EFFECT OF WHEAT-STRAW CARBON SULFUR RATIO ON MINERALIZATION OF SULFURIN SOILS UNDER SIMULATED LABORATORY AEROBIC-FLOODING CYCLES/

This study was undertaken to assess the effect of residue carbon:sulfu r (C:S) ratio on the mineralization of S in rice soils under aerobic-f looding cycles. A lab incubation study with a 3x2x2 factorial design ( two replications) was carried out for 12 weeks. The treatments were tw o rice soils, Faridpur and Joydebpur; three levels of wheat straw (Tri ticum aestivum L.) added to soils as 1) control, 2) straw C:S ratio (4 00:1), or 3) straw C:S ratio (100:1); and two moisture levels of 1) tw o weeks aerobic-flooding cycles or 2) four weeks aerobic-flooding cycl es. 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 deion ized water and analyzed for sulfate-sulfur (SO4-S) and other chemical properties at two week intervals up to 12 weeks. Cumulative SO4-S in a h soils amended with a narrow C:S ratio (100:1) of the straw had a sig nificantly 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 amend ed 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 0.1 49 mg S kg(-1) wk(-1) for unamended and straw amended soils, respectiv ely. Among the two straw C:S ratio treatments, the narrow C:S ratio re sidue treatment had the highest K-s values, indicating greater rates o f microbial S mineralization and potential to provide more plant-avail able S. The cumulative amount of C mineralized was linearly related to time of incubation and corresponded to S mineralization.