Effects of a sequential application of plant protection products on soil microbes and free-living nematodes in a field experiment

During crop growth cycle, several different plant protection products (PPPs) are often applied in combination or sequentially. Such sequential applications result in unintentional mixtures of residues that may affect ecosystem services supported by non-target organisms such as soil microbes and nematodes. This scenario of sequential PPP application is frequent in agricultural practice but rarely addressed experimentally at field scale with regard to environmental impacts. The objective of this study was to evaluate the effect of individual and sequential application of three PPPs (the herbicide clopyralid, the insecticide zeta-cypermethrin, and the fungicide pyraclostrobin) on soil microbial communities, and on the abundance of free-living nematode. Single applications (at 1× or 10× the agronomical dose) were made to triplicated field plots with each one of the PPPs or all three PPPs in sequence, with untreated plots serving as controls. Plots were sampled before each application and 7 and 28 days thereafter. The composition and abundance of the fungal community were found to be more affected compared to the bacterial community by PPP applications, while the bacterial community structure was influenced mainly by soil properties. Only transient effects of PPP applications were detected on nematode abundance. Higher-tier ecotoxicological tests such as the present field study offer greater ecological relevance compared to laboratory tests but are challenged by environmental variations that should be accounted for when evaluating the ecotoxicity of pesticides on soil microorganisms.