Effect of glyphosate on soil microbial activity and biomass

Herbicides applied to soils potentially affect soil microbial activity. Qua ntity and frequency of glyphosate application have escalated with the adven t of glyphosate-tolerant crops. The objective of this study was to determin e the effect of increasing glyphosate application rate on soil microbial bi omass and activity. The soil used was Weswood silt loam. The isopropylamine salt of glyphosate was added at rates of 47, 94, 140, and 234 mu g ai g(-1 ) soil based on an assumed 2-mm glyphosate-soil interaction depth. Glyphosa te significantly stimulated soil microbial activity as measured by C and N mineralization but did not affect soil microbial biomass. Cumulative C mine ralization, as well as mineralization rate, increased with increasing glyph osate rare. Strong linear relationships between mineralized C and N and the amount of C and N added as glyphosate (r(2) = 0.995, 0.996) and slopes app roximating one indicated that glyphosate was the direct cause of the enhanc ed microbial activity. An increase in C mineralization rate occurred the fi rst day following glyphosate addition and continued for 14 d. Glyphosate ap peared to be directly and rapidly degraded by microbes, even at high applic ation rates, without adversely affecting microbial activity.