Microbial and Photolytic Degradation of Clomazone Under Simulated California Rice Field Conditions.

摘要

This investigation was designed to determine the environmental fate processes which contribute to the overall dissipation of clomazone, a popular herbicide used on California rice fields to control aquatic weeds. This is important for determining strategies to better manage the agent, both from a farmer's and regulator's perspective. The compound's physicochemical characteristics indicate it will persist primarily in the water column, where microbial and/or photolytic degradation may drive its environmental fate. The experiments performed herein are (i) quantification of microbial degradation rates and biotransformation products formed under aerobic and anaerobic conditions, (ii) turnover in aerobic and anaerobic soil and the effects of clomazone addition on microbial communities via the addition of 13C-labelled tracer, and (iii) quantification of photolytic degradation rates and photoproducts formed by exposure to natural and artificial light.;Clomazone microbial degradation rates and the metabolic products were investigated via time-series samples that were extracted and analyzed by liquid chromatography tandem mass spectrometry (LC/MS/MS). Metabolic profiling revealed the following clomazone-derived transitions: m/z 240→125 (clomazone), m/z 242→125 (ring-open clomazone), m/z 256→125 (5-hydroxyclomazone), m/z 256→141 (aromatic hydroxyclomazone), m/z 268→125 (unknown metabolite) and m/z 272→141 (4'5-dihydroxyclomazone). Quantification results indicate an anaerobic half-life of 7.9 days, with ring-open clomazone reaching 67.4% of application at 38 d. Aerobic degradation occurred slower (t1/2 = 47.3 d), forming mostly soil-bound residues. Thus, under summer conditions, clomazone is likely to dissipate rapidly from fields via anaerobic degradation.;The allocation of clomazone to the main carbon fractions in a typical California rice field over time was investigated by addition of 13 C -- aromatic labeled clomazone to anaerobic (flooded soil) and aerobic (moist soil) sacrificial time-series soil microcosms. Samples were analyzed for concentration and 13C abundance of the following carbon fractions: CO2-C, CH4-C, bulk soil-C, chloroform fumigation extractable-C, dissolved organic-C, dissolved inorganic-C, clomazone-C, and ring-open clomazone-C. Phospholipid fatty acids (PLFA) were extracted and analyzed to depict effects of clomazone addition on changes in functional group (gram+, gram-, fungal, actinomycetes, and slow growth) profiles, and to implicate functional degraders. Results indicate clomazone will transform rapidly to ring-open clomazone, which will persist mostly in the dissolved organic phase under anaerobic conditions. Anaerobic mineralization accounted for 2.8% of applied clomazone over 90 days. Under aerobic conditions, 18.6% was found to mineralize over 140 days, with the remainder persisting in pore water and soil-bound residues. Actinomycete growth under aerobic conditions was significantly different between treatments and controls, implicating a possible relationship to clomazone degradation. Other biomarkers varied temporally, but not in response to clomazone addition; nor were the PLFA of treatments found significantly more enriched in 13C, despite significant mineralization observed in aerobic samples. Thus, aerobes may partition the agent into different cellular components than phospholipids. Overall, clomazone addition had little effect on the microbial profile, and cometabolism is thought to be the dominant degrading mechanism.;The photodegradation of clomazone under simulated California rice field conditions was investigated via time-series microcosms containing water, soil + water, and sterilized soil + water. Samples were amended with clomazone and exposed to natural and artificial sunlight over 35 days. Water and acetonitrile extracts were analyzed for clomazone and metabolites via LC/MS/MS. Results indicate direct and indirect photolytic rates to be very slow, with pseudo-first order degradation rate constants (k) calculated at 0.005+/-0.003> kwater>0 day-1, 0.005+/-0.003> ksterile>0 day-1, and 0.044+/-0.007> knon-sterile>0.010+/-0.002 day-1, depending on light type. The formation of ring-open clomazone, a microbial metabolite, correlated with clomazone degradation. This implies that direct photolysis is a minor component of overall degradation, and that California soils are void of photosensitizers capable of initiating indirect photolysis of the herbicide. Thus, microbial degradation is the primary contributor to the overall environmental fate of clomazone, and those desiring a faster photodegradtion of the compound may wish to investigate amending fields with a photosensitizing agent.