Thursday, August 9, 2007

PS 63-109: Use of nitrogen stable isotope signatures of riparian macrophyte as an indicator of human-induced N inputs to river ecosystems

Ayato Kohzu1, Chikage Yoshimizu1, Ichiro Tayasu2, and Toshi Nagata2. (1) Japan Science and Technology Agency, (2) Kyoto University

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.

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