Microplastics influence organic carbon depletion in macroaggregates and soil structural stability in the Yanhe catchment

Different land uses greatly influence the stability of soil aggregates and the spatial distribution of soil organic carbon (SOC). However, limited knowledge exists on how microplastics within aggregate fractions affect SOC retention and soil structure stability. Hence, soil aggregates, microplastic abundance, aggregate-associated SOC, total nitrogen (TN) stock, and aggregate stability were assessed under various land use types within the Yanhe catchment. Five land use types were investigated: woodland, grassland, shrubland, orchard and farmland (corn field and cereal field). The results showed that soil aggregates were predominantly distributed in the < 0.25 mm aggregate fraction (59 %–77 %) across land use types, with shrubland showing relatively greater distribution in the > 2 mm aggregate fraction (35 %). Shrubland and woodland had the highest soil structure stability, and woodland had the largest aggregate-associated TN and SOC stock (27.67 and 310.31 g m −2 , respectively), predominantly stored in the >0.25 mm aggregate fractions. In contrast, farmland, particularly corn fields, exhibited the highest microplastic abundance (330.24 items kg −1 ), which was 2.9 times greater than in shrubland. Microplastics were concentrated in > 0.25 mm aggregates, decreasing as aggregate size decreased. Structural equation modeling revealed that microplastics significantly reduced soil aggregate stability by decreasing the >0.25 mm aggregate-associated SOC stock ( p < 0.05). This effect can be attributed to the higher porosity and more aerobic conditions of macroaggregate, which promote fungal proliferation and consequently accelerate SOC mineralization. Our study highlights a mechanistic link between microplastic accumulation, SOC stock and soil structure stability, providing empirical reference to inform strategies for microplastic pollution control.