Soil sorption characteristics of benzobicyclon hydrolysate and estimated leaching risk in soils used for rice production

Environmental context The behaviour of herbicides in the environment is largely determined by the partitioning of the compounds between soil solids and soil solution. We determined that the rice herbicide-metabolite benzobicyclon hydrolysate partitions more into soil solution, and does so increasingly as pH increases. These results indicate that benzobicyclon hydrolysate is a risk for leaching in much of the rice-producing area in the US mid-South. Abstract Benzobicyclon hydrolysate (BH) is the major metabolite and active molecule in the pro-herbicide benzobicyclon (BZB), which is pending registration for use in US mid-Southern rice (Oryza sativa L.) production. The current study objectives were to (i) determine BH soil sorption coefficients; (ii) quantify relationships among BH sorption and soil properties; and (iii) estimate leaching potential using calculated retardation factors (RFs). Sorption coefficients for 10 representative Arkansas rice-production soils were determined by batch-equilibration experiments. Soil sorption (KD = 0.25–44.3 mL g−1), soil organic carbon partitioning (KOC = 28.2–7480 mL g−1), and soil organic matter partitioning (KOM = 17.9–2580 mL g−1) coefficients were negatively correlated with soil pH (r = −0.93 – −0.94). Clay and silt were significant secondary regression parameters, accounting for up to 93 % of the variation in KD in combination with pH. Clay and silt effects on sorption coefficients increased when regression analyses excluded the lowest pH soil. Soil sorption coefficients were greater in soils with clay ≥27 %, which may be a useful parameter for informing herbicide-use rates. Using the calculated RF’s, the estimated depth of leaching over the growing season exceeded the assumed 15-cm plough layer depth in eight of the 10 soils, and only two of the 10 soils had an estimated time to plough layer breakthrough less than the typical six-month growing season (April–September) under average water flux conditions. The results suggest that BH leaching below the plough layer is a potential risk for much of the rice-producing area in the US mid-South.