Impact of glyphosate on soil microbial biomass and respiration: A meta-analysis

The herbicide glyphosate is an important tool for weed management in many agricultural systems, but concerns have been raised that its increasing use impacts soil biology. At present, the influence of glyphosate on soil microbial biomass (SMB) and soil microbial respiration (SMR) is unclear, with inconsistent results across published studies. We hypothesised that differences in rates and formulation of herbicide application, presence or absence of plants, and variability in soil parameters such as pH and organic carbon (OC), may have contributed to the inconsistent results. To identify trends in the literature, we conducted a meta-analysis using linear mixed-effect and boosted regression tree models. Moderator variables included glyphosate concentration, soil pH, OC, planted or un-planted soils, field or pot experiments and time after glyphosate application. Glyphosate application, as well as moderator variables (pH, glyphosate concentration, OC and time after application) significantly affected microbial biomass and its activity. Increases in glyphosate and OC concentrations led to transitory enhancement (less than 60 days) of SMR and SMB, while respiration tended to be reduced after 60 days. Notably, field application rates (i.e. <10 mg kg−1) had no significant effect on SMR or SMB, but SMB was significantly lower at glyphosate concentrations of 10-100 mg kg−1. Ultimately, the fact that management and environmental factors regulated the soil microbial response means that generalisations about the toxicity or safety of glyphosate to SMR and SMB should be qualified with details of the conditions under which glyphosate is applied.