Glyphosate retention in grassland riparian areas is reduced by the invasion of exotic trees.

Abstract

In this study, we examined some aspects regarding the effect of willow trees (Salix fragilis L.) invasion of grassland riparian environments in the Argentinean Pampas on the runoff reduction, sedimentation and glyphosate retention in the riparian vegetationstrip (RVS). To assess the influence of willows on the filtering mechanisms, we performed runoff simulation experiments in plots of 1.5 x2.5 m, in coastal environments characterized by the presence of willowsor the lack of trees. Despite the short length of the experimentalplots, the retention of glyphosate in the controls, with no trees, washigher and reached almost 74%. Nevertheless, sediment retention did not differ significantly between the tree areas and the grassycontrols. The runoff reduction in plots with willows was of 63%. The presence of willow trees significantly altered the measured biophysical properties, such as soil moisture and aboveground biomass, compared to areas without trees. Analysis of partial correlations forenvironments with and without trees showed that the reduction inrunoff volume increased significantly with the soil sand content and the groundwater table depth, while it decreased with bulk density, soil moisture and the riparian slope. Sediment retention increased significantly with aboveground biomass, litter and root biomassand decreased with the riparian slope. In turn, glyphosate retention increased significantly with sediment retention and decreased withthe riparian slope and litter biomass. The mechanisms involving the effect of willows could not be well explained. Due to the increased intensification of agriculture, treeless RVS are important to reduce glyphosate concentration in streams and their sinks. Nevertheless, the presence of trees is also important in the context of agroecosystemsand agricultural landscapes, as they contribute to reduce the runoff flow.