Mineralization and microbial biomass formation in upland soil amended with some tropical plant residues at different temperatures

A model experiment was carried out at 15, 25, and 35°C to investigate the changes in microbial biomass and the pattern of mineralization in upland soil during 8 weeks following the addition of 8 organic materials including 6 tropical plant residues, ipil ipil (Leucaena leucocephala), azolla (Azolla pinnata), water hyacinth (Eichhornia crassipes), dhaincha (Sesbania rostrata), cowpea (Vigna unguiculata), and sunhemp (Crotalaria juncea). The amounts of CO₂-C evolved and inorganic N produced at 35°C were about 2 times larger than those at 15°C. At any temperature, the flush decomposition of C was observed within the first week and thereafter the rate of mineralization became relatively slow. A negative correlation was observed between inorganic N and C/N ratios of the added organic materials. The relationships between the amounts of cellulose or cellulose plus hemicellulose and the amount of mineralized N of the added organic materials were also negative.

The changes in the microbial biomass were affected by temperatures. The amount of biomass C and N was maximum after 42 d of incubation at 15°C, and after 7 d at 25 and 35°C, and thereafter decreased. The rate of biomass decline was slower at 15°C and faster at 35°C than at 25°C. Regardless of the temperatures, the addition of organic materials enhanced microbial biomass formation throughout the incubation periods.