Tetracycline and sulfamethazine alter dissimilatory nitrate reduction processes and increase N2O release in rice fields.

Effects of antibiotics on the transformation of nitrate and the associated N2O release in paddy fields are obscure. Using soil slurry experiments combined with 15N tracer techniques, the influence of tetracycline and sulfamethazine (applied alone and in combination) on the denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N2O release rates in the paddy soil were investigated, while genes related to nitrate reduction and antibiotic resistance were quantified to explore the microbial mechanisms behind the antibiotics' effects. The potential rates of denitrification, anammox, and DNRA were significantly (p < 0.05) reduced, which were mainly attributed to the inhibitory effects of the antibiotics on nitrate-reducing microbes. However, the N2O release rates were significantly (p < 0.05) stimulated by the antibiotic treatments (0.6-6000 μg kg-1 soil dry weight), which were caused by the different inhibition effects of antibiotics on N2O production and N2O reduction as suggest by the changes in abundance of nirS (nitrite reduction step) and nosZ (N2O reduction to N2 step) genes. Antibiotic resistance gene (tetA, tetG, sulI, and sulIII) abundances were significantly (p < 0.05) increased under high antibiotic exposure concentrations (>600 μg kg-1 soil dry weight). Our results suggest that the widespread occurrence of antibiotics in paddy soils may pose significant eco-environmental risks (nitrate accumulation and greenhouse effects) by altering nitrate transformation processes.