Dynamic changes of reactive oxygen species in paddy overlying water: mechanisms and implications

PURPOSE: The paddy soils are covered with overlying water for a long time during rice growth, which is critical to elements cycling and contaminants transformation due to the sunlight-mediated biotic processes. In this study, the reactive oxygen species (ROS) formation was investigated to reveal the associated processes in the paddy overlying water. METHODS: The dynamic changes of ROS in overlying paddy water were in situ captured in paddy plots, with biochar and straw amended in different agricultural managements. The hydrogen peroxide (H₂O₂) concentration was in situ measured through the chemiluminescence method. The accumulation of hydroxyl radical (•OH) was quantified via chemical probes, with the steady-state concentrations of •OH calculated accordingly. The As(III) oxidation experiments were performed through the indoor experiments with the simulated sunlight by 500 W xenon lamp. RESULTS AND DISCUSSION: Results indicated that the hydrogen peroxide (H₂O₂) concentration decreased from the daytime 8:00 to 16:00 in the tillering stage, falling within the range of 0 ~ 260.16 nmol L⁻¹. Abundant •OH was produced, giving the steady-state concentration of 3.94 × 10–¹⁸ ~ 3.90 × 10–¹⁷ mol L⁻¹ in different treatments. More •OH production was observed with the amendment of inorganic fertilizers than organic fertilizers, while •OH production with biochar amendment was higher than that of straw amendment. The results of quenching experiments showed that more than 70% of •OH production in paddy overlying water derived from the dissolved organic matter (DOM)-based photochemical processes. The ROS can efficiently drive As(III) oxidation in paddy overlying water, and the •OH plays the dominant role. CONCLUSION: This study elucidated that photochemically produced ROS dynamically changed during rice growth, which was highly affected by the agricultural managements and might be of great significance to pollutants transformation in paddy fields.