Effects Of Single And Combined Cd-Polyethylene Pollution On Soil Organic Carbon And Microbial

Heavy metal pollution is one of the main environmental pollution problems of farmland soil in China.Cadmium（Cd）is a kind of highly toxic element and one of the most harmful heavy metals.microplastics（MPs）are a new soil pollutant,which can harm soil environmental health.The changes of soil organic carbon pool and the mechanism of carbon dioxide fixation to participate in the process of global carbon neutralization are hot issues at present.There are relatively few studies on the corresponding carbon neutralization of Cd and MPs pollutants.This study took Northeast phaeozem and Tianjin Tidewater soil as the research object,and adopted the method of exogenously adding heavy metal Cd and polyethylene microplastics（PE）to the two soils to study the changes and internal relations of soil available Cd,organic carbon components and soil bacterial community structure under the single and compound addition of different concentrations of Cd and PE.Based on the research results,to explore the characteristics of soil carbon dioxide emission and the changes of bacterial and fungal community structure under specific concentrations of single and combined Cd and PE pollution.From the perspective of achieving the"dual carbon"strategy of soil,this study provides a scientific basis for the impact of heavy metal and PE pollution on soil organic carbon and microorganisms.The main findings are as follows:（1）The effects of Cd and PE pollution on the composition of available Cd and organic carbon in soil were studied based on orthogonal experiments.The results showed that the addition of PE significantly increased the content of available Cd in black soil,but decreased the concentration of available Cd in tidal soil at the first day.The organic carbon（SOC）and mineral bound organic carbon（MAOC）were decreased by Cd treatment in black soil.Soluble organic carbon（DOC）first increased and then decreased with the extension of culture time,and tended to be stable after 15th day of culture.The SOC content increased with the increase of PE concentration in the tidal soil.From the perspective of time,MAOC showed a trend of first increasing and then decreasing.The effect of Cd and MPs on EOC was the same in the two soils,which was Cd>Cd PE>PE,the addition of PE was able to mitigate the effect of Cd on soil EOC.The content of particulate organic carbon（POC）in phaeozem was several times higher than that in fluvo-aquic soil,and the order of the influence of the two pollutants on POC was Cd PE>Cd>PE（fluvo-aquic soil）,Cd>Cd PE>PE（phaeozem）,composite treatment is reflected in the antagonistic effect.The MAOC content of both soils was greatly affected by MPs,there is a synergistic effect of Cd and PE on MAOC.（2）The effects of Cd and PE pollution on the structural characteristics of soil microbial were studied based on orthogonal experiments.The results showed that the influence of Cd and PE combined pollution on microbialαdiversity of phaeozem was greater than that of single Cd or MPs,while the influence of MPs and Cd combined pollution on microbialαdiversity of moist soil was not significant.In phaeozem,the addition of Cd significantly increased the simpson index,but the addition of MPs had no significant effect on microbialαdiversity.Under the combined treatment of MPs and Cd,theαindex was reduced to varying degrees compared with the control.The highest proportions of Proteobacteria,Acidobacteriota,Gemmatimonadota,and Actinobacteriota were found in the two soils.At the genus level,Sphingomonas,RB41,Subgroup＿7,and Vicinamibacteraceae were common genera with high proportions in both soils.The effects of Cd and PE on soil bacterial community structure were as follows:Cd>Cd PE>PE（phaeozem）and Cd PE>Cd>PE（fluvo-aquic soil）.（3）The effects of single and combined pollution of Cd and PE on the mineralization characteristics of soil organic carbon were studied by the laboratory organic carbon mineralization culture experiment was used.The results showed that in the fluvo-aquic soil,the microbial biomass carbon content of PE treatment was the lowest at the first day(151.24 mg·kg^-1),while that of Cd treatment was the lowest at 10th day(157.49 mg·kg^-1),and there was no significant difference between the microbial biomass carbon content of each treatment at 25th day and 45th day.However,the microbial biomass carbon content in phaeozem decreased with time,and was higher in the Cd and PE treatments than in the CK treatment at 45th day.The CO₂ emission flux in both soils was higher in the early stage and tended to be stable in the later stage.Phaeozem has higher organic matter and higher emissions than fluvo-aquic soil.The addition of two pollutants in phaeozem will increase the cumulative CO₂ emissions,and the two pollutants have a promoting effect on soil CO₂emissions,and the effects of Cd and PE on the cumulative CO₂ emissions show a superposition effect,and the promoting effect of Cd PE treatment is the strongest.Cd and Cd PE treatments had no effect on CO₂ emissions from the fluvo-aquic soil,while PE treatment alone had an inhibitory effect.（4）The effects of single and combined Cd and PE pollution on soil bacterial and fungal community structure were studied by the laboratory organic carbon mineralization culture experiment was used.The results showed that Cd and MPs could affect the structure of soil bacterial and fungal communities before and after cultivation in the mineralization experiments of soil organic carbon contaminated by single concentration of Cd and PE.The presence of Cd in phaeozem reduced the relative abundance of Firmicutes in soil,but had little effect in fluvo-aquic soil.The presence of PE can significantly increase the relative abundance of Proteobacteria and Cyanobacteria in phaeozem.Under the treatment of Cd PE,the relative abundance of Proteobacteria and Cyanobacteria in the two kinds of soil increased significantly.The presence of Cd significantly increased the relative abundance of Proteobacteria,while the composite treatment with PE added alleviated this trend.Two soil dominant fungal phyla were Ascomycota and Chytridiomycota.The genus-level fungal communities were quite different between the two soils.Fusarium was more abundant in phaeozem,and Chaetomium was significantly higher in fluvo-aquic soil than in phaeozem.