ORGANIC-MATTER, MICROBIAL BIOMASS AND ENZYME-ACTIVITY OF SOILS UNDER DIFFERENT CROP ROTATIONS IN THE TROPICS

Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mi neralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusio n of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial b iomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg(-1) soil in a pearl millet-wheat-fa llow rotation to 256 mg kg(-1) soil in a pearl millet-wheat-green manu re rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soi l enzyme activity. There was a good correlation between microbial biom ass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatas e activity of the soil under different crop rotations was little affec ted. The results indicate the green manuring improved the organic matt er status of the soil and soil microbial activity vital for the nutrie nt turnover and long-term productivity of the soil.