Degradation of S-metolachlor and its Effects on Soil Enzymes and Microbial Communities in Vegetable Field Soil

S-metolachlor is a promising alternative to metolachlor. However, the extensive use of S-metolachlor as herbicide in vegetable fields in China has caused concerns about its environmental fate. Here we investigated the effects of temperature, relative humidity (RH) and PH on the degradation rate of S-metolachlor in vegetable soil. The degradation rates of S-metolachlor increased with increasing temperatures and RH and either acidic or basic PH facilitates S-metolachlor degradation. The degradation of S-metolachlor under these conditions followed the first-order kinetics resulting in the half-lives (T-1/2) ranging from 12.18 d to 70.71 d at 5–35°C, and 27.28 d to 53.72 d at RH 30–90%; and 29.62–19.69 d at pH 6–8. Stronger response of soil enzymes including catalase, dehydrogenase, urease and cellulase to S-metolachlor was detected in soil with high organic matter. PLFA profiles showed that, totally, the microbe populations including actinomycetes, fungi and bacteria increased gradually in the first 14 days after the treatment and decreased from 14d to 28d after the treatment. All the S-metolachlor treatments caused the increase of aerobe and anaerobe. High S-metolachlor concentration, 13.9 mg/kg, could cause significant variation at the first 2 weeks, stimulating growth of the entire soil microbial community. These findings might have practical implications for the fate of S-metolachlor residue in vegetable fields. Environmental factors, especially temperature, relative humidity and pH should be considered in combination with the appropriate application dose of S-metolachlor for achieving satisfactory weed-control efficacy, reducing runoff, and minimizing effects on environmental quality.