INFLUENCES OF GLUCOSE, NITROGEN AND PLANT RESIDUES ON THE IMMOBILIZATION OF SULFATE-S IN SOIL

Sulphur immobilization and microbial transformations were monitored in a clay loam following 12 treatments using S-35-labelled SO42--S (SO42--S-35), glucose, N and plant residues (rape leaves and straw). Over a 102 day incubation, the immobilization of SO42--S-35, presented as a percentage of that added, was inversely related to its addition rate. Additions of glucose and plant residues increased the immobilization of SO42--S-35. The extent to which SO42--S-35 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 SO42--S-35, whilst the effect of N was insignificant in the absence of a C amendment. Over the first 3 days, the amounts of added SO42--S-35 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 SO42--S-35 remaining as biomass-S-35 decreased. Decrease in biomass-S-35 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 SO42--S. However, N addition enhanced the mineralization of soil organic-S, increasing the concentration of SO42--S in soil.