Parâmetros microbianos em solo sob aplicação do herbicida glifosato em microcosmos

Abstract

Glyphosate is considered a potent post-emergent herbicide widely used in agriculture, especially in soybean cultivation, but is not metabolized in plants, and the microorganisms are responsible for its degradation. The soil microorganism is responsible for the execution and control of several fundamental functions, and soil contamination by pesticides can both inhibit part of the microbiota by the toxicity of its components, but also boost native populations capable of degrading such compounds. In this sense, it is of great importance to evaluate the microbiological parameters, since they provide information on the presence and activity of microorganisms, as well as on the intensity, type and duration of the effects of pesticide contamination on soil metabolic activity. The objective of this work was to evaluate different microbial parameters in Red Latosol without recent history of contamination under application of glyphosate in microcosms. The concentration of active ingredient (ai) as glyphosate acid equivalent (36.5%) and the maximum dose indicated in the formulated commercial product package (6 L ha-1) were considered for calculation of the dose used, and two concentrations of glyphosate: recommended dose (DR: 2.37 mg ai kg-1 soil) and 10 times the recommended dose per hectare (10 × 23,70 mg ai kg-1 soil). The microcosms were assembled using sterile glass bottles (3.5 L) with hermetic closure, adding 1,250 kg of soil in each flask, contemplating a layer of 7.5 cm. For each dose, the microcosms were maintained in quadruplicate, as well as microcosms without application of the herbicide (SA), also in quadruplicate. Incubations were performed for 28 days at room temperature, with no incidence of light. As indicators of microbial activity, soil respiration, total hydrolytic activity, dehydrogenase activity, total heterotrophic counts, microbial biomass carbon and metabolic quotient were used. In the experimental conditions used, the results indicated a tendency of greater respiration in the microcosms with application of glyphosate, being 15% and 54% higher in DR and 10X treatments than in SA. The results of hydrolytic activity, dehydrogenase and total heterotrophic counts fluctuated over the 28 days, however, with no significant difference between the treatments. The carbon of the microbial biomass presented a tendency of decrease soon after the application of glyphosate in comparison with SA; however, similar values were observed at the end of 28 days. At the beginning of the incubations, the metabolic quotient showed a trend of higher values in the presence of glyphosate, potentially indicating greater stress of the microbiota. However, this parameter was similar between the treatments at the end of the experiments. Under the conditions of the experiment, the results indicated that, at the highest recommended dose of the formulated herbicide (6 L ha-1) and also in a dose ten times higher (60 L ha -1), the potential effects on the microbiota are transient .