| The long-term abuse of veterinary antibiotics induces the production and spread of antibiotic resistance genes(ARGs)in the environment.Soil is becoming an important repository for ARGs.At the same time,the presence of heavy metals in soil also exerts a selective pressure on the persistence of ARGs,which can be absorbed and accumulated through the roots of the crops,thus threatening human health through food chain transmission.Biochar is a common soil amendment.Because of its large specific surface area,well-developed pore structure and abundant functional groups,it is of great significance to maintain soil ecological balance and is applied widely to farmland as a soil restoration agent.Biochar is also oxidized by biological and abiotic factors in the environment,and aging,that is,its surface structure and physical and chemical properties change,which in turn affects the repair and control of pollutants in soil.Therefore,the impact of aging on the accumulation of vegetables ARGs in heavy metals-antibiotic complex contaminated soil has been clarified,which is helpful to accurately evaluates the environmental benefits of long-term application of biochar.In this study,fresh biochar(pristine biochar,BC)was prepared from corn straw,and the aged biochar(oxidized biochar,OBC)was obtained by oxidant-drying/wetting-freezeing/thawing alternating cycle method.The contaminated soil was thoroughly mixed with two kinds of biochar at different doses(0%,1%,2%,5%)to set up a pot experiment of pakchoi(Brassica chinensis L.).The abundance of ARGs(tet X,tet W,sul2,erm B),class I integrants(int I1),and bioavailable concentration of heavy metals(Pb,Cu,Cd)in pakchoi rhizosphere and non-rhizosphere soil were compared,the changes of available heavy metals in soil were analyzed,and the migration and transformation of ARGs in soil-crop systems were explored.On this basis,the impacts of BC and OBC on soil organic matter and enzyme activities were investigated to reveal the bio-efficiency of biochar on ARGs and heavy metals in complex contaminated soil before and after aging.The main results of this study are as follows:(1)The absolute abundance of ARGs and int I1 genes in soil was as follows: int I1 > sul2 > erm B > tet X > tet W.The addition of BC and OBC were basically consistent with the changes in absolute abundance and relative abundance of ARGs in rhizosphere soil,that was,the abundance of tet W,sul2 and erm B genes decreased,while the abundance of tet X and int I1 increased.Moreover,both biochars changed the distribution of ARGs and int I1 in rhizosphere soil.The non-rhizobial ARGs abundance of BC treatment was higher than that of rhizosphere,while the OBC treatment was the opposite.At the same dose,the abundance of ARGs in rhizosphere soil treated with OBC was higher than that of BC,indicating that BC had better impedance effect on ARGs expression.In addition,ARGs and int I1 genes can be migrated into plants through rhizosphere soil,and biochars application can significantly reduce the accumulation of ARGs and int I1 genes in plants,and BC had stronger control ability than OBC.(2)Biochar can effectively control the migration of heavy metals in soil and plant system.Compared with the control without biochar,the bioavailable contents of Pb,Cu and Cd in rhizosphere and non-rhizosphere soils was significantly decreased(P<0.05),and there was no difference between rhizosphere soil and non-rhizosphere soil.Compared with BC,OBC was easier to fix heavy metals in soil,and the availability of metals decreases obviously,which indicated that biochar had a certain long-term stability in soil.And the fixation ability of the two biochars to heavy metals was: Pb > Cu > Cd.In addition,compared with BC,OBC reduced the ability of plants to absorb heavy metals,effectively reduced the bioavailability of heavy metals.Pearson's correlation analysis showed that available Cu and available Pb in rhizosphere soil were significantly correlated with tet X(r=-0.555-0.602,P < 0.01),and available copper was significantly correlated with int I1(r=-0.634,P < 0.01),indicating that the presence of heavy metals in plant rhizosphere had a significant impact on the migration and diffusion of ARGs in soil.(3)Biochar has a certain improvement effect on the soil organic matter content and soil enzyme activity of compound contaminated soil.Compared with non-rhizosphere soil,BC and OBC significantly increased the content of organic matter in rhizosphere soil(P<0.05),and BC was better than OBC.Biochar inhibited soil urease activity to some extent,and there was no difference between rhizosphere and non-rhizosphere soil.The alkaline phosphatase and sucrase activities in soil were increased by both biochars,and rhizosphere enzyme activity was higher than that of non-rhizosphere,and there was significant difference only in the proportion of 5% addition(P < 0.05).OBC had a more obvious promoting effect on enzyme activity than BC.In addition,Pearson's correlation analysis showed that soil organic matter and soil enzyme activity had certain effects on the distribution of soil ARGs and heavy metals. |
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