Differential effects of microplastics on soil organic carbon via lignin phenols and amino sugars in soil aggregates

Accumulation of plant- and microbial-derived carbon together determines the stability of soil organic carbon (SOC) pools. To explore the accumulation characteristics and comparative contributions of organic carbon from diverse origins within soil aggregates exposed to microplastic pollution, we conducted corn pot experiments using biodegradable (polylactic acid; PLA) and conventional microplastics (polyethylene; PE) to examine the effects of microplastic pollution on microbial and plant derived carbon in macroaggregates (0.25–2 mm) and microaggregates (< 0.25 mm). Exposure to PE and PLA microplastics diminished the microbial necromass carbon in macroaggregates and its contribution to SOC, whereas their effects on microbial necromass carbon in microaggregates fluctuated depending on microplastic concentration. Due to the increased turnover rates and structural modifications experienced by biodegradable microplastics, PLA treatments yielded superior microbial necromass carbon content than PE treatments. PE reduced plant-derived carbon and its contribution to SOC across both aggregates, whereas PLA exerted a dosage-related effect on plant-derived carbon. Remarkably, 87%–408% of additional SOC in PLA treatment was primarily derived from other carbons, possibly indicating the inclusion of microplastic derived carbon in SOC assessments, thereby inflating SOC estimates. Under microplastic pollution, plant derived carbon dominated the accumulation of SOC in macroaggregates, whereas microbial derived carbon dominated SOC accumulation in microaggregates. Furthermore, macroaggregates exhibited higher SOC concentrations than microaggregates, with PLA treatments demonstrating superiority over PE treatments. Remarkably, microplastics supported the stability of SOC in microaggregates without exerting similar effects on macroaggregates.