Pesticide effects on the diversity and function of non-target soil microbes: Truths, lies and regulatory issues

Pesticides constitute an integral part of conventional farming. Considering the key role of soil microbes on ecosystem functioning, assessing the effects of pesticides on them is of prime importance. EU legislation regarding pesticide soil microbial toxicity relies on simple C and N mineralization tests which do not provide a reliable toxicity assessment. Pesticides nature and their concentrations in the environment are two factors which affect their soil microbial toxicity: Fungicides usually exhibit a more prominent effect, while pesticide agricultural uses result in relatively low soil exposure rates. So far, studies have focused on the impact of synthetic pesticides onto soil microbes at artificially high rates, while little is known regarding the soil ecotoxicity of biological pesticides. We have studied the impact of biological vs synthetic pesticides at realistic application rates using a multidisciplinary approach including i) biochemical tools to determine functional effects ii) culture-independent tools to determine effects on the diversity of soil microbes (PLFAs, DGGE, TRFLP) and iii) Pesticide dissipation measurements which reveal the level of pesticide residues in soil on a temporal basis. RNA-based methods allow the identification of effects which are not otherwise visible. Higher resolution molecular tools like SIP and -omics will provide insights on pesticide soil microbial toxicity. Overall, pesticides, when used at recommended doses, are unlikely to have a persistent effect on soil microbes. Based on the current knowledge, a new regulatory framework including the use of new tools is needed for a realistic estimation of the impact of pesticides on non-target soil microbes.