Abstract
The stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in sediment and water quality were monitored in the Koyoshi River watershed in Akita, Japan and used in describing the change in water quality and nitrogen balance in the watershed. Although the degree of organic pollution had worsened downstream (DOC: 2.0 mgC · l-1 to 2.8 mgC · l-1; NO3--N: 0.22 mg · l-1 to 0.31 mg · l-1), the values of δ15N and δ13C downstream (1.5 ‰ and -26.3 ‰, respectively) were lower than those in the upper river basin (3.2 ‰ and -24.0 ‰, respectively). It was deduced that there have been nitrogen loading in the upper river basin and that humic substances have contributed to the decrease in the isotope ratios in the middle basin. The seasonal variation in δ15N in sediment indicates the influence of fertilizer applied in rice fields. The amount of nitrogen removal from water was estimated by isotope fractionation analysis with ammonium concentration and δ15N in sediment, and found to be reasonable considering the estimated amount of fertilizer normally applied in rice fields.
