SEASONAL RESPONSES IN MICROBIAL BIOMASS CARBON, PHOSPHORUS AND SULFURIN SOILS UNDER PASTURE

The response of the soil microbial biomass to seasonal changes was inv estigated in the field under pastures. These studies showed that over a 9-month period, microbial biomass carbon, phosphorus and sulphur (bi omass C, P, S), and their ratios (C:P, C:S, and P:S) responded differe ntly to changes in soil moisture and to the input of fresh organic mat erials. From October to December (1993), when plant residues were larg ely incorporated into the soils, biomass C and S increased by 150-210% . Biomass P did not increase over this time, having decreased by 22-64 % over the dry summer (July to September). There was no obvious correl ation between biomass C, P, and S and air temperature. The largest amo unts of biomass C and P (2100-2300 mu g and 150-190 mu g g(-1) soil, r espectively) were found in those soils receiving farmyard manure (FYM or FYM+NPK) and P fertilizer, whereas the use of ammonium sulphate dec reased biomass C and P. The C:P, C:S, and P:S ratios of the biomass va ried considerably (9-276:1; 50-149:1; and 0.3-14:1, respectively) with season and fertilizer regime. This reflected the potential for the bi omass to release (when ratios were narrow) or to immobilize (wide rati os) P and S at different times of the year. Thus, seasonal responses i n biomass C, P, and S are important in controlling the cycling of C, P , and S in pasture and ultimately in regulating plant availability of P and S. The uptake of P in the pasture was well correlated with the s um of P in the biomass and soil available pools. Thus, the simultaneou s measurement of microbial biomass P and available P provide useful in formation on the potential plant availability of P.