DEGRADATION OF ISOPROTURON IN SOIL IN RELATION TO CHANGES OF MICROBIAL BIOMASS AND ACTIVITY IN SMALL-SCALE LABORATORY AND OUTDOOR STUDIES

The soil degradation of isoproturon under standardized laboratory cond itions was compared to that carried out in an outdoor experiment using lysimeters. After application of C-14-labelled formulated isoproturon (1.5 kg A.I./ha), radiochemical analysis, as well as microbial invest igations, were performed to relate changes in soil microbial biomass t o its capability of degrading isoproturon. The results showed that the microbial biomass, as well as its dehydrogenase activity, varied unde r field conditions due to fluctuations of temperature and soil moistur e. In the laboratory experiment the microbial biomass decreased during the 100 day experimental period, this reduction being the result of t he experimental conditions. Consequently, the herbicide was degraded m ore quickly in the outdoor experiment where leaching, formation of (CO 2)-C-14, uptake by plants, and photolysis also took place, than in the laboratory experiment. Further microbiological investigation in the l aboratory experiment showed that specific populations known to be resp onsible for the metabolic degradation of the compound were enhanced in the isoproturon-treated soil.