Effects of pesticides on soil bacterial, fungal and protist communities, soil functions and crop quality in vineyards

Abstract:1. Pesticides can have unwanted effects on non-target organisms and thereby change community composition. Such changes might be particularly important in soil microbial communities which drive many ecosystem functions, and the quality of crops. 2. Over three study years, the community composition of microorganisms was affected by herbicide application but a loss of operational taxonomic units (OTUs) was only observed in fungi and protists. By contrast, soil copper contamination reduced OTU richness of bacteria and protists in some years but had no significant effect on fungal richness. Copper also changed the community composition in all three groups of soil microorganisms. While we found no effect of copper on ecosystem functions, herbicide application reduced microbial respiration and biomass by about 39% and 45% respectively. However, decomposition rates remained virtually unchanged by any pesticide. In 40% of the vineyards without herbicide application yeast assimilable nitrogen (YAN) levels in grape must were below the critical threshold of 140 mg/L, whereas in vineyards with herbicide application it was only 20%. No other grape quality parameter was affected by either herbicide application or copper contamination. 3. Synthesis and applications. Application of pesticides led to changes in richness and composition of soil microbial communities and directly reduced some ecosystem functions (microbial biomass and respiration), but not all (decomposition). Grape quality parameters can indirectly be enhanced by pesticide application, highlighting the trade-off between the interests of nature conservation and the interests of the farmer, in terms of crop quality. Balancing these two diverging interests requires the establishment of alternative vineyard management allowing reduced pesticide application.