Résumé
To face water scarcity, the reuse of treated wastewater (TWW) appears to be a fair and sustainable solution for irrigation of agro-ecosystems. However, TWW can modify soil endogenous communities with the supply of TWW exogenous bacteria, antibiotic resistance genes, and antibiotics likely to induce a selection pressure favouring the dissemination of antimicrobial resistances. Furthermore, due to the low biodegradability of some antibiotics, they can accumulate and reach critical concentrations in soils. To determine a threshold equivalent to the PNEC for soils, increasing antibiotic concentrations in TWW were applied to soil microcosms (0, 10, 100, 500 and 1000 g/kgdry soil). To do this, sulfamethoxazole (SMX) and clarithromycin (CLA), were chosen. For SMX, most of the effect were observed within the first incubation week. Several concentration thresholds were established. (i) Between 100-500g/kgdry soil, a 1st threshold was identified, above which DT50 decreased by a factor of 5. (ii) From 10-100g/kgdry soil, bacterial selection and inhibition were observed. (iii) A positive correlation between the sul1 and intl1 genes abundance and the SMX concentration was identified. Three levels of resistance were noted within the 1st week according to SMX concentration: 0-10g/kgdry soil, 100-500g/kgdry soil, 1000g/kgdry soil. After 1.5 month, these two genes still exhibited higher number of copies for SMX concentration above 10-100g/kgdry soil (+0.5-1 log) suggesting resistance transfer. For CLA, the first results pointed out a much slower degradation (DT50 > 30 days). We can therefore expect stronger effects on the associated resistance genes and a long-term disturbance of soil microbial communities. The results are undergoing. These experiments enabled to gain a better understanding of the dynamics related to the inputs of these antibiotic and identified a key SMX concentration between 10 and 100g/kgdry soil which disturbs soil endogenous communities and whose residual effects are still noticeable after 1.5 months.