Mechanism of the effect of pH and biochar on the phytotoxicity of the weak acid herbicides imazethapyr and 2,4-D in soil to rice (Oryza sativa) and estimation by chemical methods

The existing form of an ionizable organic compound can simultaneously affect its soil adsorption and plant bioactivity. In this experiment, the adsorption and bioactivity of two weak acid herbicides (WAHs), imazethapyr and 2,4-D, were studied to explore the predominant mechanism by which the soil pH and the addition of biochar can influence the phytotoxicity of WAHs in soil. Then, the WAH concentration extracted by hollow fiber-based liquid-phase microextraction (CHF-LPME), the in situ pore water concentration (CIPW) and the added concentration (CAC) were employed to estimate the phytotoxicity. The results showed that with increased pH from 5.5 to 8.5, the phytotoxicity of the WAHs to rice increased about 1-fold in the soil, but decreased in aqueous solutions, the IC50 values for imazethapyr and 2,4-D at pH 5.0 were 3- and 2-fold higher than that at pH 8.0. In addition, the soil adsorption decreased, indicating that the adsorption process was the dominant factor for the variation of the phytotoxicity of the WAHs in the tested soil instead of the decreasing bioactivity. The concentration that inhibits plant growth by 50% (IC50) calculated by the CAC in different pH and biochar soils ranged from 0.619 to 3.826 mg/kg for imazethapyr and 1.871-72.83 mg/kg for 2,4-D. The coefficient of variation (CV) of the IC50 values reached 65.61% for imazethapyr and 130.0% for 2,4-D. However, when IC50 was calculated by CIPW and CHF-LPME, the CVs of the IC50 values decreased to 23.51% and 36.23% for imazethapyr and 40.21% and 50.93% for 2,4-D, respectively. These results suggested that CIPW and CHF-LPME may be more appropriate than CAC for estimating the phytotoxicity of WAHs.