Riparian vegetation strips (RVS) reduce surface runoff volume and retain sediments, pesticides and nutrients that are transported across them from adjacent crop-fields (CF). The ability of these strips to retain glyphosate has been demonstrated using experimental plots, but the spatial variability of that process is unknown. In this work, the influence of microtopography inside the RVS on the retention of glyphosate (and its major metabolite, AMPA), phosphorus and nitrogen were analyzed within a RVS of agricultural landscapes. Retention levels inside and outside preferential flow pathways (PFP) were compared under presence and absence of a tree stratum. Soil glyphosate + AMPA concentration within PFP was 88-fold higher than outside. Phosphorus and nitrogen soil concentrations, clay and bulk density were also higher inside than outside the PFP. The tree stratum did not modify soil concentration of glyphosate + AMPA, phosphorus, nitrogen, clay content, nor the morphometry of the PFP. Bulk density and clay content recorded in adjacent CF and in PFP, in addition to the high glyphosate, phosphorus and nitrogen concentrations in PFP soil, are consistent with a hydraulic connection between the CF and the PFP. These results contrast with some conclusions obtained from experimental studies under uniform plots and emphasize the importance of taking into account the genesis and structure of PFP in the design, evaluation and management of the filtering function of RVS.

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Background: The use of glyphosate on glyphosate-tolerant soybean crops led to improved control of a wide range of weeds, which resulted in reduced costs with the no-till system. The emergence of the first herbicide-resistant weeds have driven an increase in glyphosate applications, and even though those soybean materials have a low sensitivity to glyphosate, the rhizobial symbionts could be affected by the herbicide, and plants might be indirectly injured.Objective: This study aimed to determine the effect of multiple glyphosate applications throughout the soybean crop cycle on plant growth, nodulation and biological nitrogen fixation (BNF). Methods: The effects of one, two and three treatments of a recommended dose of glyphosate on BNF and growth of glyphosate-tolerant soybean plants were evaluated in greenhouse and field experiments.Results: Two or more applications of glyphosate inhibited the BNF and growth of soybean plants. Under controlled conditions, at least one glyphosate application at V1 affected the number and mass of nodules per plant, and successive applications in advanced phonological stages resulted in the inhibition of nodule growth. With two and three sprayings of glyphosate, the proportion of N derived from the air in plants was reduced by 41% compared with the treatment without glyphosate. In field experiments, detrimental effects of three sequential applications of glyphosate on number of nodules per plant (-25%), biomass production (-21%) and grain yield (-36%) were detected. Conclusions: Multiple glyphosate applications of glyphosate inhibited the BNF and growth of soybean plants and it could be as damaging as +weed interference. Nomenclature: Glyphosate; soybean, Glycine max (L.

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