Altered litter chemistry under pollution predicts fungal-mediated decomposition decline in a subtropical forest

Ecological risk assessments typically prioritize direct toxicity, often overlooking indirect cascades that reshape ecosystem function. By incubating field-collected litter in a standardized, metal-free environment, we assessed the integrated 'litter legacy' (encompassing pollution-altered substrate quality and resident microbes) independent of direct soil toxicity. We found that litter from polluted sites exerted a strong constraint on decomposition, driven by a dual legacy: a shift in resource stoichiometry (characterized by a stress-induced low C:N ratio) and a pre-simplified fungal community. Specifically, this legacy effect acted as a selective filter that perpetuated the loss of fungal diversity and specialized decomposers, while bacterial communities showed high functional redundancy. Structural equation modeling revealed that this fungal diversity loss, maintained by the litter legacy, was the primary driver of functional decline. Our study highlights that the 'resource-mediated legacy' acts as a persistent ecological filter that decouples soil improvement from functional recovery, suggesting that restoring the functional integrity of polluted ecosystems requires strategies that explicitly address plant defense traits and fungal community re-establishment.