Mineralization-immobilization of sulphur in a soil during decomposition of plant residues of varied chemical composition and S content

Background and aims The recycling of plant residues can be an important source of available sulphate in soils. This study investigates the effects of soil sulphate availability on residue decomposition and the relationships between plant residue quality and S and C mineralization Methods A soil incubation experiment examined rape-seed straw mineralization after adding sulphate at rates of 20 and 50 mg S-SO4 kg−1 soil in a loamy soil. Soils amended with wheat straw, tall fescue, mustard or beech leaf residues were incubated. Net C and S mineralization were measured continuously during a 175-day incubation at 20 °C and gross S mineralization and immobilization were quantified using 35S soil labeling. Results The addition of sulphate did not change C mineralization, indicating that soil S supply was high enough to supply microbial needs during decomposition. The chemical quality of the residue significantly modified the rate of decomposition and the gross and net S mineralization, with a high release of sulphate from mustard and fescue residues but little net change in soil sulphate with the other residues. Conclusions The chemical composition and C/S ratio of plant residues are important criteria for predicting both the dynamics and the amount of sulphate available during residue decomposition.