Bioavailability-based estimation of phytotoxicity of imazaquin in soil to sorghum

The injury to sensitive rotational crops caused by residual herbicides from their applications has happened frequently. The assessment of activity of residual herbicide in soil has been of agronomic concern. To accurately determine the toxicity and obtain a comparable concentration that inhibits growth by 50% (IC50) in different soils, the concentrations of imazaquin in CaCl2 (CCaCl2) and H2O (CH2O) extraction and in in situ pore water (CPW) were adopted for the estimation of IC50 to sorghum. The IC50 values based on CCaCl2 and CH2O were 0.06 mg L(-1) in soil Ansai (AS) to 0.13 mg L(-1) in soil Huajiachi (HJC), 0.32 mg L(-1) in soil AS to 0.71 mg L(-1) in soil HJC, respectively, with variation coefficients of 38.32 and 34.93%. However, CPW-based IC50 values ranged from 0.90 mg L(-1) in soil Xiaoshan (XS) to 1.09 mg L(-1) in soil HJC with a variation coefficient of 6.96%. This implies that the IC50 based on imazaquin concentration in in situ pore water is almost identical in the five soils. With further studies with more herbicides and plant species, this method might be expanded for the estimation of phytotoxicity of residual herbicide(s) to plant(s).