[Effects of Polyethylene Microplastics with Different Particle Sizes on Soil Organic Carbon Characteristics and Mineralization in Agricultural Soil].

Abstract

The contamination of agricultural fields by microplastics （MPs） has emerged as a prominent issue of current concern. A 180-day indoor soil cultivation experiment was conducted to investigate the effects and potential mechanisms of various particle sizes [millimeter-scale （mMP）, micrometer-scale （μMP）, and nanometer-scale （nMP）] of polyethylene microplastics （PE-MP） on the mineralization of soil organic carbon （SOC） and changes in its characteristic components. The results indicated that while PE-MP initially （0-13 days） promoted SOC mineralization, it inhibited it over the long term （180 days）, with larger PE-MP particles exhibiting a more pronounced inhibitory effect. PE-MP significantly increased the proportion of SOC distribution within large （>2 mm） and small （0.25-2 mm） aggregates while reducing it in micro-aggregates and silt-clay fractions, with larger-sized PE-MP （mMP and μMP） showing a more pronounced effect, suggesting that the physical protection of SOC by larger aggregates may be a key mechanism for reducing SOC mineralization. Larger-sized PE-MP （mMP） significantly increased the content of active organic carbon [readily oxidizable organic carbon （ROC） and dissolved organic carbon （DOC）] in soil, whereas smaller-sized PE-MP （nMP） significantly decreased DOC content. PE-MP notably increased the content of light fraction organic carbon （LFOC） while significantly decreasing that of heavy fraction organic carbon （HFOC）, with the degree of reduction becoming more pronounced with decreasing particle size. Furthermore, PE-MP significantly reduced the humification degree of dissolved organic matter （DOM）, with smaller particle sizes leading to higher reductions. Correlation analysis and structural equation modeling further revealed that particle size mediated the differential impacts of PE-MP on the contents of various organic carbon characteristic components, degree of DOM humification, and the proportion of SOC in large and small aggregates, thereby influencing the ultimate intensity of SOC mineralization （cumulative CO₂ emissions）. In summary, larger-sized PE-MP facilitated SOC retention and inhibition of its mineralization in agricultural fields； however, as particle size decreases, this beneficial effect may diminish and even become detrimental. These findings provide valuable scientific theoretical foundations for the scientific assessment of the environmental effects of PE-MP in agricultural fields and for effective pollution prevention and control measures.