Phosphorus requirements of microbial biomass in a regosol and an andosol

Abstract

The critical P concentration in microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) and minimum amount of available P to obtain the critical P concentration in the microbial biomass of a granitic regosol and an andosol of Japan were examined. Phosphorus was applied as KH 2PO 4 at rates of 0, 25, 50, 200 and 400 mg P kg −1 soil to a regosol and 0, 25, 100, 400 and 800 mg P kg −1 to an andosol together with 2000 mg C (rice straw) and 200 mg N (ammonium sulphate) kg −1. With increasing P application, the available P in soil markedly increased in the regosol and gradually increased in the andosol. The amount of microbial biomass C and P increased with available P up to 76 and 29 mg P kg −1 in the regosol and andosol, respectively, and either remained constant or was slightly decreased at a higher available P value. The concentration of P in the microbial biomass was higher in the regosol (29 to 89 mg P g −1) than in the andosol (13 to 32 mg P g −1), assuming that 1 g of dry biomass contained 0.5 g C. The microbial biomass C to P ratio was higher in the andosol (16 to 38) than in the regosol (6 to 17). The critical P concentration in microbial biomass was estimated to be 62 mg P g −1 biomass in the regosol and 19 mg P g −1 in the andosol. The corresponding minimum value of available P in soil to increase microbial biomass was estimated as 38 and 6 mg P kg −1 soil in the regosol and andosol, respectively. The specific respiration of microbial biomass was also very high at those P concentrations which were considered optimum in both soils to increase not only the amount of microbial biomass C and P but also microbial activity. These were 38 mg P kg −1 soil in the regosol and 6 mg P kg −1 soil in the andosol.