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

Abstract

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.

Field dissipation of S-metolachlor in organic and mineral soils used for sugarcane production in Florida

“…Although the physiochemical properties of the organic soil is different from soils in previous S-metolachlor field dissipations studies, the half-life of 50 to 126 d is within the range of 13 to 142 d reported in previous field studies (Frank et al 1991;Huang et al 2017;Martínez et al 1997;Mueller et al 1999; Figure 2. Dissipation of S-metolachlor in the top 7.5 cm of organic soil in sugarcane fields in 2013 to 2014 and 2014 to 2015 growing seasons at the Everglades Research and Education Center in Belle Glade, FL.…”

“…This shows that persistence of S-metolachlor in the second season was 2.3-fold that of the first season. S-metolachlor half-lives of 13 to 142 d under field conditions depending on soil type and prevailing environmental conditions have been previously reported (Frank et al 1991;Huang et al 2017;Martínez et al 1997;Mueller et al 1999;Shaner and Henry 2007;Shaner et al 2006;Westra et al 2014;Wu et al 2015). An exponential decay model, with the exception of Shaner et al (2006) and Westra et al (2014), described dissipation dynamics of S-metolachlor in these previous studies.…”

“…Constant moisture levels and decreasing temperatures also reduce S-metolachlor degradation (Braverman et al 1986;Walker and Brown 1985;Zimdahl and Clark 1982). In field studies, the half-life of S-metolachlor can be up to 142 d depending on the soil type and properties, and the prevailing environmental conditions (Frank et al 1991;Huang et al 2017;Martínez et al 1997;Mueller et al 1999;Shaner 2014;Shaner and Henry 2007;Westra et al 2014;Wu et al 2015). Currently, no information is available on field dissipation and persistence of S-metolachlor on organic and mineral soils used for sugarcane cultivation in Florida.…”