Influences of glucose, nitrogen and plant residues on the immobilization of sulphate-S in soil

Sulphur immobilization and microbial transformations were monitored in a clay loam following 12 treatments using 35S-labelled SO 4 2−S (SO 4 2− 35S), glucose, N and plant residues (rape leaves and straw). Over a 102 day incubation, the immobilization of SO 4 2− 35S, presented as a percentage of that added, was inversely related to its addition rate. Additions of glucose and plant residues increased the immobilization of SO 4 2− 35S. The extent to which SO 4 2− 35S was immobilized was positively correlated with the C-to-S ratio of the amendments, irrespective of their origins (glucose and plant residues). In the presence of a C source (glucose or plant residues), N significantly increased the immobilization of SO 4 2− 35S, whilst the effect of N was insignificant in the absence of a C amendment. Over the first 3 days, the amounts of added SO 4 2− 35S immobilized were linearly correlated (1-to-1 relationship) with the amounts of added S recovered in the soil microbial biomass (labelled). With further incubation, the proportions of immobilized SO 4 2− 35S remaining as biomass- 35S decreased. Decrease in biomass- 35S was thought to be due to the conversion of biomass-S into soil organic-S. Glucose addition increased the immobilization (microbial utilization and incorporation into the soil organic matter) of native soil SO 4 2−S. However, N addition enhanced the mineralization of soil organic-S, increasing the concentration of SO 4 2−S in soil.