Effects of glyphosate application and nitrogen fertilization on the soil and the consequences on aboveground and belowground interactions

The application of nitrogen (N) and herbicides are commonly used to fertilize crops and protect them against weed development, but are also considered as soil and environment pollutants. Even so, the individual and combined non-target effects of N fertilizers and herbicides on multitrophic interactions within agrosystems are not well known. From soil samples collected in the field, we examined the effects of the direct application of glyphosate and/or N fertilization on microbial activities and soil nutrient status. In addition, we investigated the increase in biomass and, nutrient acquisition of the bean (Phaseolus vulgaris) and the consequences of the applications of N and glyphosate on the performance of the herbivore aphid (Aphis fabae). From soils that did (N+) or did not receive (N0) synthetic N fertilization over a 6-year period, we assessed the effects of glyphosate (CK, without glyphosate; FR, field rate of glyphosate) and N fertilization (N+, with N fertilization; N0, without N fertilization) applications in a mesocosm experiment for 75-days. Following the 75 day treatment, the biological and physiological consequences, both belowground and aboveground were determined. The growth of arbuscular mycorrhizal fungi (AMF) and dehydrogenase activity, were negatively affected following N+ fertilization and the application of the FR of glyphosate, while in the absence of glyphosate, alkaline phosphatase (AIP) activity was reduced. Functional microbial responses were unaffected by both N and glyphosate, even when applied in combination. Conversely, the N fertilization significantly increased the nitrate content (NO3−) in the CK soils and the total N in the FR soils, compared to CK/N0 and FR/N0 soils. The combined effects of glyphosate and nitrogen fertilization (FR/N+) significantly decreased the soil C:N ratio, but significantly increased nitrification compared to CK/N0 and FR/N0 soils. The FR/N+ treatments positively affected plant performance, improving the total chlorophyll, sucrose, ammonium, amino acid content, and pod biomass, compared to the CK/N0 and FR/N0 soils. Unlike glyphosate, which did not appear to exert an effect when applied alone or in combination, N fertilization significantly increased aphid nymph survival. The non-metric multidimensional scale allowed us to establish belowground and aboveground interactions with glyphosate and N fertilization. We conclude that glyphosate and N fertilization have negative effects on soil microflora and potential pests, but do not necessarily affect belowground and aboveground interactions, and may offer equal or superior benefits to crop productivity.