Soil management affects copper and zinc export in runoff from Northern vineyard plots (Rouffach, Alsace, France)

Zinc-based and Cu-based fungicides are widely used in European vineyards to prevent fungal diseases. The soil management and rainfall characteristics influence the runoff export of copper (Cu) and zinc (Zn) from vineyard plots, although quantitative field studies are mostly missing. The runoff exports of Zn and Cu from Northern vineyard plots (Rouffach, Alsace, calcareous-loamy soils with pH = 8.0) under conventional and alternative soil management were compared during two contrasting vine-growing seasons. The conventional management included standard soil maintenance with chemical weeding, whereas the alternative management involved the conversion to organic farming with a mechanical soil management. Cu in top-soil, i.e. the first 5 cm, reached 103.6 ± 19.1 and 70.8 ± 7.8 kg Cu ha^-1 for conventional and alternative plots, respectively, mirroring accumulation since decades. Zn in top-soil reached 70.8 ± 16.4 and 63.9 ± 7.8 kg Cu ha^-1 for conventional and alternative plots, respectively. No Zn applications on the conventional and alternative plots were recorded during the two growing seasons. Overall, our results emphasize that destructuring of the surface soil layer and tillage preceding a storm event largely influences water flows and exports of both dissolved and solid-bound Cu and Zn. Plowing work on the organic plot a few days before a most intense storm event resulted in significant mass export, accounting for 99% and 95% of the total mass exported during the vine-growing season for Cu and Zn, respectively. However, grass cover on one out of two inter-rows limited runoff volumes to a maximum runoff coefficient of up to 1.4% over the two vine-growing seasons. The seasonal export of Cu and Zn occurred mainly by surface runoff as the monthly water storage of the soil was lower than the water holding capacity, thereby limiting vertical flows in both management modes. The mass export of solid-bound Cu and Zn contributed to more than 95% of total export of both Cu and Zn from the vineyard plots. The seasonal Cu and Zn exports ranged from 0.001 to 0.05% of historical Cu and Zn in the top-soil, raising the issue of both Cu and Zn accumulation in vineyard soils. Altogether, this study underscores that soil mechanical management preceding a storm event largely affects Cu and Zn export, although Cu and Zn fluxes in runoff from the alternative management are globally lower.