Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin

Nitrogen (N) saturation of upland forests has been assumed to be a substantial N source downstream. However, removal processes of N, including assimilation and denitrification in the downstream area, have not been clarified. To evaluate the N removal processes, nitrate (NO3−) and organic N concentra...

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Veröffentlicht in:Hydrology 2017-09, Vol.4 (3), p.43
Hauptverfasser: Shinozuka, Ken’ichi, Chiwa, Masaaki, Tayasu, Ichiro, Yoshimizu, Chikage, Otsuki, Kyoichi, Kume, Atsushi
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Sprache:eng
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Zusammenfassung:Nitrogen (N) saturation of upland forests has been assumed to be a substantial N source downstream. However, removal processes of N, including assimilation and denitrification in the downstream area, have not been clarified. To evaluate the N removal processes, nitrate (NO3−) and organic N concentrations, as well as nitrogen isotope ratio (δ15N) and oxygen isotope ratio (δ18O) of NO3− were measured along three rivers of Tatara River Basin, Japan where upland forests have already been N-saturated. Geographic information system (GIS) based topographical analysis was also conducted to evaluate the land use as urban area in relation to topography. In two of the three rivers, NO3− concentrations did not increase from upstream to downstream, despite the potential non-point N sources of urban areas. In another river, NO3− concentrations rather decreased. The values of δ15N and δ18O of NO3− and organic N concentrations suggested the presence of denitrification and assimilation over N pollutants in the river whose watersheds have a lower percentage of urban area. The lower percentage of urban area could be explained by the lower topographic index. This study concluded that the NO3− leaching from upland N-saturated forests was substantially assimilated or denitrified in the downstream area.
ISSN:2306-5338
2306-5338
DOI:10.3390/hydrology4030043