Chemodiversity of soil dissolved organic matter affected by contrasting microplastics from different types of polymers

Chemodiversity of dissolved organic matter (DOM) is a crucial factor controlling soil nutrient availability, greenhouse gas emissions, and pollutant migration. Microplastics (MPs) are widespread pollutants in terrestrial ecosystems in many regions. However, the effects of MPs on DOM chemodiversity are not sufficiently understood, particularly under different types of polymers. Using UV-Vis spectroscopy, 3D fluorescence spectroscopy, and Fourier-transform ion cyclotron resonance mass spectrometry, the effects of three prevalent MPs [polyethylene, polystyrene, and polyvinyl chloride (PVC)] on the chemical properties and composition of soil DOM were investigated via a 310-d soil incubation experiment. The results showed that MPs reduced the aromatic and hydrophobic soil DOM components by more than 20%, with PVC MPs having the greatest effect. Furthermore, as MP contents increase, the humification level of soil DOM significantly decreases. MPs increased DOM molecules with no heteroatom by 8.3%-14.0%, but decreased DOM molecules with nitrogen content by 17.0%-47.8%. This may be because MPs cause positive "priming effect," resulting in the breakdown of bioavailable components in soil DOM. This is also related to MPs changing microbial richness and diversity and enriching microbial communities involved in lignin compositions degradation. In the presence of MPs, soil DOM chemodiversity depended on soil pH, electrical conductivity, dissolved organic carbon, soil organic matter, bacterial Shannon, and fungal Chao index. Specifically, DOM in MP-contaminated soils featured more lipids and less condensed aromatics and proteins/amino sugars, thereby conferring a lower DOM aromaticity and higher lability.