Critical sulphur concentration and sulphur requirement of microbial biomass in a glucose and cellulose-amended regosol

The critical S concentration and S requirement of the soil microbial biomass of a granitic regosol was examined. S was applied at the rate of 0, 5, 10, 20, 30 and 50 mug, S as MgSO4. 7H(2)O, together with either 3000 mug glucose-C or 3333 mug cellulose-C, 400 mug N, and 200 mug P g(-1) soil and 200 mug K g(-1) soil. Microbial biomass, inorganic SO42--S, and CO2 emission were monitored over 30 days during incubation at 25 degreesC. Both glucose and cellulose decomposition rates responded positively to the S made available for microbial cell synthesis. The amounts of microbial biomass C and S increased with the level of applied S up to 10 mug S g(-1) soil and 30 mug 8 S g(-1) soil in the glucose- and cellulose-amended soil, respectively, and then declined. Incorporated S was found to be concentrated within the microbial biomass or partially transformed into soil organic matter. The concentration of S in the microbial biomass was higher in the cellulose- (4.8-14.2 mg g(-1)) than in the glucose-amended soil (3.7-10.9 mg g(-1)). The microbial biomass C:S ratio was higher in the glucose- (46-142:1) than in the cellulose-amended soil (36-115:1). The critical S concentration in the microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 5.1 mg g(-1) in the glucose- and 10.9 mg g(-1) in: the cellulose-amended soil. The minimum requirement of SO42--S for microbial biomass formation was estimated to be 11 mug S g(-1) soil and 21 mug S g(-1) soil for glucose- and cellulose-amended soil, respectively. The highest levels of activity of the microbial biomass were observed at the SO42--S concentrations of 14 mug 8 S g(-1) soil and 17 mug S g(-1) soil, for the glucose and cellulose amendments, respectively, and were approximately 31-54% higher during glucose than cellulose decomposition.