Effects Of Microplastics On Loessal Soil Aggregate Stability And Soil Organic Carbon Mineralization

With the increase of the use years of agricultural film in the arid and semi-arid areas of Northwest China,the content of microplastics in the soil gradually increased.In order to find out the effect of microplastics on the stability of farmland soil aggregates,farmland soil was collected in this study,and 25μm polyethylene microplastics（PE-MPs）and 1 mm PE-MPs were added at different concentration ratios(0.06,0.6,1.92,3.20,6.40 g·kg^-1)were pressed into soil aggregates and then cultured indoors.The stability of the aggregates,The mineralization rate of aggregate organic carbon and the change characteristics of soil microbial biomass carbon,and a structural equation model was constructed to quantify the influence of various factors on soil aggregates,in order to provide a basis for farmland soil microplastic management and soil conservation.The main conclusions are as follows:1.Microplastics can increase the microbial biomass in soil aggregates,and the effect of 1 mm microplastics with larger particles is more obvious.Adding 25μm PE-MPs to the soil with high organic matter level can increase the soil microbial biomass carbon content by 24.40%～93.50%within the range of 0～6.4 g·kg^-1.However,adding 1 mm PE-MPs to soil with high organic matter level could increase soil microbial biomass carbon content by 29.76%～109.89%.Adding 1 mm of PE-MPs to the soil with low organic matter level can increase the soil microbial biomass carbon content by24.87%～72.29%within the range of 0～6.4 g·kg^-1.2.Smaller particle size microplastics can promote soil organic carbon mineralization rate,while larger particle size microplastics can inhibit soil organic carbon mineralization rate.The microplastics with smaller particle size significantly improved the organic carbon mineralization rate of soil aggregates,and the 25μm PE-MPs increased the organic carbon mineralization rate of soil aggregates at low and high organic matter levels by5.30%～38.63%,respectively.and 7.92%～15.84%.Microplastics with larger particle size significantly reduce the mineralization rate of soil aggregate organic carbon,and 1 mm PE-MPs reduce the rate of soil aggregate organic carbon mineralization at low and high organic matter levels by 1.51%～31.06%,respectively.and 22.77%～33.16%.3.The effect of microplastics on soil aggregate stability is related to soil organic matter level,microplastic particle size and microplastic concentration.Microplastics will reduce the stability of poor soil aggregates with low organic matter content and increase the risk of soil structure degradation.However,microplastics can enhance the aggregate stability of soil with high organic matter content,and microplastics with larger particle size have more obvious enhancement effect.The addition of microplastics increased the stability of soil aggregates as the laboratory incubation time increased.Compared with 25μm PE-MPs,the effect of adding 1 mm PE-MPs on the water stability of the aggregates was more obvious.Adding 1 mm of PE-MPs to soils with low and high organic matter levels increased the mean mass diameter（WMWD）of water-stable aggregates by 1.22%～78.15%and 2.72～34.05,respectively.When adding 25μm PE-MPs to soils with low and high organic matter levels,the mean mass diameter（DMWD）of force-stable aggregates was increased by 14.16%～59.60%and 9.24%～46.31%,respectively.Adding 1 mm PE-MPs The improvement of DMWD was 33.44%～65.66%and31.45%～49.23%respectively.The water stability of soil aggregates after adding microplastics is related to the particle size of microplastics.When 25μm PE-MPs were added to soils with low and high organic matter levels,the total amount of soil water-stable aggregates(WR_0.25)decreased by4.17%～13.36%and 0.16%～21.06%after 6 months of indoor cultivation,respectively.%;adding 1mm PE-MPs increased WR_0.25by 3.88%～19.05%.The water stability of soil aggregates after adding microplastics was related to the level of soil organic matter.Similarly,when 1 mm PE-MPs was added,after 6 months of indoor culture,the WMWD decreased by 4.39%～28.08%at low organic matter levels;while at high organic matter levels,WMWD increased by 5.73%～61.15%.4.Microplastics indirectly affect aggregate stability by affecting soil microbial biomass carbon content and the mineralization rate of soil organic carbon.Diameter of microplastics indirectly affects the content of water-stable aggregates by affecting the rate of soil organic carbon mineralization and soil microbial biomass carbon content,with indirect effect coefficients of-0.55 and 0.13;And directly affecting the content of water-stable aggregates,the level of soil organic matter will indirectly affect the proportion of water-stable aggregates by affecting the mineralization rate of soil organic carbon and soil microbial biomass carbon,and the indirect effect coefficients are 0.65 and0.44,respectively。...