Use of stable nitrogen isotope signatures of riparian macrophytes as an indicator of anthropogenic N inputs to river ecosystems

Deterioration of aquatic ecosystems resulting from enhanced anthropogenic N loading has become an issue of increasing concern worldwide, and methods are needed to trace sources of N in rivers. Because nitrate from sewage is enriched in ¹⁵N relative to nitrate from natural soils, δ¹⁵N values of stream nitrate (δ¹⁵N_nitrate) should be an appropriate index of anthropogenic N loading to rivers, as should the δ¹⁵N values of riparian plants (δ¹⁵N _plant) because they are consumers of nitrate. We determined the δ¹⁵N values of stream nitrate and six species of riparian macrophytes in 31 rivers in the Lake Biwa Basin in Japan. We then tested the correlation between these values and various land-use parameters, including the percentage of land used for residential and agricultural purposes as well asfor natural areas. These δ¹⁵N values were significantly positively correlated with land use (%) that had a high N load (i.e., residential or agricultural use) and significantly negatively correlated with forest (%). These findings indicate that δ¹⁵N values of stream nitrate and riparian plants might be good indicators of anthropogenic inputs of nitrogen.