Temporal dynamics of soil dissolved organic matter influenced by biochar and microplastics: Insights from molecular and optical signatures

Microplastic (MP) pollution in soil has raised concerns about its impact on the dynamics of dissolved organic matter (DOM). Biochar (BC) influence MP-induced DOM dynamics, the specific mechanisms, especially for iron-modified biochar (FeBC), remain poorly understood. Here, we investigated the effects of BCs and FeBCs on DOM composition in paddy soil amended with biodegradable (polybutylene succinate, PBS) and conventional (polystyrene, PS) MPs over a 60-day incubation, using combined optical (EEM-PARAFAC) and ultrahigh-resolution mass spectrometric (FT-ICR MS) characterization. Unmodified BCs increased DOM solubilization and increased the relative enrichment and abundance of aromatic DOM components under the present incubation conditions, whereas FeBCs accelerated microbial activity (biological index, BIX: 1.11–1.47 vs. 0.76 in control) and DOM turnover. In PBS-amended soils, FeBC enhanced dissolved organic matter release (DOC: 474 mg/kg in PBS+FeBC₆ vs. 279 mg/kg in control), leading to the accumulation of labile, low-molecular-weight organic fractions (molecular lability boundary index, MLB_L index: 0.211 vs. 0.152 in control), and molecular turnover (time-decay slope: 0.069). In contrast, PS-enriched soils retained more aromatic, lignin-like, and recalcitrant DOM fractions. PBS treatments showed time-decay patterns (slope=0.007–0.069) consistent with rapid DOM transformations, while PS-amended soils maintained chemically stable DOM pool (slope=0.006–0.017). Thermodynamic modeling showed that PBS-induced transformations were largely spontaneous (ΔG° ≤ 0), while PS required energy input (ΔG° > 0). These findings highlight how BC and Fe modification distinctly regulate MP-induced DOM dynamics, with implications for carbon persistence and soil health.