Soil Microbial Biomass and Enzyme Activities as Influenced by Organic Manure Incorporation into Soils of a Rice-Rice System

A fertilizer management study involving incorporation of poultry manure, farm yard manure, sesbania and gliricidia into soils of a rice-rice system was initiated in May 1993. In order to determine the effects of organic manure incorporation on soil microbial biomass and enzyme activity, soils were collected from the respective plots at the end of the second rice crop in February 1996. and were incubated with and without the respective organic manure at the rate of 15 Mg ha−1 at 25°C, under submergence. The total viable microbial counts, bacteria, actinomycetes, N flush, biomass C and activities of urease, amidase, acid and alkaline phosphatase, dehydrogenase and L-glutaminase were determined after 60 days of incubation. Soils freshly amended and soils previously amended with organic manures registered significantly greater microbial biomass and enzyme activity than the unamended control. The microbial biomass and enzyme activity, however, varied with the type of organic manure incorporated into the soil. Except for acid phosphatase, which showed slight inhibition, all the other enzymes were activated to different degrees by organic manure incorporation. A significant and positive relationship of enzyme activity with organic C and total N suggested that the addition of organic manure to soils increased C turnover, N availability and microbial activity which in turn led to greater enzyme synthesis and accumulation in the soil matrix.