Effects of atrazine on soil microbial indicators and the evaluation of herbicide attenuation in microcosms

Purpose Soil microbiota performs essential functions related to soil functioning and quality. Atrazine is a widely used herbicide, presenting potential risks to (agro)ecosystems. Thus, we aimed to investigate the effects of atrazine towards microbial indicators, as well as to assess herbicide attenuation in soil microcosms. Materials and methods Soil without previous atrazine uses was employed to prepare control (T1) and atrazine-spiked microcosms [T2: 7.39 mg kg(-1), recommended dose (RD); T3: 36.94 mg kg(-1); five-fold RD]. Soils were incubated for 67 days. Basal soil respiration, microbial biomass carbon, metabolic quotient, culturable heterotrophic bacteria, soil hydrolytic activity, dehydrogenase activity, and atrazine dissipation were periodically evaluated. At the end of 67 days, soils were further incubated in liquid medium containing atrazine (5 mg atrazine L-1), and herbicide attenuation was monitored during 7 days. Results Final cumulative soil respiration was higher in T3. Increased respiration rates occurred at days 1-5. Microbial biomass carbon was lower in T3 at day 31, but similar to T1 after 67 days. Metabolic quotient was higher in T3 at day 1; subsequently (days 31 and 67), values were similar between treatments. In T2 and T3, transient effects were observed toward culturable heterotrophic bacteria, soil hydrolytic activity, and dehydrogenase activity. Atrazine dissipation reached 80% (T2) and 57% (T3). Soils from microcosms were incubated in liquid medium and, after 3 days, herbicide attenuation reached 18% (T2) and > 97% (T3). Conclusions Short-term adverse effects of atrazine on microbial activities and numbers/biomass are not expected. Herbicide persistence is of concern for environmental quality and soil sustainability. Results indicate that atrazine-degrading microorganisms were enriched due to previous herbicide exposure.