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Impact écotoxicologique d’un antibiotique sur les processus microbiens du cycle de l’azote, dans des sols amendés par des produits résiduaires organiques
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The environmental concern regarding the antibioticshas increased considerably, due to their intensive uses in human and veterinary medicines and the growing interest in organic wastes recycling as soil amendments. While the occurrence and persistence of antibiotics and antimicrobial resistance in environment have become major human health issues, their ecotoxicological impacts on microbial ecosystem processes (organic matter degradation and nutrient recycling) are not well understood. The exposure (fate and bioavailability of antibiotics) and ecological characteristics of microbial communities (resistance and resilience) influence the impacts. Organic matter is a key environmental factor influencing the dissipation and the bioavailability of antibiotics, in soils. Also, organic matter and nutrient inputs influence microbial communities in amended soil. Our objectives were to investigate the effects of the sulfamethoxazole (SMX) on microbial nitrogen transformations. A dose-effect approach was performed with different doses of SMX added to different organic wastes (compost of sewage sludge -green waste (SGW), farmyard manure (FYM)), before their mixing on soil microcosms. The final concentrations of SMX in amended soils ranged from 0.022 to 2.22 mg kg-1dw, with control soil microcosms amended with SGW or FYM without SMX. Nitrogen forms, potential nitrification and denitrification activities were determined in soils, after 8, 28 and 84 days following amendments. The total and available concentrations of SMX were extracted with organic and aqueous solutions, respectively, and then quantified by UPLC-MS/MS. Only the nitrification activity was adversely impaired by SMX, following a dose-response pattern, in SGW amended soils, but not in FYM amended soils. These effects were related to the soil nitrate contents in soil-SGW, as functional output. The chemical assessment of SMX availability did not show strong differences between the SGW or FYM amended soils, in opposite to nitrification response. Actually, the acute toxicity of SMX, following a bacteriostatic action,can be only detected on growing microbial populations, such as nitrifiers, in SGW amended soils with high NH4+levels. Denitrification activity was not consistently affected by SMX, despite high nutrient levels (nitrate and organic carbon). Nitrification appeared as a sensitive indicator of SMX effects on soil microbial functioning, while denitrification seemed to be more resistant. Different level of taxonomic diversity can explain these differences. In addition, the magnitude of effects of low doses of SMX (< 0.5 mg kg-1dw) slightly decrease after 84 days of exposure, compared to days 8 and 28, in accordance with the decrease of SMX availability, measured in SGW amended soils. This highlighted resilience capabilities of soil nitrification, following short-term impairments, which could be promoted by dissipation of SMX, leading to a decrease of its bioavailability. Analytical chemistry plays an important role in understanding ecotoxicity through the assessment of exposure scenario.
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