Microplastics drive concentration-dependent shifts in soil carbon persistence by regulating microbial fungal-bacterial contribution and DOC composition

Soil organic carbon (SOC) persistence is influenced by microbial transformation of carbon (C) substrates, yet how microplastics (MPs) affect these processes remains unclear. We investigated the concentration-dependent effects of polyethylene microplastics (PE-MPs) on microbial C allocation and dissolved organic carbon (DOC) composition, using ¹³C-labeled glucose tracing combined with amino sugar biomarkers and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Three MP treatments were applied: 0% (control), 0.1%, and 1% (w/w). Results showed that 0.1% MPs significantly enhanced the accumulation of newly formed total microbial residue C, indicating a positive microbial C-pumping effect under low-level MP addition. In contrast, 1% MPs specifically promoted bacterial residue C while inhibiting fungal residue C accumulation. MPs also altered DOC molecular composition, leading to reduced modified aromaticity index (AI_mod), double-bond equivalence (DBE_wa), and the relative abundance of readily degradable compounds. Notably, 0.1% MPs elevated the proportion of thermodynamically limited transformations within DOC, accompanied by enhanced intra-group molecular transitions, collectively indicating a shift toward more efficient microbial anabolic pathways for C sequestration. Our findings reveal that MPs regulate SOC persistence by differentially modifying microbial fungal–bacterial contributions and DOC bioavailability, with low-level MPs enhancing soil C sequestration through efficient microbial residue formation.