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

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.