Analysis of imazethapyr in agricultural soils by ion exchange membranes and a canola bioassay

Abstract

Because imazethapyr residues in soils may cause plant injury to certain rotational crops, sensitive, and reliable methods for imazethapyr monitoring in soils are needed. In this study, imazethapyr analysis was investigated using two newly developed procedures: an anion exchange membrane extraction followed by an HPLC‐UV detection and a canola bioassay. Nine soils in which no previous application of imazethapyr had been made were collected from farm fields in Saskatchewan, Canada. Soils were spiked to yield imazethapyr concentrations in the range of 0–80 μg kg^‐1 dry soil and were subjected to analysis by the above procedures. In the anion exchange membrane extraction, spiked soils were shaken with the membrane strips; imazethapyr was then eluted from the membranes with a potassium chloride (KC1) solution, partitioned into dichloromethane and injected into the HPLC. This method allowed for the extraction of the ionized portion of imazethapyr from soils. In a laboratory bioassay, pre‐germinated canola seeds were planted in spiked soils and after five days of growth root and shoot growth inhibition was determined. The results of both methods were dependent on soil type. Generally, soils from depressions in the landscape yielded low imazethapyr recovery by anion exchange membrane extraction; these soils also showed low degree of imazethapyr phytotoxicity to canola growth. After imazethapyr field spraying, soils were sampled from the field at different time intervals for up to one year and analyzed in the laboratory by the above methods; also, after one year, a field bioassay was performed. Using the membrane extraction method, imazethapyr was detected only in field samples collected one week after spraying. The membrane extraction method, although very simple and cost‐efficient lacks sensitivity needed for the imazethapyr monitoring at low concentrations in agricultural soils. The 5‐day canola bioassay (root growth inhibition method) was more sensitive than the membrane extraction and showed imazethapyr presence in all field samples. However, because crop growth inhibition was more severe in the field than in the laboratory, a field bioassay may be the most reliable means to assess injury potential for certain sensitive rotational crops under field conditions.