Use of nitrogen stable isotope signatures of riparian macrophyte as an indicator of human-induced N inputs to river ecosystems

Excessive nitrogen loading due to increasing anthropogenic activities deteriorates aquatic ecosystems worldwide, providing an impetus to develop effective tools for better tracing sources and transformations of nitrogen in watersheds. Stable isotopic signatures (delta¹⁵N) of nitrate and organic pools have begun to be used as an indicator of anthropogenic inputs of nitrogen, especially that which is derived from sewage. We tested the hypothesis that delta¹⁵N values of riparian macrophyte tissues reflect those of nitrate in waters flowing in the main channel. Samples were collected in the Lake Biwa basin, Japan, over seasons at 32 rivers that drain watersheds with different human population density (HPD, 44 to 3,174 ind/km²). delta¹⁵N values of nitrate varied in the range of 0.8 – 16.1 per mil and they were positively correlated with HPD. delta¹⁵N of Paragmites japonica increased with increasing delta¹⁵N of nitrate (r = 0.71, p < 0.001, n = 228), whereas correlations between delta15N of nitrate and those of other plant taxa were weaker (Persicaria thunbergii var. thunbergii; r = 0.56, p<0.001, n = 225) or insignificant (P. communis, p = 0.11). These results suggest that major sources of nitrogen differ among riparian plant taxa, which might be related to taxon-specific variations in hydrologic connectivity between main channel and roots. We suggest that some, but not all, riparian macrophyte species are a useful recorder of delta¹⁵N of nitrate in river waters, providing an opportunity to develop a tool for assessing the inputs of sewage-derived nitrogen in a time-integral fashion.