Evaluation of the effects of a diflufenican-containing combination herbicide on microbial activities in soil

Under laboratory conditions two standard soils with or without lucerne meal, which were qualified for microbiological-ecotoxicologicaI studies, were treated within several subtrials with the herbicide 'Fenikan' (diflufenican + isorpoturon), the test compounds 'RPA 41670 H' (diflufenican) and 'Arelon flussig' (isoproturon) as well as with the reference compound 'Herbogil liquide D' (dinoterb). The amounts mixed into the soils resulted from the concentrations reached, by simulated penetration of the area-related dosages into upper soil layers, where 5 cm depth represents the single dosage. To allow dose-effect relationships dosages up to 100 fold were used. As compared to the biocidal reference compound the single and 10-fold dosages of the test compounds produced only weak effects on the biomass-related dehydrogenase activity (DHA), on the substrate-induced short-term respiration (KZA) and on the carbon mineralization (LZA). Net N mineralization, however, was dose-related increased by 'Fenikan'. By using dose-effect relations an inhibition of the DHA occurred with 'Fenikan', but to a lesser extent with RPA 41670 H'. When looking to the respiration curve of the KZA during 48 hrs incubation some herbicide effects became visible which were not detectable within the 'normal' 6 hrs CO2 measurement. By comparing microbial activities in soil with and without lucerne meal (= LIA evaluation) which allowed the elimination of additional effects of introduced nutrients by the compounds, for the DHA some dose-related inhibitions occurred with the herbicide 'Fenikan' as well as with 'Arclon flussig'. Furthermore the relationship of carbon and nitrogen mineralization showed a shift towards nitrogen by 'Fenikan' and sometimes by 'RPA 41670 H'. It seams that several factors (e.g. soil) can hamper identification of dose-related effects, especially when the test system is limited. Overall 'Fenikan' as well as the herbicides with only one of its active ingredients exerted relatively weak effects, although some clearly detectable influences occurred. Obviously the active ingredient isoproturon is mainly involved in these effects.