| Livestock manure is often used as organic fertilizer in fields because it contains nutrients needed for crop growth,but there are a lot of antibiotic residues in livestock manure.The antibiotic pollution in soil environment caused by long-term use of livestock manure has become a major concern worldwide.At present,it is not clear how sulfadiazine（SD）under single and combined pollution conditions would affect plant growth,soil bacterial community,the abundances of antibiotic-resistance genes and functional genes,and whether the effects of antibiotics would differ between rhizosphere and bulk soil.Therefore,we conducted a series of greenhouse pot experiments:（1）In a loess soil without and with phosphorus fertilizer(0 and50 mg kg–1),the effects of different sulfadiazine levels(0,1,10 and 100 mg kg–1)on maize growth and root development were investigated;（2）The effects of sulfadiazine(0,0.1 and 1mg kg–1)alone and in combination with oxytetracycline(OTC,0,0.1 and 1 mg kg–1)in a loess soil on maize growth and plant nitrogen and phosphorus nutrition were studied;（3）To investigate how sulfadiazine(0 and 1 mg kg–1)alone and in combination with oxytetracycline(0 and 1 mg kg–1)would affect the abundance of bacterial communities,antibiotic resistance genes,carbon,nitrogen and phosphorus cycling functional genes,as well as nitrogen and phosphorus content in the rhizosphere and bulk soil.The findings of the study demonstrate that:（1）The addition of sulfadiazine to soil can reduce plant dry mass and inhibit root development of maize,and root development is more sensitive than plant biomass to higher levels of sulfadiazine(10 and 100 mg kg–1).Regardless of whether phosphorus fertilizer was added or not,100 mg kg–1 sulfadiazine significantly decreased the shoot dry mass of maize.At higher sulfadiazine levels(i.e.10 and 100 mg kg–1),the root dry mass,root length,root surface area and volume of maize significantly decreased.The average root diameter and the proportion of thick roots（>4.5 mm）significantly increased while the specific root length significantly decreased with increasing sulfadiazine level in the soil.（2）The results indicate that except for 0.1SD0.1OTC,which considerably increased the dry mass and stem diameter of maize,all other treatments had an inhibitory effect on the growth of maize.Under single sulfadiazine treatment,plant dry mass,shoot height,stem diameter,and plant phosphorus content significantly decreased with an increase in sulfadiazine concentration.Both single sulfadiazine contamination and combined contamination with oxytetracycline considerably increased the root nitrogen concentration when compared with the control without SD and OTC.（3）The rhizosphere soil contained greater amounts of bacterial populations and antibiotic resistance genes than the bulk soil did.The rhizosphere effect and sulfadiazine were key factors in shaping the bacterial communities.Compared with the control without SD and OTC,the addition of sulfadiazine alone or in combination with oxytetracycline reduced the diversity of bacterial communities in both the rhizosphere and bulk soil.Sulfadiazine single and in combination with oxytetracycline increased the absolute abundance of 16S r RNA（except for bulk soil）,sul2,tnp A-03,and tnp A-04 genes in both the rhizosphere and bulk soil.（4）The abundance of carbon,nitrogen,and phosphorus cycling functional genes in rhizosphere soil is higher than that in bulk soil.Under the combined effects of sulfadiazine and oxytetracycline,the absolute abundances of abf A,acc A,frd A,acs A,and ppx genes in the rhizosphere soil and hao,pho X genes in the bulk soil increased,but the absolute abundances of xyl A,acs E,hao,gdh A,nos Z-1,nir S-3,and pho X genes in the rhizosphere soil and xyl A,acc A,frd A,acs A,gdh A,and ppx genes in the bulk soil decreased.Compared with the control without SD and OTC,single sulfadiazine pollution and combined pollution with oxytetracycline reduced the concentration of available phosphorus in the rhizosphere soil and the concentration of ammonium-nitrogen in the bulk soil,and increased the concentration of nitrate-nitrogen and ammonium-nitrogen in the rhizosphere soil.However,only the change of ammonium-nitrogen concentration in the rhizosphere soil reached was significant,and the promotive and inhibitive effects of a single sulfadiazine pollution were stronger than the combined pollution of sulfadiazine and oxytetracycline.In the bulk soil,single sulfadiazine pollution reduced the concentration of available phosphorus and total phosphorus,but under the combined effect of sulfadiazine and oxytetracycline,the concentration of available phosphorus and total phosphorus increased.In conclusion,the addition of sulfadiazine at high concentrations(10 and 100 mg kg–1)significantly inhibited the growth and root development of maize.The effect of single sulfadiazine pollution and combined pollution with oxytetracycline on the bacterial communities,antibiotic resistance genes,and functional genes abundances were different between the rhizosphere and bulk soil.The addition of sulfadiazine and oxytetracycline alone or simultaneously reduced the diversity of the bacterial communities.The effects of single sulfadiazine contamination on maize nitrogen and phosphorus nutrition and soil antibiotic resistance genes were stronger than that of combined sulfadiazine and oxytetracycline contamination.The impact of sulfadiazine alone or combined pollution with oxytetracycline on functional genes depends on the type of functional genes and whether the environment in which it operates is the rhizosphere soil or bulk soil.The findings of this study can serve as a theoretical foundation for assessing the impacts of both single and combined antibiotic pollution on plant growth,bacterial communities,antibiotic resistance genes,functional genes abundances related to carbon,nitrogen,and phosphorus cycling,as well as nitrogen and phosphorus concentrations in both the rhizosphere and bulk soil,and will be useful for guiding prudent use of livestock manure to avoid potential antibiotic pollution. |
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