Effects Of Different Concentrations Of Sulfamethazine On N₂O And NH₃ Emissions From Rice And Wheat Fields And Their Mechanisms

Nowadays,the greenhouse gases produced by agricultural systems are increasingly contributing to global climate change.Nitrous oxide(N2O)is a trace greenhouse gas closely related to global climate change.Nitrogen applied to farmland can also cause great loss through ammonia(NH3)volatilization.In recent years,with the development of intensive animal husbandry,the use of veterinary antibiotics is relatively large,and the residual antibiotics in the soil will have an activation or inhibition effect on the microbial metabolism in the process of soil nitrogen conversion,and then have an impact on the emission of N2O and NH3 in the farmland.This paper selected the typical example of veterinary antibiotics in sulfadimidine(sulfamethazine,SMZ)as the research object,through the rice-wheat fields,and conducted situ observation in the rice and wheat fields,the pig manure with without antibiotics(CM)and conventional compound fertilizer(CF)as the base fertilizer,adding different concentration of SMZ,to explore the change of different concentrations sulfadimidine on N2O and NH3 emissions and related nitrification and denitrification functional genes and the soil environmental factors.Nine treatments were set in the wheat experiment,including:no fertilizer and no antibiotics applied(rice field:RcK;wheat field:WCK),pig manure used as basal fertilizer,with the addition of 0,5,15,30 mg/kg SMZ respectively(rice field:RCM,RCM+SMZ5,RCM+SMZ15,RCM+SMZ30;wheat field:WCM,WCM+SMZ5,WCM+SMZ15,WCM+SMZ30),compound fertilizer used as basal fertilizer,with the addition of 0,5,15,30 mg/kg SMZ respectively(rice field:RCF,RCF+SMZ5,RCF+SMZ15,RCF+SMZ30;wheat field:WCF,WCF+SMZ5,WCF+SMZ15,WCF+SMZ30).Urea was applied as topdressing in the all fertilizing treatments.The results of paddy field experiments showed that the sulfamethazine did not change the seasonal pattern of N2O emissions.For the N2O flux during the whole observation period,the two-factor analysis of variance showed that there was a significant difference in the medium concentration(15 mg/kg)compared with the same fertilizer source without the application of SMZ(P<0.05).There was no significant difference for high concentration(30 mg/kg)and low concentration(5 mg/kg)SMZ(P>0.05).For the cumulative emissions of N2O,the low,medium and high concentrations of SMZ were statistically not significantly different from without the application of SMZ(P>0.1),but the medium and high concentrations increased the cumulative N2O emissions at the mean level.The medium and high concentrations of SMZ treated with pig manure as the base fertilizer were 3.47 and 4.67 times of the control;the medium and high concentrations of compound fertilizer as the base fertilizer were 1.65 and 1.52 times of the control,and the medium and high concentrations were increased the soil NH4+-N and NO3--N content.The medium or high concentration SMZ basically increased the abundance of total soil bacteria 16S rRNA,AOA amoA,AOB amoA,nirK,nirS and nosZ genes no matter SMZ is mixed with pig manure or compound fertilizer(P<0.05),except that RCM+SMZ15 treatment has no significant effect on AOB amoA,nirK and nirS genes,RCF+SMZ15 treatment has no significant effect on nirK genes,RCF+SMZ15 and RCF+SMZ30 treatment has no significant effect on nosZ genes(P>0.05).In addition,low concentration SMZ combined with compound fertilizer also significantly increased the above functional gene abundance(P<0.05),but showed no significant effect or reduced effect when combined with pig manure(P>0.05).The N2O flux during the rice period was significantly positively correlated with the abundance of total soil bacteria 16S rRNA,AOA amoA,AOB amoA,nirK,nirS genes and the content of NO3--N in soil(P<0.05),indicating that the nitrifying bacteria containing AOA amoA and AOB amoA genes and the denitrifying bacteria containing nirK and nirS genes in this study directly affected the N2O production in the rice field.Correlation analysis between the time-weighted average rate of N2O emissions from June 9th to August 24th in rice growing season and soil nitrification or denitrification process gene abundance,the contents of NH4+-N and NO3--N in soil showed that the cause of sulfamethazine promoting N2O emission was significantly improved the AOB amoA genes abundance,soil NH4+-N and NO3--N content(P<0.05).During the wheat period,SMZ and compound fertilizer mix,low concentration SMZ has no significant influence on soil total bacterial 16S rRNA,AO A amoA,AOB amoA,nirK,nirS and nosZ gene(P>0.05),while the medium concentration significantly improved the abundance of the six major genes(P<0.05),high performance is not consistent,significantly increased the abundance of AOA amoA,nirK and nosZ genes(P<0.05),had no significant effect on 16S rRNA and nirS,and significantly reduced the abundance of AOB amoA gene(P<0.05).When mixed with pig manure,except that SMZ at low and medium concentrations significantly increased the abundance of 16S rRNA and AOA amoA gene of total bacteria in soil(P<0.05),SMZ at low concentrations significantly increased the abundance of nirS gene and significantly decreased the abundance of AOB amoA gene(P<0.05),other treatments had no significant effect on the six major genes.There was a significant positive correlation between the N2O emission flux and the abundance of total soil bacteria 16S rRNA gene,nirS and nosZ gene in the wheat season(P<0.05),indicating that the denitrifying bacteria containing nirS and nosZ genes in this study directly affected the production of N2O in the wheat field.The effects of SMZ treatments on rice and wheat during the season on nirK and nirS were stronger than nosZ.Combined with the research results of the research group,it can be clarified that the reason for the high and medium concentration of sulfamethazine to promote N2O emission is to change the activity of soil nitrification and denitrification microorganisms,which directly increased NH4+-N and NO3--N content in soil in the rice season and NH4+-N content in soil in the wheat season.The results of NH3 volatilization test showed that the NH3 volatilization rule of pig manure treatment was basically consistent with that of compound fertilizer treatment throughout the rice growing season.Under the condition of pig manure as the base fertilizer,the medium and low concentration of SMZ had no significant effect on the volatilization of NH3(P>0.05).The high concentration of SMZ significantly promoted the volatilization of NH3(P<0.05),and the loss rate of NH3 volatilization increased by 1.2 times.Under the condition of compound fertilizer as the base fertilizer,medium concentration SMZ significantly promoted NH3 volatilization(P<0.05),NH3 volatilization loss rate increased by 1 time,while high and low concentration SMZ had no significant effect on NH3 volatilization(P>0.05).Except that RCM+SMZ5 treatment had no significant effect on soil urease activity(P>0.05),the other concentrations of SMZ treatment promoted soil urease activity at the average level,and SMZ promoted soil urease in compound fertilizer more than pig manure.During the entire rice growth period,there was a significant positive correlation between NH3 volatilization rate and soil urease activity(P<0.01).Correlation analysis between NH3 volatilization time weighted average rate and soil NH4+-N,NO3--N content and soil urease time-weighted average activity was carried out during the significant impact period(July 29-August 24),the results show that no matter in pig manure or compound fertilizer treatment,the SMZ significantly promoted NH3 volatilization because it increased soil NH4+-N content,and also increased soil urease activity in the treatment of compound fertilizer as base fertilizer in the topdressing period.