[Effects of the Veterinary Antibiotic Sulfamethazine on NO Emissions and the Associated Microbiological Mechanism in a Rice Field].

Veterinary antibiotics can enter into croplands with animal excrement and can have effects on nitrification and denitrification processes in the agricultural soils. A field experiment was conducted to evaluate the effect of sulfamethazine (SMZ) on NO emissions, nitrification, denitrification, and related functional gene abundances within a paddy field. Five treatments were used in the experiment, namely, no fertilizer and no antibiotics applied (CK), and pig manure used as basal fertilizer plus urea applied as topdressing with the addition of 0, 5, 15, and 30 mg·kg SMZ (SMZ0, SMZ5, SMZ15, and SMZ30, respectively). Soil and gas samples were collected and analyzed periodically throughout the rice growing season. The results showed that the SMZ did not change the seasonal pattern of NO emissions. During the entire observation period, there was a significant difference in NO fluxes between the SMZ15 and SMZ0 treatment (<0.05), but there was no significant differences in NO fluxes between the SMZ30, SMZ5, and SMZ0 treatment (>0.05). Medium and high concentrations (SMZ15, SMZ30) increased the cumulative emissions of NO at the average level, and these values were 3.47 and 4.67 times higher than that of the SMZ0 treatment, respectively; the soil NO-N content also increased. Medium and high concentrations had a significant activation effect on the gene abundances of total soil bacteria 16S rRNA, ammonia-oxidizing archaea (AOA) , and ammonia-oxidizing bacteria (AOB) during the nitrification process and the gene abundances of , , and during the denitrification process (<0.05), while the SMZ treatment with a low concentration had a slight inhibitory effect on the abundance of each gene. The ratios of abundance copies of 16S rRNA, AOA , AOB , and the genes of , , and treated by SMZ30, SMZ15, and SMZ0 were 1.58, 1.77, 2.15, 1.38, 1.33, 1.42, and 1.24, 1.37, 1.08, 1.65, 1.11, 1.64, respectively, at the average level. The abundance ratios of the six above genes treated by SMZ5 and SMZ0 were less than one and only 0.80, 0.99, 0.92, 0.76, 0.76, and 0.77, respectively. The NO fluxes were significantly and positively correlated with the abundances of the gene (<0.01), thus indicating that SMZ had an effect on NO emissions by influencing the activity of denitrifying bacteria. Therefore, the pollution of farmland by veterinary antibiotics should not be ignored, and the use of antibiotics should be controlled reasonably at the source, so as to reduce the environmental and ecological risks.