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Temporal Dynamics of Soil Dissolved Organic Matter Influenced by Biochar and Microplastics: Insights from Molecular and Optical Signatures
Summary
Scientists found that adding biochar (a charcoal-like soil additive) to farmland soil changes how plastic pollution breaks down, with iron-enhanced biochar speeding up the breakdown of biodegradable plastics into simpler carbon compounds, while regular plastic waste sticks around longer and stays more chemically stable. This matters because it shows biochar could be a practical tool for helping soils process plastic pollution faster, but the type of plastic and biochar used makes a big difference, meaning not all "eco-friendly" solutions work the same way in real farmland.
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+FeBC vs. 279 mg/kg in control), leading to the accumulation of labile, low-molecular-weight organic fractions (molecular lability boundary index, MLB 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.