Differential aging processes of microplastics in paddy soil under wet-dry alternation: Insights into chemical structure alteration and dissolved organic matter formation

Paddy soil represents a critical sink for microplastics (MPs), where frequent redox oscillations from wet-dry alternation can accelerate MPs aging, and alter dissolved organic matter (DOM) composition in paddy soil. However, this process remains poorly understood to date. Here, we systematically investigated the aging of three MPs and their structural effects on DOM in paddy soil during wet-dry alternation. Following the seventh wet-dry alternation (98 d), the weight loss of MPs followed the order polypropylene (PP, 13.83 %) > polyethylene (PE, 8.20 %) > polyethylene terephthalate (PET, 4.23 %). PET underwent limited hydrolysis, whereas PE and PP exhibited significant C-C cleavage and oxidation (C-O-C/C=O). Three MPs initiated aging via C-H bond cleavage, followed by polymer-dependent functional group transformations. Fourier transform ion cyclotron resonance mass spectrometry demonstrated a more obvious reduction in the average molecular weight of DOM in paddy soil amended by PP, while DOM exhibited the highest aromaticity and double bond equivalent in paddy soil amended by PET after the seventh wet-dry alternation. The unique molecules of DOM in paddy soil with PE and PP have a higher oxidized degree, but higher biodegradability in paddy soil with PET. These findings provide novel insights into chemical structure alterations of MPs and DOM formation during wet-dry alternation in paddy soil, contributing to more precise projections for the environmental risk of aged MPs in agricultural ecosystems.