Rimsulfuron in soil: Effect of persistence on growth and activity of microbial biomass at varying environmental conditions

The research was carried out to ascertain the effect of rimsulfuron, a solfonylurea herbicide, on soil microbial biomass growth and activity. Laboratory experiments were performed in a silty clay loam soil to relate changes of soil microbial biomass-C content and global hydrolytic activity to the rimsulfuron persistence under different conditions of temperature and soil humidity. The results showed that rimsulfuron persistence depended significantly on temperature, while it remained almost unchanged by humidity changes. A range of half-life values from 3.5 to 14.8 days was found in a temperature range from 10 degrees C to 25 degrees C, with lower half-lives at higher temperature. Persistence data were processed with the VARLEACH model, in order to predict rimsulfuron persistence under different environmental conditions. On comparing treated soils with untreated soil samples, decreases in the microbial biomass-C content and increases in the global hydrolytic activity were found to be connected with rimsulfuron persistence at the various experimental conditions. These effects persisted for a short time and, they were evident earlier at higher temperature and more persistent at lower humidity. This behaviour is discussed in terms of rimsulfuron toxicity, with the consequent release of endocellular hydrolytic enzymes from the dead microorganisms. An equation was derived to calculate the microbial biomass-C content in response to the variation of rimsulfuron persistence.