Effects Of Biodegradable/Non-Biodegradable Microplastics On The Properties And Bacterial Communities Of Cadmium-Contaminated Soil Under Continuous Dry-Wet Alternation

Microplastics and cadmium(Cd)can infiltrate soil ecosystems via a variety of routes and accumulate in the food chain,harming human health.They are currently the subject of great global attention.Microplastics and cadmium pollution are ubiquitous in soil,and their combined pollution is inevitable.Hence,research on how microplastics convert Cd and the effects of their combined pollution on soil ecology is essential for preventing soil pollution in agriculture.Rice is an important food crop and microplastics and Cd are two of the main pollutants that affect paddy soil.Rice growth is typically controlled using dryland,flooding,and drying management strategies.,however,regarding the effect of the microplastic intervention on Cd-contaminated soil in the dryland-flooding-drying cycle,uncertainty still exists.As a result,the researchers chose to focus on non-biodegradable plastics(Polyethylene,PE)and biodegradable plastics(Poly(butylene adipate-co-butylene terephthalate),PBAT).The addition percentages were chosen at 0.5%,1%,and 2 %(w/w).The impacts of biodegradable/non-biodegradable microplastics of two shapes(i.e.,film and particle)on soil characteristics,enzyme activities,Cd transformation,and soil ecological function under dryland-flooding-drying mode were investigated using a soil incubation experiment.Our research provides both theoretical and empirical support for the cleanup and management of soil contaminated by microplastics and Cd.The results are as follows:(1)PBAT and PE microplastics affected soil properties,with film microplastics showing more changes in soil properties.PBAT increased soil organic matter(0.1%～33.2%),dissolved organic carbon(DOC)(5.9%～162.1%),ammonium nitrogen(2.2%～457.0%),and reduced soil available phosphorus(1.2%～18.2%),nitrate nitrogen(0.4%～91.5%),total dissolved nitrogen(TDN)(16.3%～67.1%).PBAT increased soil pH(0.08～0.31 units)at dryland and drying stages while decreasing soil pH(0.04～0.25 units)at the flooding stage.PE decreased DOC(4.9%～24.3%)and increased TDN(4.8%～71.5%).(2)PBAT increased the activities of urease(2.7%～374.5%),dehydrogenase(16.3%～559.8%)and catalase(1.2%～56.3%)in soil,and the effect of PBAT film was more significant.However,PE had no significant effect on the activities of urease,dehydrogenase and catalase in soil.PE film decreased the activities of urease(5.3%～32.8%)in soil and increased the activities of dehydrogenase(22.4%～23.3%)at the flooding stage.(3)Different microplastics had different effects on the fractions and mobility of Cd at different stages.At dryland and drying stages,1 % and 2 %PBAT film and particle significantly promoted the conversion of reducible Cd into acid extraction Cd and reduced the Reduced partition index(IR)of Cd to soil(6.4%–12.6%);PE particle had no significant effect on the fractions of Cd in soil,while PE film reduced the oxidizable Cd in soil.Additionally,the fractions of Cd in soil changed after the flooding stage,although the impact of microplastics was minimal.In addition,stepwise regression analysis and path analysis showed that soil acid extraction Cd and reducible Cd were mainly affected by DOC content.(4)Following incubation,PBAT and PE treatments significantly increased the relative abundance of Bacteroidota(1.87%～14.65%)and Firmicutes(1.61%～9.24%);meanwhile,PBAT significantly increased the relative abundance of Proteobacteria(9.93%～17.99%),while PE significantly decreased it(3.2%～6.7%).Moreover,whether PBAT or PE,the effect of film microplastics on the bacterial community at the soil phylum level was greater than that of particle microplastics.In addition,Bradyrhizobium,Steroidobacter,Monoglobus,and Methylotenera,which perform nitrogen fixation,denitrification,and organic degradation processes,were shown to be the biomarkers in the soil treated with microplastics.The analysis of the function of soil bacteria showed that PBAT increased the functional genes of Xenobiotics biodegradation and metabolism(13.1%～24.1%),Lipid metabolism(5.4%～6.8%),Membrane transport(4.6%～5.9%)and Metabolism of terpenoids and polyketides(4.7%～5.8%).PE increased the Metabolism of other amino acids(0.8%～0.9%)and Glycan biosynthesis and metabolism(11.2%～29.8%).The results indicated that soil bacterial population and function were altered by microplastic intervention and that this had an impact on soil carbon and nitrogen cycling as well as microplastic decomposition.(5)Special bacterial groups were enriched on the surface of PBAT and PE films.Proteobacteria on the surface of PBAT film is the dominant group with a relative abundance of 87.80%,while the relative abundance of Actinobacteriota(19.83%)and Proteobacteria(34.52%)on the surface of PE film was increased,and other bacteria on the surface of microplastics was decreased,indicating that Proteobacteria and Actinobacteriota could participate in the degradation of microplastics and preferentially utilize the carbon of microplastics.They had significant competitive advantages in the ecosystem and could quickly become the dominant bacteria on the surface of microplastics.In addition,Pseudomonas,which contains a large number of pathogenic bacteria,was enriched on the surface of PBAT and PE films,with a relative abundance of 68.05% and 12.76%,respectively.