Glyphosate and terbuthylazine effects on soil functions, microbiome composition and crop performance

Herbicides are widely used for weed control in agriculture, though their fate and impact on non-target organisms like soil microbes and their function remain relatively unknown. A further complication is that herbicide effects vary depending on how they are applied and due to varying soil moisture conditions. In this study we tested the hypothesis that spraying glyphosate or terbuthylazine directly onto bare soil and when soil moisture is high would impact the soil microbial communities and their function most strongly. We measured similar amounts of glyphosate and terbuthylazine in soil whether the herbicides were directly applied to soil or first sprayed on the weed Chenopodium album and we found evidence for more rapid metabolization at high soil moisture. We found that the soil bacterial rather than the fungal community was mainly affected by a single application of the two tested herbicides. The identified shifts in community composition were independent of the modes of herbicide application but strongly dependent on soil moisture. We further found that herbicide applications only had a small impact on soil microbial function, which was approximated with analyses of the activities of N-13-acetylglucosaminidase, acid phosphatase and 13-glucosidase enzymes in soil. Finally, we also assessed the postapplication performance of the subsequent crop and found that the herbicides did not affect maize height, chlorophyll content and biomass. Overall, our study revealed that a single application of herbicides in recommended doses had minor effects on the soil microbiome with a temporal and soil moisture dependency. The latter finding points out that to avoid repercussions on non-target organisms and soil function, key research needs to solve the context-dependency of rapid herbicide degradation in soil.