Bioavailability of diuron in soil containing wheat-straw-derived char

This study evaluated the bioavailability of diuron in soil as influenced by char arising from the burning of wheat straw. The wheat char was a highly effective sorbent for diuron. The presence of 1% wheat char in soil resulted in a 7-80 times higher diuron sorption. A 10-week incubation resulted in <40% of 0.5 mg/kg diuron in 0.5% char-amended soil microbially degraded, as compared to 50% in char-free soil under the same conditions. Over the experimental range of diuron application rates from 0 to 12 mg/kg and of char contents from 0% to 1.0%, a 4-week bioassay indicated that both the barnyardgrass survival rating and the fresh weight of aboveground biomass decreased with increasing diuron application at given char contents but increased with increasing char content at potentially damaging diuron application rates. Residual analyses of bioassayed soils showed that the soils with char contents of 0.5% and higher and diuron application rates of 3.0 mg/kg and higher, as compared to those with no or low (0.05%) char and a diuron application rate of 1.5 mg/kg, had higher residual diuron levels but higher barnyardgrass survival ratings and fresh weights. These results suggest that enhanced sorption of diuron in soil in the presence of wheat char reduced the bioavailability of diuron, as manifested by reduced microbial degradation of diuron and its herbicidal efficacy to barnyardgrass. This study may have greater implication than for burning of wheat straw that field burning of vegetations may reduce bioavailability of pesticides.