Adverse impacts of Roundup on soil bacteria, soil chemistry and mycorrhizal fungi during restoration of a Colorado grassland

Glyphosate is a widely used herbicide in agricultural, domestic, and restoration settings to manage weeds and invasive plants and is the active ingredient in the herbicide formulation Roundup. Concurrently with its drastic increase in usage, concern over indirect ecosystem effects and effects on non-target species has grown. In restoration, glyphosate is often used to remove invasive plants so native plants may be re-introduced. However, successful reintroductions require soils and microbial communities that support native plant growth, and it is critical that glyphosate applications do not harm soil microbes such as mycorrhizal fungi. Despite previous studies investigating the effects of glyphosate on soils and microbial communities, comprehensive field experiments combining soil chemistry and next generation sequencing technologies to describe both bacterial and eukaryotic responses to glyphosate are limited, especially in the contexts of ecosystem restoration and soil health. We studied the effects of the glyphosate-based herbicide Roundup Promax at frequencies of 0, 2, 4, and 5 applications over the course of 12 months on soil biotic and abiotic soil health indicators in a Colorado prairie dominated by the invasive cool-season grass Bromus inermis. Here we report cascading effects on soil chemistry, with increases in nitrate and acidity and consequent decreases in calcium content and cation exchange capacity. Bacterial and archaeal communities were more affected by Roundup Promax than eukaryotic communities, with decreases in phylogenetic diversity and changes in community structure following Roundup Promax applications, particularly after five applications. More critically, the colonization of plant roots by arbuscular mycorrhizal fungi decreased significantly in plots receiving even just two applications of Roundup Promax, and dark septate endophytes decreased after four applications. Our work shows that Roundup Promax had multiple negative effects on soil biota in this field study due to either direct effects or indirect effects mediated through plant removal. Our results suggest that repeated herbicide applications are especially damaging to soil health and microbe-plant associations. These effects in turn could severely hamper the ability of native plants to establish during ecosystem restoration projects.