Reduced fluridone efficacy in soil : a possible case for reversible microbial inactivation

Abstract A reduction in efficacy of the persistent herbicide fluridone has been observed in soils with fluridone application history (FH soils). Fluridone activity, as monitored by bioassay, persisted in FH soils for ca 1 week, whereas in fluridone non-history (FNH) soils phytotoxicity could be observed even after 3 months of exposure. Incorporation of small volumes of FH soil were sufficient to confer rapid loss of herbicidal activity on larger, autoclaved FNH soil volumes. In FH soils exposed to intensive antimicrobial treatments the activity of fluridone was restored to levels similar to those observed in FNH soils. Autoclaving and to a lesser extent, other disinfestation or antimicrobial measures (γ irradiation, methyl bromide, several fungicides) used to treat FH soils, exposed test plants to phytotoxic compounds even though the herbicide was not re-applied to the soil. Fluridone adsorption coefficients ( K a ) in FH and FNH soils were similar, suggesting that physical adsorption of the herbicide does not play a key role in the observed inactivation of fluridone in FH soil. The possibility of reversible microbial inactivation of fluridone is discussed.