Effects Of Degradable Microplastics On Greenhouse Gas Emissions Of Soil And Aggregates

Biodegradable plastics such as polylactic acid（PLA）,which retain the performance of plastic products and are environmentally harmless,are currently recognized as the best solution to the problem of microplastic pollution.However,the accumulation and degradation of PLA microplastics in soil will have an impact on the carbon and nitrogen cycle in the soil,which may exacerbate the production of greenhouse gases in the soil.Soil aggregates play an important role in determining soil structure and function and regulating soil nutrient cycling,and the degradation and transformation of PLA in soil is closely related to soil aggregates,so it is necessary to study the interaction mechanism between microplastic pollution and soil,especially aggregates.In this study,three soil types,lou soil,brown soil and cinnamon soil,and soil aggregates with>2 mm,1～2 mm,0.25～1 mm and<0.25 mm granular scales were used as research objects,and indoor culture experiments were carried out to investigate the effect of PLA microplastics on soil carbon and nitrogen emissions.The main results are as follows:（1）Moderate concentrations of PLA increased the DOC content of lou soil,brown soil and cinnamon soil.The addition of PLA slowed down the decline rate of NH₄⁺-N content in lou soil,but increased the decline rate of NH₄⁺-N content in brown soil and cinnamon soil,among which the content of cinnamon soil decreased by 22.88%,26.00%and 23.48%compared with the control,and only 1.66%decreased by high concentration treatment in brown soil.In the lou soil and cinnamon soil,the fastest increase rate of NO₃^--N content was treated with low amount of PLA,the lou soil increased by 15.80%,and the cinnamon soil decreased by 4.80%;brown soil was treated with high amounts of PLA,and the content increased by 1.44%.In soil aggregates<0.25 mm,there were significant differences between soil DOC content with different concentrations of PLA microplastics,and DOC content decreased by about 13.02%,23.83%and 3.24%.Similarly,under the treatment of high concentration PLA,the DOC rate of soil aggregates with 0.25～1 mm particle level decreased the fastest,and the content increased by 11.56%compared with the control.There were significant differences between the cumulative contents of NH₄⁺-N and NO₃^--N in different particle sizes,and the contents of NH₄⁺-N in>2 mm and 1～2 mm particle aggregates treated with PLA increased compared with the control,while the other two particle levels decreased.Except for the NO₃^--N content of 0.25～1 mm aggregates treated with high concentration PLA treatment and>2 mm aggregates treated with low concentration PLA decreased by 2.63%and 2.05%,the other treatment contents increased.（2）The addition of PLA microplastics increased the soil CO₂emission flux,and the cumulative CO₂emissions of brown soil treated with high concentration PLA microplastics increased significantly by 1.11 times compared with the control group.The addition of PLA microplastics reduced the cumulative emission of N₂O,and there were significant differences in the cumulative emission of soil N₂O treated with low,medium and high concentration PLA microplastics,the lou soil N₂O emission decreased by 35.66%,66.47%and 42.13%,respectively,the emission of N₂O in brown soil was reduced by 45.57%of the low PLA treatment,and the N₂O emission after adding low concentration PLA in cinnamon soil increased by about 12.50%compared with the control group.（3）The effect of PLA on CO₂emissions of soil aggregates had a dose effect,and the CO₂emissions of 1～2 mm soil aggregates with low concentration of PLA microplastics were significantly reduced by 52.18%compared with the control group.The cumulative CO₂emissions of soil aggregates at the>2 mm,0.25～1 mm and<0.25mm granular levels increased by 8.97%,18.31%and 10.94%,respectively.The addition of PLA to aggregates reduced the N₂O emissions of aggregates,and the largest reduction in N₂O cumulative emissions was 0.25～1 mm granular level soil aggregates,and after adding low,medium and high concentrations of PLA,the cumulative emissions were reduced by 45.99%,57.91%and 42.47%,respectively.