Influence of Phosphate and Copper on Reductive Dechlorination of Thiobencarb in California Rice Field Soils

Abstract

The potential for reductive dechlorination of the herbicide thiobencarb (TB) by microbes and its prevention in saturated anaerobic rice field soils was examined in laboratory microcosms. TB is effective in controlling both annual grasses and broadleaf weeds. In anoxic microcosms, TB was effectively degraded within 30 days to its dechlorinated product, deschlorothiobencarb (DTB), in two Sacramento Valley rice field soils. TB dechlorination, and subsequent degradation, followed pseudo-zero- (lag phase) and first-order (degradation phase) kinetics. Logistic regression analysis (r² > 0.841) produced a half-life (t_1/2) in nonsterile soils ranging from 10 to 15 days, which was also observed when microcosms were amended with low concentrations (40 mg L^-1) were added to the microcosms to determine if copper toxicity to dechlorinating microbes is concentration dependent within the range used. After 30 days, the low-copper-amended soils closely resembled the nonsterile experiments to which no Cu²⁺ was added while the high-copper-amended microcosms were similar to the sterile experiment. Microcosms were also separately amended with 5.7 g L^-1 phosphate (PO₄^2-; as KH₂PO₄), a nutrient regularly applied to rice fields. Phosphate-amended experiments also showed TB degradation, but no DTB formation, indicating the phosphate played a role, possibly as a microbial inhibitor or an alternative electron acceptor, in limiting the dechlorination of TB. In summary, TB dechlorination was inhibited at high Cu(OH)₂, CuSO₄·5H₂O, and KH₂PO₄ concentrations. Keywords: Microcosms; herbicide; fungicide; anaerobic; half-life