Soil-Dependent Microbial Metabolic Strategies Shape Microplastic-Induced Molecular Transformation of Dissolved Organic Matter

Abstract Microplastic contamination fundamentally alters terrestrial carbon cycling, yet the mechanisms linking polymer exposure to the formula-level transformation of dissolved organic matter (DOM) remain poorly resolved. Here, we integrate ultrahigh-resolution mass spectrometry with genome-resolved metagenomics to track dissolved organic matter turnover in contrasting agricultural soils exposed to conventional and biodegradable microplastics. In the two contrasting soils tested, DOM molecular trajectories diverged strongly with the soil background. Biodegradable microplastics were associated with extensive loss of putative lignin-like precursor formulas in a nutrient-limited loess soil. This loss coincided with highly expanded, precursor-dominated paired mass distance networks of up to 3882 edges and 2448 nodes, in which nodes present assigned molecular formulas and edges represent mass-difference-inferred putative transformations. Overwhelming negative microbial correlations characterized by 5247 negative versus 801 positive links were observed. Functionally, this pattern co-occurred with a community-wide shift toward resource-acquisition strategies, with normalized traits increasing by up to 205%. Conversely, in carbon-rich black soil, microplastics promoted selective molecular reworking and net product accumulation. Paired mass distance networks remained compact with approximately 1000 nodes, and metagenome-assembled genome networks were dominated by positive associations at 72.2%. These more stable dynamics were accompanied by a microbial shift toward growth-yield traits. By proposing a conceptual soil-polymer-microbiome framework derived from two contrasting soils, our findings caution against assuming that biodegradable microplastics are universally ecologically benign and suggest that they may intensify turnover of native organic matter in resource-limited soils, a possibility that requires direct testing with isotope-based approaches.