Effect Of Microplastic Derived From Plastic Film On Soil Organic Carbon Pool And Carbon Dioxide Emission In Aridareas

Plastic film mulching has been a useful method to improve crop production,but plastic residues in cropland soil will fragmentize into microplastics（<5 mm）and could be a major source of microplastic in soil environments.The microplastics probably disguising a hidden contribution to soil carbon storage due to the main component of microplastics are carbon.Moreover,the accumulation of microplastics in the soils largely affected soil physicochemical properties and microbial activities,which would potentially influence the production of carbon dioxide（CO₂）in the soil.However,until now,the impact of microplastics originated from plastic film on soil organic carbon and CO₂emission remain unclear.In this study,we extracted and analyzed the carbon content originated from plastics in soils,and evaluated the contribution of microplastic-carbon to soil organic carbon in the mulched fields.Based on microcosmic experiment,investigated the mechanisms of microplastic effect on soil CO₂emission under the different microplastic concentration.The object of this study is to estimate the contribution of microplastic-carbon to soil carbon sequestration and investigate the impact of microplastic on soil greenhouse gas production.This project would assist in achieve carbon neutrality project in 2060 and improve agricultural sustainability.Results of this thesis are following:（1）The abundance of microplastics ranged from 4198 to 47420 pieces kg^-1(average12589 pieces kg^-1).The polymer type of microplastics are mainly polyethylene（PE）,and the dominated form of microplastic particles was was films（74.0%）,followed by beads（24.2%）and fibers（1.8%）.Small size microplastics（<500μm）accounted for 94.3%.Moreover,we found positive relationship between the soil salt content with the proportion of 200-500μm microplastics and negative relationship with the proportion of 20-100μm microplastics,indicating that soil salinization probably induced the fragment of plastic film into microplastics with 200-500μm size,but inhibited its further crushed into smaller microplastics.The average carbon content of microplastic was 8.67 mg C kg^-1,and its contribution to soil organic carbon pool was about 1.59‰（0.05-14.24‰）.（2）The effects of different doses of low-density polyethylene（LDPE）microplastics on CO₂emission in dryland soil were evaluated by microcosm experiment.Compared with the soils without microplastics,low doses（0.01%and 0.1%）of both virgin and aged microplastics had negligible effects on soil organic carbon decomposition,whereas a high dose（1.0%）of these two microplastics significantly（p<0.05）accelerated the production of CO₂in soils by 15-17%,showing a dose effect.The presence of microplastics did not significantly affect soil dissolved organic carbon and microbial biomass carbon.Exposure to virgin microplastics significantly reduced the functional genes related to hemicellulose（abf A and man B）degradation（p<0.05）,whereas increasing the aged microplastics concentrations significantly decreased the abundances of functional genes encoding starch（sga）,hemicellulose（abf A,man B and xyl A）,and cellulose（cex）hydrolysis（p<0.05）.The1.0%concentration of microplastics increased the metabolic entropy（q CO₂）of soil microorganisms,indicating that microorganisms were stressed in the process of growth and metabolism,and more organic matter needed to be decomposed to obtain substrates and energy to maintain their own growth,which may be the key driving mechanism for promoting soil CO₂emission.