The influence of glyphosate on soil CO2 respiration and microbial communities structure in histosol and fluvisol

Carbon dioxide is the most important among all greenhouse gases and microbial respiration is one of the fundamental soil processes, that contribute to atmospheric CO₂ pool. Many factors influence the respiration of soil microbiota. With regard to agricultural soils, the most discussed are the type of cultivation and fertilisation. Much less attention is paid to herbicides, which are widely used and comprise a frequent contaminant of the soil environment. The most commonly used herbicide worldwide is glyphosate (GFP). In 2018, annual  GFP consumption exceeded 8.25 x 10⁸ kg and is still increasing. GFP is a foliar herbicide that is used in agriculture in the form of commercial formulations (GFC). Due to imprecise dosing, leaf washoff or with plant necromass, GFC reaches the soil environment where can potentially modulate activity and abundance of soil microorganisms. Possible modes of action include toxicity (e.g. due to inhibition of the shikimate pathway) or stimulation due to the fact that GFP is biodegradable and can potentially be a source of all major biogenic elements C, N and P. Due to the widespread use of GFC in agriculture, it may be an important driver of soil-related CO₂ emissions, but knowledge in this area is still fragmentary.  

In the current study, a wide range of GFP and GFC (0 – 10 000µg g^-1) doses were used to determine its effect on microbial respiration in two agriculturally used soils (histosol and fluvisol). In parallel, the qualitative and quantitative composition of the soil microbial community was studied. Pure glyphosate (GFP) and a commercial formulation (GFC) containing adjuvants in addition to glyphosate were tested.

For both soils, the exposure to GFP and GFC resulted in an increase in microbial respiration. However, this effect was greater for GFC, indicating the important role of adjuvants in shaping the environmental effect of the herbicides used. In accordance with the respiration results, a significant increase in the total bacteria count was found in both studied soils. Qualitatively, communities' structures were not significantly transformed, even under the influence of high doses of the tested preparations.