Aerobic versus Anaerobic Microbial Degradation of Clothianidin under Simulated California Rice Field Conditions

Microbial degradation of clothianidin was characterized under aerobic and anaerobic California rice field conditions. Rate constants (k) and half-lives (DT50) were determined for aerobic and anaerobic microcosms, and an enrichment experiment was performed at various nutrient conditions and pesticide concentrations. Temperature effects on anaerobic degradation rates were determined at 22 ± 2 and 35 ± 2 °C. Microbial growth was assessed in the presence of various pesticide concentrations, and distinct colonies were isolated and identified. Slow aerobic degradation was observed, but anaerobic degradation occurred rapidly at both 25 and 35 °C. Transformation rates and DT50 values in flooded soil at 35 ± 2 °C (k = −7.16 × 10–2 ± 3.08 × 10–3 day–1, DT50 = 9.7 days) were significantly faster than in 25 ± 2 °C microcosms (k= −2.45 × 10–2 ± 1.59 × 10–3 day–1, DT50 = 28.3 days). At the field scale, biodegradation of clothianidin will vary with extent of oxygenation.