Combined effects of oxygen and temperature on nitrogen removal in a nitrate-rich ex-paddy wetland

Temperature is generally considered to be the primary factor controlling the nitrogen removal rate (NR) in nitrate (NO₃^-)-rich submerged sediments. Temperature stimulates both sediment oxygen (O₂) respiration, to create anaerobic conditions, and microbial photosynthetic activity, to provide the organic carbon required for denitrification and expand the uppermost aerobic layer, i.e., the O₂ penetration depth (OPD). The OPD serves as a diffusion barrier for NO₃^- to the underlying anaerobic layer for denitrification. The complex effects of O₂ and temperature on the NR are unclear under field conditions with a wide range of temperatures and O₂ suppliers. This study aimed to determine the combined effects of O₂ and temperature on the NR in an NO₃^--rich, riparian ex-paddy wetland ("yatsu" environment) under long-term bare soil conditions. We used three years of field monitoring with occasional O₂ microprofile measurements from undisturbed submerged soil cores. We observed vertical supersaturated O₂ concentration plateaus up to 4.2 mm depth, which confirmed the presence of underground O₂ producers, i.e., photosynthetic microorganisms forming habitat in the soil, and very large OPDs of up to 42.9 mm. A multiple regression analysis showed that temperature and dissolved O₂ concentration in the flooded water were the key positive and negative influences, respectively, on the NR (332 kg N ha^-1 year^-1 on average), in association with the total N input. Microbial photosynthesis appeared to remain active regardless of the season, providing O₂ to increase OPD and partly suppress the NR; however, photosynthesis has increased the soil C content and appears to have positively contributed to a sustained NR during the 20 years of bare soil conditions. Our results suggest that temporal no vegetation-shade (bare soil) conditions with periodic weed cutting is recommended to effectively remove N from the watershed, while maintaining high temperatures and soil organic C in yatsu environments.