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SOIL BIOLOGY & BIOCHEMISTRY

Soil Nitrogen Mineralization and Microbial Biomass Relation, and Nitrogen Conservation in Humid-Tropics

^a Central Agricultural Research Institute, Post Box 181, Port Blair-744101Andaman, India
^b Resear. Centre of Central Soil and Water Conserv., Resear. and Training Institute, Kota, Rajasthan, India

^* Corresponding author (cbpandey5{at}rediffmail.com).

The reciprocal relationship between microbial biomass C (MB-C) and net nitrification in the seasonally dry tropics can reduce nitrate (NO₃^–) leaching losses, and be regarded as a mechanism of N conservation. The objective of the present study was to determine the influence of high seasonally prolonged rainfall on water-filled pore space (WFPS), net-nitrification rates (NNR), net-ammonification rates (NAR), pool sizes of NH₄⁺–N and NO₃^––N, and MB-C in three major land-use systems (moist-evergreen, semi-evergreen forests, and home garden) in the equatorial humid tropical climate of South Andaman, India. Soil properties were examined to determine a mechanism of N conservation in the humid tropical soils. Across the land use systems, WFPS ranged from 68 to 84% during the wet season and 25 to 47% during wet season dry spells (48–72 h after wet season high rainfall). The rate of net-nitrification and NO₃^––N pool declined 10 to 17 times and 4 to 6 times, respectively during the high rainfall wet season in all the land use systems compared with wet season dry spells. On the contrary, ammonification rate and NH₄⁺–N pool increased 7 to 34 times and 2 to 3 times, respectively during the wet season across land use systems compared with the wet season dry spell. Microbial biomass C was positively correlated with the NNR, but it was inversely correlated with the NH₄⁺–N pool. The above observations suggest that NO₃^– N is likely reduced to NH₄⁺ via a dissimilatory pathway during rainy periods (anaerobic conditions), which conserves N in the plant available form (NH₄⁺) in the soils of the humid tropics.

Abbreviations: MB-C, microbial biomass carbon • NH₄⁺–N, ammonium N pool • NAR, net ammonification rate • NMR, net mineralization rate • NNR, net nitrification rate • NO₃^––N, nitrate N pool • WFPS, water-filled pore space