| Due to the rapid development of industrialization and urbanization,soil heavy metal pollution has become an important environmental problem in China,seriously threatening soil productivity,agricultural safety and human health.As the most active and sensitive index in soil ecosystem,microorganisms play an important role in promoting soil quality and plant health.Once the soil is polluted by heavy metals,it will not only lead to the change of microbial community structure,but also affect the functional diversity and various biochemical processes of the soil ecosystem.In order to better understand the health of soil and the relationship between heavy metal pollution and microbial community,typical heavy metal compound contaminated farmland downstream of Dachang mining area in Guangxi was taken as the research object.High-throughput sequencing technology was adopted to analyze microbial diversity,community structure composition and function,and explore the interaction and mechanism between microorganisms and heavy metals,so as to provide a basis for soil environmental evaluation and remediation of polluted farmland.(1)The study area was mainly polluted by Sb,Cd,As,Zn and Pb,with the average contents of 451.09,7.77,247.96,1182.41 and 954.35 mg·kg-1,respectively.100%,100%,97.5%,80%and 75%of the points for Sb,Cd,As,Zn and Pb exceeded the risk screening values in China's agricultural land soil control standards,respectively.The results of the potential ecological risk assessment showed that the farmland in the study area showed a strong ecological risk on the whole.Sb and Cd had the highest contribution rate(71.52% and 23.02%,respectively).From the perspective of the spatial distribution,Cd and Zn contents had a similar spatial distribution.There were two obvious high value zones distributed in the northwest and the middle.While,As,Sb and Pb contents were similarly distributed and decreased from southwest to northeast.In terms of land use types,the contents of As,Pb and Sb were higher in dryland soil than in paddy soil,which were 1.47,2.03 and 1.88 times of that in paddy soil,respectively.The contents of Zn and Cd in paddy soil were higher than that in dryland soil,which were 1.32 and 1.13 times,respectively.Source analysis showed that the pollution of Sb,Pb and As mainly came from the input of human mining activities,while the pollution of Cd and Zn was the result of the comprehensive action of human mining activities and natural factors.Soil heavy metal pollution also causes soil acid pollution,which leads to the increase of the variation of soil available heavy metal content.Compared with the corresponding total content,the variation degree of available Cd,Pb and Zn content was significantly increased.In both paddy soil and dryland soil,the available Cd,Pb and Zn contents were significantly correlated with the total amount of As,Sb,Pb and pH,and their spatial distribution patterns are similar.The average contents of available Zn,Cd and Pb in dryland soil were significantly higher than that in paddy soil,which were 3.75,1.96 and 4.25 times of that in paddy soil respectively.However,the available Sb content in paddy soil was higher than that in dryland soil.(2)In paddy soil,Proteobacteria,Chloroflexi,Acidobacteria and Actinobacteria were dominant bacterial phyla,and sum of average relative abundance was 80.73%.The dominant bacterial orders were norank?c?Acidobacteria,Anaerolineales and Rhizobales,with a prevalence of 10.38%,10.09%and 5.00%,respectively.Ascomycota was the most abundance fungal phylum,with an average relative abundance of 77.20%.The dominant fungal orders were Hypocreales,Sordariales,unclassified?k?Fungi and unclassified?p?Ascomycota,and the average relative abundance were 31.48%,12.91%,10.71%and 10.04%,respectively.The pollution of heavy metals As,Sb,Pb as well as available Cd and Zn has a great influence on the structure and diversity of microbial community in paddy soil,while bacteria of Chloroflexales,Gemmatimonadales,Myxococcales,Solibacterales,Subgroup?7 and fungi of Agaricales had strong tolerance to these heavy metals.The bacteria of Acidimicrobiales,Gaiellales,norank?c?S085 and Solirubrobacterales might decrease the accumulation of Cd in rice grains,while Gemmatimonadales,Myxococcales Solibacterales and fungi of Rozellomycota might also decrease the As.(3)In dryland soil,Proteobacteria,Chloroflexi,Acidobacteria and Actinobacteria were also the most dominant bacterial phyla,accounting for 76.78%of gene sequencing.The dominant bacterial orders were norank?c?Acidobacteria,Rhizobiales,Bacillales and Anaerolineales,and their average relative abundance were 9.09%,6.84%,5.75%and 5.19%,respectively.Ascomycota showed the highest average relative abundance(63.35%)in fungal communities.The dominant fungal orders were Hypocreales and Mortierellales,with an average abundance of 29.81%and 18.68%,respectively.The available Cd,Pb and Zn contents had a great influence on the structure and diversity of microbial community in dryland soil,while bacteria of Acidobacteriales,Gemmatimonadales,JG30-KF-AS9,Ktedonobacterales,Myxococcales,Planctomycetales,Solibacterales,Subgroup?7 and fungi of Tremellales had strong tolerance to these metals.Physical and chemical properties such as pH and organic matter play an important role in regulating the adaptation of microorganisms to heavy metal pollution.(4)There were significant differences in microbial diversity and community structure between paddy soil and dryland soil.There were 82.19%of shared bacterial OTUs in dryland and paddy soil,while the number of fungal OTUs only accounted for 47.65%.The bacteria of Anaerolineales,Myxococcales,norank?c?KD4?96,norank?c?SBR2076,43F-1404R,Desulfuromonadales,Acidiferrobacterales,norank?c?Subgroup?7 and fungi of unclassified?p?Ascomycota,unclassified?c?Agaricomycetes,Agaricales and unclassified?c?Sordariomycetes were obviously enriched in paddy soil.However,the bacteria of Bacillales,Rhizobiales,Rhodospirillales,Micromonosporales,Desulfurellales and fungi of Mortierellales,Tremellales,unclassified?c?Dothideomycetes and Xylariales were abundant in dryland soil.(5)A total of bacterial 41 subfunctional groups from 6 metabolic pathways were found in farmland soil contaminated by heavy metals.Among these bacterial functions,the main metabolic function groups were Carbohydrate metabolism,Amino acid metabolism and membrane transport.Heavy metal pollution was the main factor driving the change of bacterial metabolic function.The heavy metal resistance functions of bacteria in dryland soil were Glycan biosynthesis and metabolism,Cell motility and Biosynthesis of other secondary metabolites,which were significantly positively correlated with most heavy metals.Cd and Zn were the main elements that affected the metabolic function of bacteria in paddy soil,and Amino acid metabolism,Xenobiotics biodegradation and metabolism,Lipid metabolism and Biosynthesis of other secondary metabolites were the heavy metal resistance functions.The soil fungi communities could be classified into 8 ecological function groups,and the average abundance of them differed greatly between dryland and paddy soil.The unassigned ecological function group in dryland soil accounted for 19.83%,while it was 42.82%in paddy soil.The dominant ecological functional groups of dryland soil fungi were Saprotroph(27.65%),Pathotroph-Saprotroph-Symbiotroph(26.83%)and Pathotroph-Symbiotroph(17.65%).The dominant ecological functions of fungi in paddy soil were Saprotroph(29.46%)and Pathotroph-Saprotroph-Symbiotroph(18.21%).Each ecological function group had a great degree of variation,and their responses to soil environmental factors were different.The influence of soil physical and chemical properties was greater than that of heavy metals.The patho-Saprotroph-symbiotroph had strong tolerance to Cd and Zn in paddy soil.However,there was a significant positive correlation between the available heavy metal contents and Symbiotroph in dryland soil. |
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