The Effects of Glyphosate and Atrazine Mixture on Soil Microbial Population and Subsequent Impacts on Their Fate in a Tropical Soil

Glyphosate applied in association with atrazine provides the best cost/benefit for weed control for genetically modified corn. Therefore, the aim of this work was to evaluate the effects of applying glyphosate in mixture with atrazine on soil microbial population and on herbicides fate in a representative Oxisol from Brazil. The treatments consisted in applying the recommended field rate of glyphosate in association with 0, 1, and 2 times the recommended field rate of atrazine (and vice versa), plus the control (without herbicides application). The presence of atrazine temporarily (21 days) decreased soil microbial biomass (SMB) and increased soil carbon mineralization (SCmin, up to 13 times) and microbial metabolic quotient (qCO(2)) due to the stresses caused by its toxicity. When the mixture was applied independent of the rates, SMB was recovered and the amounts of extractable and non-extractable C-14-residues were the same for both herbicides at 63 days. These results suggest that glyphosate may mitigate atrazine's temporary impact on soil microbes by supplying them nutrients during their adaptation.