Oxygen-Stable Isotope Ratios of Nitrate and Nitrate Concentration in Stream Water during the Course of Nitrogen Saturation

We measured major inorganic ions and stable isotope ratios of nitrogen and oxygen of nitrate in the stream water in the Chichibu region during the course of nitrogen saturation. Nitrate concentration showed a high west-east gradient in the study area. Stable isotope ratios of oxygen in nitrate showed clear relationships with nitrate concentration. A direct relationship was apparent in the lower range of nitrate concentrations in that the stable isotope ratio of oxygen increases as the nitrate concentration increases. On the other hand, oxygen stable isotope ratios of nitrate hardly increase in the middle to higher concentrations of nitrate. This phenomenon may suggest that the oxygen stable isotope ratios of nitrate reflect a phase of the nitrogen saturation in the forest ecosystems.