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Abstract
Atrazine (ATZ) and S-metolachlor (S-MET) are two herbicides widely used, often as mixtures. The present work examined whether the presence of S-MET affects the ATZ-biodegradation activity of the bioaugmentation bacterium Pseudomonas sp. strain ADP in a crop soil. S-MET concentrations were selected for their relevance in worst-case scenarios of soil contamination by a commercial formulation containing both herbicides. At concentrations representative of application of high doses of the formulation (up to 50 µg g−1 of soil, corresponding to a dose approximately 50× higher than the recommended field dose (RD)), the presence of pure S-MET significantly affected neither bacteria survival (∼107 initial viable cells g−1 of soil) nor its ATZ-mineralization activity. Consistently, biodegradation experiments, in larger soil microcosms spiked with 20× or 50×RD of the double formulation and inoculated with the bacterium, revealed ATZ to be rapidly (in up to 5 days) and extensively (>96%) removed from the soil. During the 5 days, concentration of S-MET decreased moderately to about 60% of the initial, both in inoculated and non-inoculated microcosms. Concomitantly, an accumulation of the two metabolites S-MET ethanesulfonic acid and S-MET oxanilic acid was found. Despite the dissipation of almost all the ATZ from the treated soils, the respective eluates were still highly toxic to an aquatic microalgae species, being as toxic as those from the untreated soil. We suggest that this high toxicity may be due to the S-MET and/or its metabolites remaining in the soil.
Highlights
Diverse pesticides, fertilizers and organic components used in commercial formulations can be found in agricultural soils due to intensive use in crop cultivation or accidental spills
Following inoculation with viable cells of the bioaugmentation bacterium (1.360.56107 CFU g21 soil dry weight) of soil spiked with mixtures of [14C]ATZ plus increasing doses of non-labeled ATZ from the double formulation Primextra S-Gold (56, 206 or 506RD), ATZ mineralization started rapidly
ADP performance When considering in situ bioremediation strategies, bioaugmentation may fail in the field as a result of the susceptibility of the specialized degrading microorganisms to high concentrations of non-target compounds; other plausible reasons may include microbial competition for a limiting nutrient and/or inhibition of degradation through catabolic repression/competitive inhibition phenomena in the soil [1,2,26,27]
Summary
Diverse pesticides (and their metabolites), fertilizers and organic components used in commercial formulations can be found in agricultural soils due to intensive use in crop cultivation or accidental spills. As part of a framework for the rational bioremediation of ATZcontaminated land, we recently presented evidences that a cleanup strategy, combining soil bioaugmentation with this bacterial strain and biostimulation with citrate [17], was effective at a larger microcosms scale [14,15] It led to the rapid removal of ATZ from a natural crop soil spiked with a commercial formulation containing ATZ as single active ingredient at doses mimicking worst-case scenarios (e.g. spills and/or concentrated hotspots) [14,15]. Assessment of the ecotoxicity of soil samples and eluates/leachates from the bioremediated soils proved the effective soil decontamination in less than 10 days, contributing to significantly diminish the toxicity impact in the aquatic compartment [14]
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