Diverse Impacts of Microplastic-derived Dissolved Organic Matter at Environmentally Relevant Concentrations on Soil Dissolved Organic Matter Transformation

Dissolved organic matter (DOM) is critical to soil ecosystems, with its dynamics influenced by exogenous substances like microplastics (MPs)-derived dissolved organic matter (MPs-DOM) from agricultural mulches. However, the impacts of MPs-DOM, especially at environmentally relevant concentrations, on soil DOM dynamics remain unclear. Here, we examined DOM transformation in yellow (YS) and black (BS) soils upon the addition of MPs-DOM, leached from biodegradable and nonbiodegradable mulches under ultraviolet irradiation (UV-MPs-DOM) and dark conditions (D-MPs-DOM), at environmentally relevant concentrations (3 mg C/kg). Results showed that extraction conditions, rather than mulch type, predominantly affected the bioavailability of MPs-DOM. UV-MPs-DOM, enriched in lipid-like and protein/amino sugar-like compounds, promoted soil DOM transformation. In YS, characterized by lower microbial diversity, UV-MPs-DOM enhanced DOM lability more than D-MPs-DOM. Conversely, in BS, with a diverse microbial community, UV-MPs-DOM with high bioavailability not only directly altered soil DOM composition but also was rapidly metabolized by the soil microbiome, particularly Proteobacteria, thereby resulting in increased soil DOM recalcitrance. However, the low bioavailability of D-MPs-DOM primarily exerted direct effects, contributing to its accumulation and increase in soil DOM lability. These findings provide novel evidence that MPs-DOM at environmentally relevant concentrations can alter soil DOM through distinct pathways, highlighting its potential long-term ecological risks.