Nutrient requirements (N, P, S) of microbial biomass formation in a regosol of Japan

Abstract

The critical N, P and S concentrations in the soil microbial biomass (biomass) and minimum available N, P and S in soils to obtain the critical N, P and S concentration in the biomass was examined. A granitic regosol was mixed with organic C (2 g C kg −1 soil as rice straw (Exp. 1, 2) or 3 g C kg −1 soil as cellulose) and mineral nutrients (4 levels of N (Exp. 1), 5 levels of P (Exp. 2), 5 levels of S (Exp. 3)). The soils were adjusted to 40% of WHC, and then incubated at 25 °C for 30–60 days. Biomass C, N, P, S, available N (NH 4 +NO 3), P, S in soil and CO 2 -C evolved were measured at 0, 5, 10, 20, 30 and 60 days after amendment. With increasing N, P, and S application, biomass C, N, P and S markedly increased up to 200 mg N, 200 mg P and 30 mg S kg −1 soil, respectively and then either remained constant or slightly decreased at higher levels. The critical N, P and S concentration in biomass (defined as that required to achieve 80 % of the maximum synthesis of biomass C) was estimated to be 85, 62, and 11 mg g −1 biomass, respectively. The minimum values of available N, P and S in the granitic regosol to obtain the critical N, P and S concentration in the biomass was estimated as about 185, 38 and 22 mg kg −1 soil, respectively.