Vol. 29. No. S/IO, pp. 1609-1612, 1997 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-0717/97 $17.00 + 0.00 SOO38-0717(%)002982 0
PII:
SHORT COMMUNICATION NITROGEN MINERALIZATION AND MICROBIAL BIOMASS-N IN A SUBTROPICAL HUMID FOREST OF MEGHALAYA, INDIA A. K. DAS,‘* L. BORAL,* R. S. TRIPATHI’ and H. N. PANDEY2 ‘South Asian Regional Network, School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India and %partment of Botany, School of Life Sciences, North-Eastern Hill University, Shillong 793 014, India (Accepted 5 October 1996 I
The natural nitrogen supply for plants and microorganisms results principally from the mineralization of organic N compounds (Runge, 1983). This and process occurs in two steps: ammonification
nitrification. They play a key role by making N available for plants and microbes. In forest ecosystems most of the N on the forest floor is contained within the litter layer and the soil organic fraction (Rosswall, 1976; Tate, 1987). Less than 1% of the total soil N is in inorganic forms readily available for plant uptake. Therefore, in an unfertilized forest soil N availability is determined by the rate at which organic N pools are mineralized. The mineralization of N in the field is often balanced by microbial immobilization. The microbial biomass N is an important repository of plant nutrients that is more labile than the bulk of soil organic matter (Jenkinson, 1988). It can contribute substantial amounts of nutrients in the soil (Marumoto er al., 1982). The soil microbial biomass is in a constant state of turnover and dead microbial cells are readily mineralized by the remaining microflora. Recognition of the importance of soil microorganisms has led to increased interest in measuring the nutrients held in their biomass (Jenkinson and Powlson, 1976; Martikainen and Palojarvi, 1990; Singh et al., 1989, 1991). We have studied the seasonal variation of the NH4-N and NO,-N pools, N mineralization rates and microbial biomass-N in the soil of a subtropical humid broadleaved forest of northeast India. The study was carried out at Mawphlang, 25 km southwest of Shillong (25” 25’N 91” 41’E at an altitude of 1825 m in Meghalaya, northeast India. The climate has typical monsoonal character. The year can be subdivided into a cold winter (NovemberFebruary), spring (March-April) and rainy season *Author for correspondence.
(May-October). Mean air temperatures in the winter, spring and rainy seasons are 12.4, 16.6 and 19.9”C, respectively. The annual rainfall averages 2500 mm, a major portion of which occurs during the rainy season. The forest is undisturbed subtropical, humid and broad-leaved and represents a relic climax vegetation (Bor, 1942). The dominant tree species are Quercus dealbata L., Q. grzjithii H. and Schima khasiana Dyer. while Symplocos chinensis (Lour.) Druce and Daphne shillong Banerji are the main shrub components (Barik et al., 1992). The soil of the study area is a Ferralsol, the texture loamy sand with a mesic temperature regime. The ranges for different soil properties are: pH, 4.8-5.0 (determined in a 1:2.5 soil-water suspension); organic C, 7.7-10.3% (determined by Walkley and Black’s rapid titration method); total Kjeldahl nitrogen (TKN), 0.76-0.94% (determined by acid digestion of the samples on a block digester followed by distillation and titration in a Tecator kjeltek auto 1030 analyser); available P, 5-7 pg g-’ (determined by the chlorostannous dilute acid flouride extractant (Jackson, 1958). Five paired soil samples were collected randomly from the study site from 0 to 5 and 5 to 10 cm depth. The samples were taken in December 1990, March 1991 and July 1991. The soil moisture content ranged from 35 to 71% (determined gravimetrically at 105C) and soil temperature from 9 to 18°C. One part of each pair was placed in a polythene bag, sealed and inserted back into the soil pit for field exposure. The other part was analysed after air drying. Nitrogen mineralization in the soil was determined by an in situ buried polythene bag technique (Eno, 1960). Inorganic N (NH4 and NOs) was extracted with 1 M KC1 at initial and 30 days of field exposure and then estimated by the distillation method (Allen, 1989). Microbial biomass-N was determined in the field moist sample, using chloro-
1609
Short Communication
1610
form fumigation-extraction method as proposed by Brookes et al. (1985). One-way ANOVA was used to test the statistical significance amongst the various seasons; the Fisher’s LSD at the P < 0.05 level was used to determine further the significance between any two seasons; and a paired Student’s ttest was used to test if depthwise mean values differed significantly (Zar, 1984). The lowest NH4 and NO3 concentrations were observed during the rainy season and the highest during the winter. Extractable NH4 was always higher than the extractable N03. The average annual NH4 concentrations were 39 and 24 pg N g-’ for O-5 and 5-10cm depths, respectively. The corresponding values for NO3 were 16 and 11 pg N g-’ (Fig. 1). Net N-mineralization exhibited pronounced seasonal changes. The rate was highest during the rainy season (peak growing period) when soil moisture content and soil temperature were also high. Ammonification was much faster than nitrification in all the three seasons. NHd-N contributed to more than 70% of N-mineralization during winter and spring. During the rainy season this was reduced to 60%. The highest mineralization, ammonification and nitrification rates were 56, 34 and 22 pg N g-’ 60-
LSD
mineralization
: 8m
40
z ah E + +* g
20
3
__ LSD D
2.57
m 4.32 Ammonification
LSD “”
LSD m ;
2.23
IZ‘J 0.73 g m
40 - Nitrification
z
Winter
Spring
Rainy
Seasons
Fig. 2. Net mineralization, net ammonification and net nitrification at two soil depths (m, O-5cm; ?,?5-10cm) during winter, spring and rainy seasons;* indicates significant (P < 0.05) difference between depths.
0
Winter
spring
30 day-’ in the O-5 cm and 33, 19 and 14 pg N g-’ 30 d-’ in 5-10 cm depths, respectively. The difference in N-mineralization, ammonification and nitrification was significant among the seasons (P < 0.05) (Fig. 2). Rainy
Seasons
Fig. 1. Seasonal patterns of extractable NH4 and NO3 at two soil depths (m, O-5 cm; W, S-10 cm);* indicates significant (P < 0.05) difference between depths.
The microbial biomass-N was high in the winter and low in the rainy season (Table 1). The average values of microbial N, 236 pg N g-’ soil for O-5 soil for 5-10 cm depths, and 164pg Ng-’
I611
Short Communication Table I. Seasonal pattern of microbial biomass-N (ag g-‘) at two soil depths in the subtropical humid forest and other forest types. Values are mean standard error Authors
Soil depth
Forest type O-5 cm
5-IOcm
Subtropicalhumid Winter Spring Rainy LSD (P = 0.05)
271 rt 6.09 232 + 3.75 205 t 0.29 15.67
194 + 2.33 169 + 5.12 129 + 4.58 14.44
Present study
Amazon
(O-IO cm)
27-119
Livingston ef al. (1988)
Finland deciduous
(O-10 cm)
180
Martikainen and Palojarvi (1990)
Dry tropical mixed
(O-IO cm)
53-88
accounted for 2.25 and 2.50% of the total soil N, respectively. The marked seasonality in the rates of net Nmineralization must be related to low soil temperature, since the soil moisture content is seldom limiting in this forest due to the thick litter layer, which does not allow the soil to dry out completely. This is in contrast to the situation reported for the dry deciduous forests of India (Singh et al., 1989), where the litter layer is prominently seen only during the dry season. In our study the net ammonification rate was greater than the net nitrification rate. The low nitrification rates in forest soils were also reported by Nadelhoffer et al. (1983); Richards er al. (1985) and Saxena and Ramakrishnan (1986). The availability of ammonium and phosphorus in soil have been suggested as a general mechanism regulating nitrification in some terrestrial ecosystems (Ellenberg, 1977; Richards ef al., 1985). In our study the low nitrification could be due to low P availability: NH4 was always present in a higher quantity than NO. The microbial biomass-N in our study is higher than the values reported in several evergreen rain and deciduous forests (Table 1). The higher microbial biomass-N in the soil may be considered as nutrient conservation strategy, as suggested by Tate (1987). The proportion of microbial biomass-N to the soil N (2.25-2.50) in this forest is within the range reported in the literature (Stevenson, 1986). The reduced microbial N and increased microbial turnover in the rainy season may result from feeding by microbiovore populations (Singh et al., 1991). It is pertinent to mention here that nutrients released from microbial biomass may not necessarily be taken up by plants. Thus, N released from microbial biomass may be transferred to the next generation, fixed by soil colloids or taken up again by soil organisms (Tate, 1987) leading to the conservation of N in the microbial biomass. Higher N-mineralization rates and microbial biomass-N in the upper soil layer (O5 cm) compared to the sub-soil (5-10 cm) could be related to the availability of resources (more organic matter) as reported by Maggs (1991).
Srivastava
(I 992)
Our results add to knowledge on the mineralization potential of a humid subtropical broad-leaved forest of northeast India that it is characterized by high ammonification and low nitrification rates. Low nitrification rates would be helpful in conserving N loss through leaching and high ammonification rates would favour the growth of heterotrophic microorganisms, which in turn may enhance microbial immobilization of N as a conservation strategy and source of labile pool of nutrients in the soil. Acknowledgements-This research was supported by the University Grants Commission, New Delhi through the DRS Programme sanctioned to the Botany Department of N.E.H.U., Shillong.
REFERENCES S. E. (1989) Chemical Analysis of Ecological Materials, 2nd Edn. Blackwell Scientific Publications,
Allen
Oxford. Barik S. K., Pandey H. N., Tripathi R. S. and Rao P. (1992) Microenvironmental variability and species diversity in tree fall gaps in a subtropical broadleaved forest. Vegefatio 103,31-40. Bor N. L. (1942) The relict of veeetation of Shillone olateau, Assam. indian Forest Recoyd 3, 152-195. _ ’ Brookes P. C., Landman A., Pruden G. and Jenkinson D. S. (1985) Chloroform fumigation and release of soil N: a rapid direct extraction method to measure microbial biomass N in soil. Soil Biology and Biochemistry 17, 837-842.
Ellenberg H. (1977) Stistoff als Standortsfaktor, insbesonderfur mitteleupaische Ptianzen geselleschaffen. Oecolgia Planrarum 12, 1-22.
Eno C. E. (1960) Nitrate production in the field by incubating the soil in polythene bags. Soil Science Society of America Proceedings 24,277-279.
Jackson M. L. (1958) Soil Chemical Analysis. Prentice Hall, Englewood Cliffs. Jenkinson D. S. (1988) Determination of microbial biomass carbon and nitrogen in soil. In Advances in Nitro.gen Cvclinn in Agricultural Ecosvsrems (J. R. Wilson, Ed.) pp. 368-386. C.A.B. . International, Wallingford. Jenkinson D. S. and Powlson D. S. (1976) The effect of biocidal treatments on metabolism in soil, V. A method for measuring soil biomass. Soil Biology and Biochemistry 8, 209-2 13.
1612
Short Communication
Livingston G. P., Vitousek P. M. and Matsoff P. A. (1988) Nitrous oxide flux and nitrogen transformation across a landscape gradient in Amazonia. Journal of Geographical Research 93D, 1593-l 599.
Maggs J. (1991) Nitrogen mineralization and nitrification in rain forests of contrasting nutrient status and physiognomic structure near lake Eaacham, north east Queensland. Australian Journal of Ecology l&47-5 1. Martikainen P. J. and Palojarvi A. (1990) Evaluation of the fumigation-extraction method for the determination of microbial C and N in a range of forest soils. Soil Biology and Biochemistry 22, 797-802.
Marumoto T., Anderson J. P. E. and Domsch K. H. (1982) Mineralization of nutrients from soil microbial biomass. Soil Biology and Biochemistry 14, 469475.
Nadelhoffer K. J., Aber J. D. and Melilleo J. M. (1983) leaf litter production and soil organic matter dynamics along a nitrogen availability gradient in South Wisconsin (U.S.A.). Canadian Journal of Forest Research 13, 12-21.
Richards B. N., Smith J. E. N., White G. J. and Charley J. L. (1985) Mineralization of soil nitrogen in three forest communities from the New England region of New South Wales. Australian Journal of Ecology 10,429-441. Rosswall T. (1976) The internal nitrogen cycle between microorganisms, vegetation and soil. In Nitrogen, Phosphorus and Sulfur-Global Cycles (B.H. Stevensson
and R. Soderlund, Eds), pp. 157-167. Ecological Bulletin, Stockholm. Runge M. (1983) Physiology and ecology of nitrogen nutrition. In Physiological Plant Ecology III. Responses to the Chemical and Biological Environment, Vol. 12C (0. L. Lange, P. S. Noble, C. B. Osmond and H. Ziegler, Eds) pp. 163-200. Springer, Berlin. Saxena K. G. and Ramakrishnan P. S. (1986) Nitrification during slash and bum agriculture (Jhum) in north-eastern India. Acta-Oecologia-Oecologia Plantarum 7, 307319.
Singh J. S., Raghuvanshi A. S., Singh R. S. and Srivastava S. C. (1989) Microbial biomass acts as a source of plant nutrients in dry tropical forest and Savanna. Nature (London) 338,499-500.
Singh R. S., Srivastava S. C., Raghuvanshi A. S., Singh J. S. and Singh S. P. (1991) Microbial C, N and P in dry tropical Savanna: effects of burning and grazing. Journal of Applied Ecology Z&869-878.
Srivastava S. C. (1992) Influence of soil properties on microbial C, N and P in a dry tropical ecosystem. Biology and Fertility of Soils 13, 176180. Stevenson F. J. (1986) Cycles of the Soil. Wiley, New York. Tate R. L. III (1987) Soil Organic Matter. Biolological and Ecological Efsects. Wiley, New York. Zar J. H. (1984) Biostatistical Analysis, 2nd Edn. Prentice Hall, Engelwood Cliffs.