Biochar Application Enhances Photochemical Reactivity of Dissolved Organic Matter to Stimulate CO2 Emissions in Paddy Water

Abstract Dissolved organic matter (DOM) plays a pivotal role in regulating photochemical reactivity and carbon turnover in flooded paddy systems. While biochar (BC) is recognized as a promising strategy for soil carbon preservation, its impact on DOM evolution and photochemical carbon cycle in paddy water remains unknown. Here, we demonstrated that BC amendment continuously enhanced the photogeneration of hydroxyl radicals (•OH) and low-energy excited triplet-state DOM (3DOM*-L) in paddy water, increasing CO2 emissions by an average of 35.5% while playing a minor role in CH4 formation over a 34-day incubation. The enhanced photoactivity was associated with the enrichment of the photoactive molecular pool characterized by highly aromatic compounds and polyphenols with higher aromaticity, unsaturation, and oxygenated degrees. The proposed mechanisms in enriching photoactive DOM of paddy water depended on soil types, i.e., BC facilitated the biotic reprocessing of inert, highly recalcitrant aromatics into metabolites associated with photoactivity in sandy soil, whereas abiotic processes, including BC-induced dissolved black carbon release and mineral-associated organic carbon desorption, likely played more important roles in loam soil. Enhanced anaerobic fermentation further supplied photolabile substrates for amplifying mineralization. Extrapolation estimations suggested that BC-enhanced photochemistry may release an additional 3.56–15.47 Tg CO2 yr–1 globally, potentially offsetting ∼0.7–7.0% of BC’s CO2 mitigation potential. The findings revealed a previously unrecognized photochemical pathway of BC-involved carbon loss across the soil-water continuum, with implications for evaluating the net climate benefit of biochar as a carbon management strategy.