Repeated annual glyphosate applications may impair beneficial soil microorganisms in temperate grassland

Due to the worldwide use of the herbicide glyphosate, there is a growing interest in understanding its impact on beneficial soilmicroorganisms. However, most studies have been focused on evaluating the effects on these microorganisms of a single application in agricultural crops, despite the fact that repeated applications is a common scenario in different production systems. We evaluated the impact of four annual glyphosate applications on arbuscular mycorrhizal fungi (AMF), dark septate endophytes (DSE) and free-living diazotrophs in a temperate grassland. Sub-lethal (0.8 l ha−1) and recommended field doses (3 l ha−1) were analyzed. AMF viable spores and free-living diazotrophs densities were reduced by 56% and 82% respectively, after the fourth application even at sublethal dose. While total AMF root colonization in Lolium arundinaceum was not affected among treatments, arbuscules percentage was reduced in plants grown in plots treated with 3 l ha−1. A similar response was detected in DSE root colonization. Considering the role they have in structuring plant communities, these deleterious effects on beneficial soil microorganisms might negatively impact on grassland productivity and diversity. It is necessary to investigate the resilience of the microbial community in order to develop a long-term strategic management of glyphosate applications that would achieve the desired objectives without irreversibly affecting soil biota.