Elevated CO2 impacts mercury migration in Chinese paddy soils: Critical roles of pH, dissolved organic matter and the biological pump.

Elevated CO₂ (eCO₂) alters soil biogeochemistry, yet its impact on mercury (Hg) dynamics in low-background paddy soils remains poorly understood. Here, we investigated Hg transformation in 11 Chinese paddy soils under eCO₂ (approximately 600 ppm) using open-top chambers. The results revealed a striking "decoupling paradox": while grain methylmercury (MeHg) concentrations often decreased due to growth dilution, the total MeHg accumulation surged significantly (e.g., +656.8% in Yanbian), driven by a biomass-stimulated "biological pump." Mechanistically, eCO2 induced rhizosphere acidification and promoted local oxidation. Crucially, Random forest modeling identified tyrosine-like dissolved organic matter (C4) as the dominant predictor for root MeHg uptake, suggesting that protein-like DOM quality may be an important, albeit statistically inferred, modulator of MeHg bioavailability in the rhizosphere. Consequently, eCO₂ accelerates Hg mobilization from "clean" soils into the food web. Current concentration-based indices (Target Hazard Quotient) mask this intensified flux. These findings highlight that future climate scenarios may activate latent soil Hg risks, necessitating a shift toward flux-based risk assessment frameworks.