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

Abstract

Environmental contextThe 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. AbstractBenzobicyclon 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=0.25–44.3mLg−1), soil organic carbon partitioning (KOC=28.2–7480mLg−1), and soil organic matter partitioning (KOM=17.9–2580mLg−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.

Migration behavior of benzobicyclon hydrolysate and associated influencing factors in different agricultural soils

“…Previous studies have shown that BH has high hydrolytic stability and solubility in water, with a half-life greater than 1 year (25 °C), while the solubility of BH increases with increasing solution pH (Williams et al 2017). Furthermore, different agricultural soils have different BH adsorption values (Willett et al 2020). These physical properties might lead to the BH leaching into groundwater or other water bodies, posing a huge environmental risk.…”

“…These physical properties might lead to the BH leaching into groundwater or other water bodies, posing a huge environmental risk. The adsorption and desorption behavior of BH in soil has been reported (Willett et al 2020). However, its leaching and migration in soil and associated influencing factors have not been explored.…”