RIMSULFURON IN SOIL - EFFECT OF PERSISTENCE ON GROWTH AND ACTIVITY OFMICROBIAL 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 r elate changes of soil microbial biomass-C content and global hydrolyti c activity to the rimsulfuron persistence under different conditions o f temperature and soil humidity. The results showed that rimsulfuron p ersistence depended significantly on temperature, while it remained al most 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 rims ulfuron persistence under different environmental conditions. On compa ring treated soils with untreated soil samples, decreases in the micro bial biomass-C content and increases in the global hydrolytic activity were found to be connected with rimsulfuron persistence at the variou s experimental conditions. These effects persisted for a short time an d, they were evident earlier at higher temperature and more persistent at lower humidity. This behaviour is discussed in terms of rimsulfuro n toxicity, with the consequent release of endocellular hydrolytic enz ymes from the dead microorganisms. An equation was derived to calculat e the microbial biomass-C content in response to the variation of rims ulfuron persistence.