Barley straw decomposition and S immobilization

Sulphur deficiencies have become increasingly recognized in both soils and crops. The return of plant residues low in S to low S soils may lead to decomposition processes being limited by S availability. In a laboratory incubation, barley straw that was low in S was added to four low S soils, with or without added sulphate-S. Straw samples with varying concentrations of S were also added to one of the soils. Decomposition was monitored as CO2 production, and S immobilization was measured by the change in phosphate-extractable-S. Decomposition was limited by S in all four soils, but this was dependent on the straw S content. With sufficient S, approximately 40% of the straw C was respired over 25 days at 25°C, but this was up to 30% less where S was limiting. Mineralization occurred at a straw S content of 0.15% irrespective of any S addition. At 0.11% S, there was a balance where decomposition was not retarded in the absence of added S, but immobilization occurred if S was added. However, at 0.07 and 0.04% S, there was increased immobilization, and decomposition was retarded if S was not added. The critical S content for immobilization was estimated to be 0.13% or a C-to-S ratio in the residue of 340. For potential limitation of residue decomposition, the S content would have to be slightly less than this with a C-to-S ratio in the range 400–650 (0.11-0.07% S). In S-deficient areas and depending upon the soil, the incorporation of plant residues low in S may lead to reduced plant growth and retarded residue decomposition.