Influence of Phosphate and Copper on Reductive Dechlorination of Thiobencarb in California Rice Field Soils

Abstract

The potential for reductive dechlorination of the herbicide thiobencarb (TB) by microbes and its prevention in saturated anaerobic rice field soils was examined in laboratory microcosms. TB is effective in controlling both annual grasses and broadleaf weeds. In anoxic microcosms, TB was effectively degraded within 30 days to its dechlorinated product, deschlorothiobencarb (DTB), in two Sacramento Valley rice field soils. TB dechlorination, and subsequent degradation, followed pseudo-zero- (lag phase) and first-order (degradation phase) kinetics. Logistic regression analysis (r2 > 0.841) produced a half-life (t(1/2)) in nonsterile soils ranging from 10 to 15 days, which was also observed when microcosms were amended with low concentrations (<3 mg L(-1)) of copper (Cu2+; as the fungicides Cu(OH)2 and CuSO4.5H2O). High Cu2+ concentrations (>40 mg L(-1)) were added to the microcosms to determine if copper toxicity to dechlorinating microbes is concentration dependent within the range used. After 30 days, the low-copper-amended soils closely resembled the nonsterile experiments to which no Cu2+ was added while the high-copper-amended microcosms were similar to the sterile experiment. Microcosms were also separately amended with 5.7 g L(-1) phosphate (PO4(2-); as KH2PO4), a nutrient regularly applied to rice fields. Phosphate-amended experiments also showed TB degradation, but no DTB formation, indicating the phosphate played a role, possibly as a microbial inhibitor or an alternative electron acceptor, in limiting the dechlorination of TB. In summary, TB dechlorination was inhibited at high Cu(OH)2, CuSO4.5H2O, and KH2PO4 concentrations.

Microbial Degradation of Clomazone under Simulated California Rice Field Conditions

“…Some clomazone degradation and compound 2 formation were observed in anaerobic samples after 19 days, possibly due to the stimulation of spores resistant to autoclaving, which has been previously documented in rice field soil , . To illustrate the potential impact this may have on first-order kinetic calculations, two regression lines are plotted in Figure : clomazone degradation through (A) 19 days (no observed parent degradation) and (B) 38 days (equilibrium time).…”

Photolytic versus microbial degradation of clomazone in a flooded California rice field soil

“…A pilot study, where soil was subjected to (a) triple autoclaving, (b) microwave irradiation and (c) sodium azide amendment, revealed that autoclaving was the most effective inhibiter of clomazone degradation, while the other two methods had little effect (data not shown). Two previous studies6, 16 found anaerobic spore rejuvenation in autoclaved flooded soil microcosms to cause degradation of clomazone and thiobencarb, which confounded the controlling effects of sterilization. However, the former found autoclaving to be effective over 28 days, which is twice the length of the regulatory rice field water holding time in the Sacramento Valley, and is sufficient time to test whether the compound will degrade significantly via photolysis while the fields are flooded.…”

Biodegradation of Clomazone in a California Rice Field Soil: Carbon Allocation and Community Effects

“…Since most clomazone-C under both soil conditions was recovered in DOC or respired CO 2 , and not in PLFA, this suggests the compound is degraded by cometabolic processes. As with thiobencarb, another popular chloroaromatic herbicide, amendment with organic carbon, phosphate, and copper are expected to alter the dynamics of clomazone biodegradation.…”