Glyphosate-based herbicides alter soil carbon and phosphorus dynamics and microbial activity

Abstract

Glyphosate is among the most used herbicides worldwide. However, both the active ingredient and additives of the commercial formulations may adversely affect chemical and biological processes in soil. Therefore, the aim of this work was to evaluate the effect of glyphosate and the commercial formulation (CH) on soil C, N and P dynamics, and microbial enzyme activity in two soil management conditions: a nopal plot (NP) with an history of 5 years of glyphosate application, and in an abandoned plot (AP) with a history of previous agricultural management without glyphosate applications. We performed three experiments: a) glyphosate and CH effects on phosphorus sorption by performing a non-biological P fixation experiment; b) glyphosate and CH effects on microbial activity, C, N and P dynamic, and the biodiversity of soil microorganisms in terms of biomarker fatty acids after a 27 day incubation experiment; and c) glyphosate and CH degradation was tested isolating bacteria from the incubated soil samples and growing in a defined media with potassium phosphate, glyphosate or CH as sole source of P. Our results suggested that other compounds from CH may be affecting P sorption, because when the herbicide was applied, P sorption was reduced in the NP, but in AP soil no significant effects were observed. In contrast, in the AP soil both glyphosate and the CH reduced C mineralization. Besides, CH application reduced Pmic concentration and laccase activity, but increased the specific activity of phosphonatase and phosphodiesterase in the AP soil. Additionally, the microbial community in NP soil revealed higher diversity index than the AP soil, but major changes in microbial groups were shown only when CH was applied. In vitro assay with native bacterial isolates confirmed their susceptibility to the commercial herbicide in both NP and AP soils. In conclusion, our results demonstrated changes on some processes related to soil C and P dynamics when pure glyphosate and a commercial glyphosate were applied, along with changes in microbial activity and community structure, being stronger the effects with the commercial formulation used. Further research should be done for test the effects of glyphosate on microbial stoichiometry and enzyme stoichiometry.