Rice Root Fe Plaque-Induced Hydroxyl Radicals Increase Paddy Soil CO₂ Emissions

Poorly crystalline iron (Fe) oxides often accumulate on rice root surfaces, forming Fe plaque. While this plaque has been implicated in soil organic carbon (SOC) transformation, its specific role in regulating SOC mineralization is still insufficiently understood. In this study, we hypothesize that Fe plaque induces the generation of hydroxyl radicals (•OH), which disrupt the "enzyme latch" mechanism and promote CO₂ emissions. The results showed that seedlings with Fe plaque showed a 150% higher •OH concentration, 50% higher laccase activity, 25% higher SOC mineralization rate, and 20% lower phenolic concentration in comparison to those without Fe plaque. These effects disappeared when a •OH scavenger was applied or oxygen availability was reduced. Moreover, the exogenous addition of Fe(II) in Fe plaque-free soils reproduced the increases in •OH, laccase activity, and SOC mineralization, confirming the role of Fe-driven •OH production. These findings demonstrate that Fe plaque-induced •OH simultaneously disrupts phenolic inhibition and accelerates SOC mineralization. This study identifies a previously underappreciated oxidative mechanism by which Fe plaque destabilizes SOC in paddy soils and highlights the role of mineral-radical interactions in controlling carbon turnover.