Effect of wheat straw carbon:sulfur ratio on mineralization of sulfur in soils under simulated laboratory aerobic-flooding cycles

This study was undertaken to assess the effect of residue carbon:sulfur (C:S) ratio on the mineralization of S in rice soils under aerobic-flooding cycles. A lab incubation study with a 3x2x2 factorial design (two replications) was carried out for 12 weeks. The treatments were two rice soils, Faridpur and Joydebpur; three levels of wheat straw (Triticum aestivum L.) added to soils as 1) control, 2) straw C:S ratio (400:1), or 3) straw C:S ratio (100:1); and two moisture levels of 1) two weeks aerobic-flooding cycles or 2) four weeks aerobic-flooding cycles. The organic residues (25 mg straw g(-1) soil) were mixed with 30 g soil and an equal amount of glass bead mixtures (1:1) and transferred into a leaching tube. Soils in leaching tubes were leached with deionized water and analyzed for sulfate-sulfur (SO4-S) and other chemical properties at two week intervals up to 12 weeks. Cumulative SO4-S in ah soils amended with a narrow C:S ratio (100:1) of the straw had a significantly higher accumulation of SO4-S over the control or the wider C:S ratio (400: 1). The first-order rate constants (K-s)in soils amended with wheat straw under aerobic-flooding cycles ranged from 0.112 to 0.160 mg S kg(-1) wk(-1) and the K(s )values ranged from 0.030 to 0.149 mg S kg(-1) wk(-1) for unamended and straw amended soils, respectively. Among the two straw C:S ratio treatments, the narrow C:S ratio residue treatment had the highest K-s values, indicating greater rates of microbial S mineralization and potential to provide more plant-available S. The cumulative amount of C mineralized was linearly related to time of incubation and corresponded to S mineralization.