Exploration of the nitrification process in surface sediment of a hypereutrophic intertidal zone via hydroxylamine monitoring

Hydroxylamine is considered an indicator of nitrification, as it is both produced and consumed during the ammonium (NH₄⁺) oxidation process. In this study, to explore the site of active nitrification and elucidate the factors controlling nitrification, hydroxylamine concentrations in the seawater and porewater of the hypereutrophic Yatsu tidal flat, eastern Japan, were measured using the iodine oxidation method with a Sep-Pak C₁₈ cartridge. The concentrations over approximately 2 years indicated that nitrification was more active in the surface sediment than in seawater. This would support denitrification in the surface sediment via active nitrification–denitrification, which removes N. In addition, multiple linear regression analysis indicated that the concentration in porewater was positively correlated with the seawater parameters of nitrate (NO₃⁻) concentration, NH₄⁺ concentration, and temperature, while among porewater parameters, it was negatively correlated with the phosphate (PO₄³⁻) concentration and positively correlated with the NH₄⁺ concentration. These relationships between hydroxylamine concentration and environmental factors suggest that porewater nitrification increases with higher temperature and NH₄⁺ supply from seawater and porewater, as well as under oxic conditions. Our results demonstrate that hydroxylamine is a useful indicator of sediment nitrification in the hypereutrophic intertidal zone.