Effects of the herbicides metazachlor and dinoterb on the soil microflora and the degradation and sorption of metazachlor under different environmental conditions

Environmental conditions may modify pesticide effects on non-target soil mi cro-organisms due to their influence on pesticide persistence, bioavailabil ity and interactions with microbial metabolism. This study investigated the relationship between effects of the herbicide metazachlor and its degradat ion and availability in soils with different organic C contents (soil A=1.3 %, soil B=7.1%), incubated at 20 degreesC and 30 degreesC. Relative differe nces between dehydrogenase activity (DHA) in soil treated with metazachlor and untreated soil ranged from -16.6 to +18.9% with a greater impact for so il A, probably due to greater pesticide availability and potential for the microflora to recover from adverse effects. Relative effects on substrate-i nduced, short-term respiration (SIR, -11.8 to +6.0%) and N mineralisation ( N-min, -24.4 to +35.8%) were similar in both soils. Soil temperature did no t consistently influence the effects of metazachlor on DHA and SIR. N-min w as more strongly affected at 20 degreesC than at 30 degreesC, which was pot entially caused by larger pesticide concentrations at 20 degreesC or a smal ler potential for the microflora to reproduce destroyed biomass. Additional studies with the herbicide dinoterb showed stronger effects on DHA, SIR an d N-min than for metazachlor (-57.7 to +78.2%) and no clear influence of so il and temperature. Results showed that environmental factors may influence pesticide effects on micro-organisms. Clear relationships are not always f ound due to interactions between these factors, pesticide persistence, bioa vailability and microbial metabolism.