Repeated annual application of glyphosate reduces the abundance and alters the community structure of soil culturable pseudomonads in a temperate grassland

Abstract

Grasslands include some of the most endangered and less protected ecosystems on the planet, such as the Pampa of South America. In the Flooding Pampa where native grasslands were not replaced by crops, glyphosate is frequently sprayed in late-summer every year to increase winter forage productivity for cattle without knowing the impacts on both aboveground and belowground grassland genetic resources. Here, we evaluated the effect of this practice on the abundance and community structure of members of the genus Pseudomonas, one of the most diverse bacterial taxa of the plant rhizosphere broadly recognized by their benefits on plant growth and protection, and on soil remediation. In a field experiment, we analyzed the impact of a single application or of a repeated annual application during 4 consecutive years of the recommended dose of commercial glyphosate (Glacoxan®, 3 l ha-1 and 1440 g active ingredient ha-1). In a greenhouse experiment, we tested whether exposure of bare soil to either commercially formulated or pure glyphosate has a direct impact on the resident pseudomonas community in the absence of plants. In the field, we found that the repeated annual herbicide application resulted in a reduction in the abundance and a shift in the community structure of culturable pseudomonads, with evidence of an enrichment of glyphosate-metabolizing lineages. On the other hand, a single application of glyphosate to bare soil under greenhouse conditions did not affect the abundance nor the community structure of pseudomonads. Considering that these plant-beneficial bacteria are natural inhabitants of the rhizosphere and that their fitness is promoted by plant root exudates, the observed alterations are most likely associated with the drop in the vegetation cover directly provoked by the effect of glyphosate. For the first time, we report on the consequences that a reiterated glyphosate application has on a diverse soil reservoir of bacteria that contributes to crucial ecosystem functions in a temperate natural grassland under animal production. Our findings strengthen the notion that this agronomical practice has an unavoidable imprint on non-target microorganisms that are relevant components of the plant-soil feedbacks and that may be instrumental for ecosystem restoration. We conclude that it is necessary to develop sustainable management practices to avoid affecting key functional microorganisms and exploit their contributions to society.