Decadal Biochar Legacy Stabilizes Paddy Soil Carbon by Reshaping Redox-Driven Reactive Oxygen Species Dynamics

Biochar (BC) displays great potential in stabilizing soil organic carbon, yet its decadal legacy for carbon preservation under redox oscillations remains unclear. The study comprehensively investigated how decadal BC (D-BC) application restrained carbon mineralization of paddy soil via reshaping reactive oxygen species (ROS) dynamics. In situ fluorescence imaging and quantification results showed that D-BC soil inhibited hydroxyl radical (•OH) generation by 22.3% than fresh BC (F-BC) input soil, but elevating hydrogen peroxide (H2O2) and superoxide (O2•–) accumulation by 36.2% and 25.4% during redox oscillation, respectively. This resulted from its weakened microbial iron reduction and the lower reactivity of Fe(II), which significantly limited Fenton reactions. In contrast, a shift in the microbial community to redox-adapted taxa, lower catalase activity, and enriched dissolved inorganic carbon collectively facilitated the biogenic origin of H2O2 in D-BC soil. Scavenging and correlation analyses revealed that reduced •OH generation led to net suppression in CO2 emission from D-BC soil by >30% via upregulating the enzymatic latch, while enhanced H2O2 accumulation imposed oxidative stress on carbon-degrading microbes. Laboratory verifications confirmed the roles of surface oxidation and persistent free radical attenuation of aged BC in ROS redistribution. The failure of decadal straw (D-Straw) return to reproduce these phenomena suggested that the decadal BC legacy stemmed from a particular redox impact, rather than exogenous carbon input alone. The findings uncovered a new mechanism of BC-induced carbon stability, advancing understanding of the carbon cycle within redox-dynamic zones.