RELEASE OF SULFUR FROM RICE RESIDUES UNDER FLOODED AND NON-FLOODED SOIL-CONDITIONS

Increased rice cropping intensity, due to shorter maturity varieties a nd shorter turn around times between crops, has lead to an increase in burning of rice straw. This can have significant short and long term effects on the productivity of the rice cropping system. A glasshouse experiment was conducted using an Aquic Haplustalf soil to study the r elease of S from different rice residues (straw and ash) under differe nt water management conditions (non-flooded and flooded), and with and without the addition of S fertilizer. Rice straw residues (0.086 and 0.108%S) and ash produced from them (0.168 and 0.374%S), labelled with S-35, were used to investigate the importance of residue form and res idue S concentration for the release of S. A loss of approximately 60% of the S from low-S straw and 40% from high-S straw was recorded on b urning. The results showed that the addition of 6 kg S ha-1 as gypsum significantly increased grain yield across treatments. The addition of gypsum resulted in an increased S release from high-S straw residue u nder non-flooded conditions as measured by plant uptake, suggesting a priming effect of gypsum on S release from residues. The incorporation of straw and ash resulted in significantly increased grain yield in s ome but not all treatments. The percentage recovery of S in plants was higher in ash than in straw treated soil, but since S had been lost d uring burning, the absolute amount recovered (soil+plant) was higher i n straw than in ash-treated soil. The S-35 data indicate that if there is a critical S level for mineralization of rice straw, this level is less than the 0.086% in the residues used in this experiment.