Microbial activity, community composition and degraders in the glyphosate-spiked soil are driven by glycine formation

Widely-used glyphosate may produce aminomethylphosphonic acid (AMPA), glycine and sarcosine. To date, little is known about effects of these degradation products on soil microorganisms and their potential degraders. Here, we incubated a soil spiked either with 2-¹³C-glyphosate, ¹³C-AMPA, ¹³C₃-sarcosine or ¹³C₂-glycine for 75 days. Respiration (CO_{2 tot}) and mineralization rates of the compound (¹³CO₂) were estimated in addition to phospholipid fatty acids (PLFAs_tot and ¹³C-PLFAs) as biomarkers to identify four groups of microorganisms (Gram-negative & Gram-positive bacteria, actinobacteria, fungi). 16S/ITS rRNA amplicon sequencing was also conducted to identify the microbial community at the phylum and genus level. The CO_{2 tot} and ¹³CO₂ rates were highest on day 2 in all treatments, as follows: glycine (CO_{2 tot}: 1.09 μmol g^-1; ¹³CO₂: 18 %) > sarcosine (CO_{2 tot}: 0.89 μmol g^-1; ¹³CO₂: 8.5 %) > glyphosate (CO_{2 tot}: 0.67 μmol g^-1; ¹³CO₂: 2.2 %) > AMPA (CO_{2 tot}: 0.53 μmol g^-1; ¹³CO₂: 0.3 %). Both the PLFAs_tot and ¹³C-PLFAs were highest in glycine (PLFAs_tot: 0.054-0.047 μmol g^-1; ¹³C-PLFAs: 0.2-0.4 %) and glyphosate (PLFAs_tot: 0.049-0.047 μmol g^-1; ¹³C-PLFAs: 0.1-0.3 %) treatments compared to sarcosine and AMPA treatments. Gram negative bacteria were major microbial group of soil microbiome as well as primary degraders of all compounds. In contrast, Gram-positive bacteria, actinobacteria and fungi could have been consumers of primary degraders. Certain genera e.g. Gemmatimonas, Arenimonas and Massilia showed increased abundance in certain treatments indicating their potential involvement in biodegradation. Based on similar time-dependent microbial activity and shifts in abundances of (¹³C-)PLFAs and 16S rRNA genera, we deduced that glyphosate was mainly degraded to glycine, and presumably at elevated amounts. We reported for the first time that the glycine presumably altered microbial activity and community composition rather than glyphosate directly. Future studies should thus also consider the potential impacts of degradation products of the parent compound on soil microbiomes.