Aplicação do herbicida atrazina em solo: efeitos sobre indicadores microbianos e análise de sua atenuação

Abstract

Atrazine is one of the main herbicides used in agricultural practice. Its indiscriminate use represents a growing problem, with potential risks to ecosystems and human health. The microbiota performs essential functions related to the dynamics of organic matter and nutrients in soils, and its degrading capacity is used in the bioremediation of xenobiotics such as atrazine. In this sense, the effects of atrazine towards soil microbiota were investigated, as well as the attenuation of this herbicide. Soil without a history of atrazine contamination was used to prepare control microcosms (T1) and microcosms contaminated with atrazine (T2, 7.39 mg kg 1 , recommended dose; T3, 36.94 mg kg-1, five-times the recommended dose). Microcosms were incubated for 67 days, with periodic evaluations of microbial indicators and atrazine attenuation. Soil respiration, indicative of microbial activity, showed increased rates at the beginning of incubations (days 1-4), particularly higher in contaminated soils; in the course of the experiments, respiratory rates were similar to control. The final cumulative respiration was higher at T3, and similar between T1 and T2. The microbial biomass carbon was not affected by atrazine in T2 when compared to T1; in T3, biomass was lower than T1 at day 31, recovering to values similar to T1 after 67 days. The metabolic quotient, an indicator of microbial stress, was higher in T3 at day 1; subsequently, values were similar between treatments. Transient effects have also been observed for fluorescein diacetate hydrolysis (which measures the total soil hydrolytic activity), dehydrogenase activity (which assesses the oxidative capacity of the microbiota), and enumeration cultivable heterotrophic soil bacteria. Even though the microbial indicators used have returned to control values, the observed variations, particularly in T3, may indicate effects on microbial diversity, which should be investigated. Atrazine dissipation reached 80% and 59% in T2 and T3, respectively, after 67 days. In absolute values, more atrazine was dissipated in T3, but also more atrazine persisted in this treatment, which may represent a potential risk to soil and water quality. Incubation of soils in liquid medium containing atrazine (5 mg L-1) resulted, after 7 days, in dissipations of 35% and >99% for soils from microcosms T2 and T3, respectively. This suggests the existence of atrazine-degrading microorganisms in the investigated soil, which may have acted in herbicide dissipation. Investigations regarding degrading microorganisms gain relevance in the context of their applicability in the bioremediation of atrazine-contaminated soils.