Abstract

The objective of this study was to evaluate the effects of glyphosate on soil microbial activity under appropriate and water deficit conditions. The factors consisted of four treatments with different Roundup Ready^® soybean cultivars, two herbicidal treatments [glyphosate (1.080 and 1.800 g a.e. ha^-1) + control] and two soil moisture treatments (appropriate and water deficit conditions), plus two soybean conventional varieties subject only to the soil moisture treatments. It was evaluated total organic carbon (TOC), basal respiration rate (CO₂), microbial carbon biomass (MCB), metabolic quotient (qCO₂), microbial quotient (qmic), nodule number (NN), and dry weight of nodules per plant (DWNP). Except for TOC, all other microbiological parameters were significantly influenced by the application of glyphosate, and the effects are more evident at a 1.800 g a.e. ha^-1 dose in Roundup Ready^® soybean cultivars associated to water deficit condition. The soil microbiota stability was favored by the absence of glyphosate application and water deficit condition on soil. MCB and qCO₂ showed to be quite sensitive to changes resulting from the use of glyphosate. In general, in conventional cultivars there was lesser effect of water deficit conditions on the microbiological attributes, when compared to treatments with glyphosate application under the same conditions.

Key words: Glycine max (L.) Merrill, inhibitors of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPs), microbiota, Roundup Ready^® soybean seed.