Effects of a Sequential Application of Plant Protection Products on Soil Microbes and Free-Living Nematodes in a Field Experiment

During crop growth cycle, often several different plant protection products (PPPs) are 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 prior to each application, and 7 and 28 days thereafter. The fungal community’s composition and abundance were found to be more susceptible than the bacterial community to PPP applications, while the bacterial was mainly driven by the in-field heterogeneity of soil properties. Transient effects of PPP applications were detected on nematode abundance. Higher tier ecotoxicological studies offer greater ecological relevance compared to the standard laboratory tests but are challenged by environmental variations that should be accounted for when evaluating the ecotoxicity of pesticides on soil organisms.