Influence of organic carbon on reductive dechlorination of thiobencarb in California rice field soils.

The herbicide thiobencarb is suspected of causing delayed phytotoxicity syndrome (DPS) in rice plants. While the ultimate agent appears to be its dechlorinated product (deschlorothiobencarb), the influence of organic carbon on the formation of deschlorothiobencarb in California rice field soils has not been investigated. Thus, two different soils were compared for their ability to reductively dechlorinate thiobencarb with carbon augmentation: one from the eastern Sacramento Valley, which has historically displayed DPS, and one from the western Sacramento Valley, which has not. Rice straw was homogenized into samples of each soil to produce 0, 0.33 or 2% organic carbon augmentation. During 90-days of anoxic incubation, substantial deschlorothiobencarb production was measured in both soil types. However, only the thiobencarb degradation rate in the eastern valley soil was positively correlated with carbon content. Thus, other characteristics of DPS-resistant soils may limit deschlorothiobencarb formation.