Influence of innovative agricultural practices on reducing pesticide and nitrate transfer in maize monoculture

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Inglés. Maize represents a third of the cultivated area in Ariège, southwestern France. Controlling pests requires the use of pesticides that are dispersed in soils and natural waters. However, some agricultural practices may modify their transfer into the environment: reduced tillage, organic amendment, pesticide application management. The objective of this study was to evaluate the influence of these innovative agricultural practices on the pesticide and nitrate transfer in irrigated maize monoculture by studying the soil solution for four consecutive years. On each study site, a reference modality was compared to: (1) a reduced tillage modality; (2) a reduced tillage modality with addition of organic matter; and (3) a localized pesticide application modality (i.e., herbicide application only on the maize rows to reduce the dose applied). In total, nitrates and 32 pesticides (including 16 herbicides, 3 insecticides, and 6 fungicides) were analyzed in soil solutions at 50 cm soil depth collected by suction cups. Pesticide concentrations showed a high variability within each molecule (with an average factor of 4000) and between the molecules measured (with an average factor of 1000). The highest concentrations (median concentration >1 μg L-1 and maximum of 2331 μg L-1) was observed for S-metolachlor and its metabolites. The results did not show significant differences between the modalities in the experimental conditions tested. Using partial least square regression and decision trees, environmental variables such as organic carbon content, soil pH, and soil water stock, however, were dominant in controlling the transfer of pesticides by limiting or promoting their presence in the soil solution. The temporal factor was also a key factor, favoring the dissipation and/or degradation of molecules (in particular for S-metolachlor, glyphosate, and azoxystrobin), while the concentrations observed during the fallow period were relatively higher than those measured at the end of the previous culture. The trends observed were, however, largely dependent on the physicochemical properties of the molecules studied, controlling their dynamics in the environment.