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

Abstract

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 4.2mm 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 42.9mm. 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 (332kgNha-1year-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 20years 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.