Effects Of Continuous Application Of Livestock Manure On The Accumulation Of Heavy Metals And The Diversity Of Soil Microbial Community In Vegetable Garden Soil

Livestock manure provides a variety of nutrients and organic matter for vegetable field soil,which changes the microenvironment of vegetable field soil.With the development of large-scale and intensive farms in China,untreated livestock manure has led to many heavy metals entering the soil environment,especially the pollution of heavy metals in vegetable soil caused by continuous application of livestock manure.However,there is still a lack of research data on the accumulation of heavy metals in vegetable soil and the pollution risk and evaluation caused by the continuous application of livestock manure.At the same time,the effect of continuous application of livestock manure containing heavy metals on soil microbial community diversity in vegetable fields is not clear.Therefore,in this paper,two vegetable bases located in Bishan and Beibei district of Chongqing were selected to study the effects of livestock manure types,dosage and treatment methods on the accumulation of heavy metals Cu,Zn,Pb,Ni,Cd,Cr and Hg in vegetable soil for 6 years,and the pollution degree of heavy metals in soil of vegetable soil was evaluated.The main result are as follows:(1)The application of livestock manure for 6 years increased the accumulation of heavy metals in soil of vegetable fields in monitoring sites.During the two years from March 2018 to July 2019,the heavy metal elements Cu,Zn,Pb,Cr and Hg in the vegetable garden soil of Bishan monitoring point increased by 2.99%-162.08%,3.22%-88.62%,5.28%-38.79%,5.14%-28.48% and 5.01%-191.95%,respectively,but the content of Ni decreased by 12.60%-24.27%.The Zn and Hg in the vegetable garden soil of Beibei monitoring point increased by 1.94%-8.96% and 7.52%-51.89%,respectively.The contents of Cu,Pb and Ni decreased by 5.25%-15.68%,6.83%-15.19% and 15.38%-23.31%,respectively.(2)The application of livestock manure for 6 years changed soil acidity and alkalinity in vegetable fields at monitoring sites.The soil pH of the Bishan monitoring point vegetable garden increased with the increase of the amount of pig manure applied.The application of chicken manure lowered the soil pH of the vegetable field monitored by Bishan.The treatment of decomposed chicken manure lowered the pH of the soil in the vegetable gardens monitored by Beibei.In the correlation analysis,there was a significant correlation between soil pH and heavy metal Ni at Bishan monitoring site;There was a significant correlation between soil pH and heavy metals Pb and Cr in vegetable gardens at Beibei monitoring points.There was a significant correlation between Cu-Ni and Cu-Hg(p< 0.05),and there was a very significant correlation between Cu-Pb and Pb-Ni.The heavy metals Cu in the vegetable garden soil of Beibei monitoring point had significant correlations with Zn,Pb,Ni,Cd,Cr and Hg.Pb-Ni,Pb-Cd,Pb-Cr,Pb-Hg had significant correlation(p < 0.01).Cd was not correlated with Cr but correlated with other elements.(3)Single factor pollution index indicated that after continuous application of livestock and poultry manure,six heavy metals(Cu,Zn,Pb,Ni,Cr,Hg)in the vegetable soil of Bishan monitoring point and Beibei monitoring point were at clean level,Cd in Bishan monitoring point was at severe pollution,and Beibei monitoring point was at moderate to severe pollution.And Nemerow pollution index points out that the two monitoring points are in severe pollution.(4)According to the address accumulation index method,after the continuous application of livestock manure,the soil heavy metals Ni and Cr in vegetable fields at Bishan monitoring point were not polluted,Pb was slightly polluted,and Cd and Hg were above medium-intensity pollution.Continuous application of livestock manure caused serious pollution of Pb,Cd and Hg in vegetable soil of Beibei monitoring points.(5)According to the potential ecological risk index,after the continuous application of livestock manure,the five heavy metals of Cu,Zn,Pb,Ni and Cr in the vegetable soil of Bishan monitoring point and Beibei monitoring point were at low potential risk levels,and the potential risk levels of Cd and Hg were above high.And continuous application of livestock manure makes the two-monitoring points vegetable soil comprehensive potential risk degree is very strong.(6)Under the treatment of the application of livestock manure for 6 years,the soil microbial diversity of vegetable field in Bishan monitoring point was higher than that in the control treatment,while there was no uniform law for the change of soil microbial diversity in Beibei monitoring point.Continuous application of livestock manure increased Actinobacteriota and Proteobacteria in vegetable soil at Bishan and Beibei monitoring sites(7)At genus level,the application of livestock manure for 6 years reduced the relative abundance of norank?f?JG30-KF-AS9 in vegetable soil at Bishan monitoring point,and chicken manure and pig manure increased the relative abundance of Mortierella.The application of chicken manure decreased the relative abundance of Bacillus and increased the relative abundance of Olpidium in the vegetable field of Beibei monitoring point.(8)After the application of livestock manure for 6 years,Firmicutes in the soil of Bishan vegetable garden had a significant correlation with Zn and Cd at the phylum classification level,while Chloroflexi had a significant negative correlation with Ni and pH.Monoblepharomycota was also significantly positively correlated with Cd.Zoopagomycota was significantly correlated with Zn,Cd and pH.In the soil of Beibei vegetable garden,Chloroflexi was significantly positively correlated with Zn,and Ascomycota was significantly positively correlated with pH.Heavy metal Zn generally affects the composition of soil microorganisms in vegetable gardens.