Biodynamics of Microbial Biomass Nitrogen in Organic Matter Amended Lowland Paddy Soil of Bangladesh

抄録

An incubation experiment was carried out in the laboratory at 30°C for 56 days to investigate the mineralization pattern and changes in microbial biomass in lowland paddy soil amended with four organic materials (OM). The organic materials were rice straw (Oryza sativa), dhaincha (Sesbania rostrata), water hyacinth (Eichhornia crassipes) and partially decomposed cowdung. Inorganic N (NH₄⁺, NO₃^-) released from the OM amended soils gradually increased and reached it's maximum during 42 to 56 days of incubation. N mineralization was negatively correlated with C: N ratios of the added OM. The critical C: N ratio value for N mineralization and immobilization was observed in water hyacinth (C: N=20: 1) where mineralization followed immobilization after a certain period. Microbial biomass formation gradually increased up to between 28 and 42 days of incubation and decreased thereafter. Biomass C formation ranged from 155 to 216 mg kg^-1 in OM amended soil and 95 to127 mg kg^-1 in control soil, and biomass N formation accounted for 30 to 43 mg kg^-1 in OM amended soil and 20 to 25 mg kg^-1 in control soil. No clear relationship was observed between biomass C and N formation and the C: N ratios with added OM. However, OM with high C: N ratios, like rice straw, showed higher contributions to biomass C and N formation than OMs with lower C: N ratios. The results suggest that OMs having higher C: N ratios would be better for the improvement of microbial biomass level as well as soil fertility under tropical conditions.