Effects Of Antibiotics And Humic Acid On Antibiotic Resistance Genes Under Heavy Metal Stress

The frequent and large application of organic fertilizers such as livestock and poultry manure in agricultural production activities has led to increasingly serious pollution problems with heavy metals,antibiotics and antibiotic resistance genes(ARGs).Humic acid(HA)is a common organic substance in soil,which can improve soil environment,activate microbial activity,and promote plant growth.It is also a commonly used soil remediation material.Therefore,in this study,cadmium(Cd)and sulfadiazine(SD)were used as representatives of heavy metals and antibiotics,and HA was selected as an additive,the effects of Cd and SD single and combined pollution and HA addition to Cdcontaminated soil on ARGs and microbial community structure were studied,and the relationships between environmental factors,mobile genetic elements(MGEs)and microbial community and ARGs were analyzed.The main conclusions are as follows:(1)In the composite contaminated soil of Cd and SD,SD activated the activity of Cd,and Cd led to an increase in the residual concentration of SD.Cd and SD increased the abundance of total MGEs and total ARGs,and the higher the concentration of Cd and SD was,the greater the synergy was.(2)Proteobacteria and Luteimonas,which account for the largest proportion of bacteria,had the highest abundance in C8S100 in the treatment groups of Cd(8 mg/kg)and HA(100 mg/kg),which may have contributed to the highest abundance of total ARGs and total MGEs in the C8S100 treatment,suggesting that Cd and SD altere the microbial community and further stimulate the expression of ARGs.(3)The addition of high concentrations of HA(50 mg/kg)to Cd-contaminated soil reduced the abundance of ARGs.At the 2 mg/kg Cd level,the relative abundance of total ARGs increased by 0.13-fold in the treatment with HA 10 mg/kg and decreased by 0.12-fold in the treatment with HA 50 mg/kg;at the level of 8 mg/kg Cd,the relative abundance of total ARGs increased by 0.09-fold in the treatment with HA 10 mg/kg and decreased by 0.18-fold in the treatment with HA 50 mg/kg.It showed that the addition of high concentrations of HA to Cd-contaminated soils can effectively abate ARGs.(4)Adding a high concentration of HA(50 mg/kg)to Cd-contaminated soil,the abundance of Proteobacteria,Gemmatimonadetes,Acidobacteria,etc.at the phylum level and the bacteria Luteimonas,which accounted for the largest proportion at the genus level,all decreased.This might be the the main reason for the decrease of ARGs abundance in soil after adding high concentration of HA to Cd-contaminated soil.(5)MGEs had the greatest impact on ARGs,especially int I1,which played a crucial role in the spread and expression of ARGs.