| Recently,with the rapid development of intensive agriculture and large-scale livestock farming,heavy metals and antibiotics have entered and accumulated in agricultural soils along with organic fertilizers.The combined pollution of heavy metals and antibiotics poses a serious threat to ecological and environmental safety and human health.Soil nitrification dominated by ammonia-oxidizing archaea（AOA）and ammonia-oxidizing bacteria（AOB）,is a central part of nitrogen cycling.It has been reported that nitrification process links nitrogen fixation and denitrification,influencing greenhouse gas nitrous oxide（N2O）emissions and nitrate-nitrogen leaching.The combined pollution of heavy metals and antibiotics in soils significantly affects the microbial community structure,which inevitably affects the microbially mediated nitrification process.However,the effects of biochar remediation,and combined contamination of heavy metal and antibiotics on nitrification remain elusive.Therefore,the incubation experiments were conducted as follows:（1）The effects of biochar on Cd availability and nitrification acticity,as well as the effects of nitrification inhibitor dicyandiamide（DCD）on nitrification in Cd-contaminated red soil and fluvo-aquic soil;（2）The effects of heavy metal(cadmium Cd,5mg·kg-1)alone or in combination with antibiotic(oxytetracycline OTC:0.1,1mg·kg-1)on nitrification activity and nitrifying communities.The soil samples were collected along the eastern China from north to south,including Inner Mongolia,Shandong,Zhejiang and Hainan.The results could provide a comprehensive insights into the effects of heavy metals and antibiotics on nitrification and its related microorganisms,as well as a theoretical basis for soil remediation in heavy metals and antibiotics contaminated soils.（1）Biochar significantly reduced the available cadmium content in red soil and fluvo-aquic soil.The remediation effect of biochar in red soil was more significant and lasted longer than that in fluvo-aquic soil.Accordingly,we concluded that biochar was more effective in the remediation of contaminated acidic red soil than alkaline fluvo-aquic soil.DCD significantly inhibited nitrification activity in heavy metal Cd-contaminated soil,while biochar alleviates the inhibition effects of DCD on nitrification,especially in fluvo-aquic soil.（2）DCD significantly reduced AOB-amoA gene abundance in both red soil and fluvo-aquic soil.The AOB-amoA gene copy number was 78.64%higher in the BCD-treated red soil and 23.10%higher in the fluvo-aquic soil than DCD-treated soils,indicating that biochar mitigated the inhibitory effects of DCD on AOB.In the red soil,the abundance of AOB-amoA gene in biochar treated soils was 34.28%higher than that in CK soils,indicating that biochar stimulated AOB growth in red soil.The reasons could be due to the reduction of acidity of red soil by biochar.In addition,the application of DCD significantly inhibited the growth of AOB and had no significant effect on AOA in the heavy metal contaminated soils,coupling the results that DCD significantly inhibited nitrification activity,thus we speculated AOB,but not AOA,were the main players in the nitrification process in heavy metal contaminated soils remediated by biochar.（3）The OTC pollution increased the available Cd content.The Cd availability was increased with the increasing of OTC concentration,which could be explained by the fact that OTC could interact with Cd2+through functional groups.（4）Both single and combined contamination of Cd and OTC inhibited soil nitrification.The inhibitory effect diminishing with incubation time.The inhibition effect of combined contamination on nitrification increased with increasing of OTC concentration,and was more severe that of Cd or OTC single contamination.Both single and combined contamination of Cd and OTC significantly inhibited AOA and AOB growth,and the inhibitory effects were congruous with those on nitrification activity.The AOB were mainly affiliated with Nitrosospira and the AOA mainly grouped with Crenarchaeota,indicating the potential importance of Nitrosospira AOB and Crenarchaeota AOA in nitrification in the heavy metals and antibiotics contaminated soils.In conclusion,biochar could significantly reduce the available cadmium content,especially in acidic soils.DCD significantly inhibited nitrification activity and AOB amoA gene abundance,while exerted little effects on AOA abundance.Therefore,we speculated that AOB,rather than AOA dominated the nitrification process in heavy metal contaminated soils.in heavy metal Cd-contaminated soil,and together with biochar addition could alleviate the inhibition effects.Both single and combined treatments of heavy metal cadmium（Cd）and antibiotic oxytetracycline（OTC）inhibited soil nitrification,and combined pollution had a more significant inhibition effect than single pollution. |
