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Hydrology 2017, 4(3), 43; doi:10.3390/hydrology4030043

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

1
Kyushu University Forest, Kyushu University, 394 Tsubakuro, Sasaguri, Fukuoka 811-2415, Japan
2
Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto 603-8047, Japan
3
Faculty of Agriculture, Kyushu University, 6‑10‑1 Hakozaki, Higashi‑ku, Fukuoka 812-8581, Japan
*
Author to whom correspondence should be addressed.
Received: 16 July 2017 / Revised: 10 September 2017 / Accepted: 11 September 2017 / Published: 14 September 2017
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Abstract

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. View Full-Text
Keywords: mixed land use; nitrogen eutrophication; nitrogen-saturated forest; non-point source; stable isotope; topographic index mixed land use; nitrogen eutrophication; nitrogen-saturated forest; non-point source; stable isotope; topographic index
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Shinozuka, K.; Chiwa, M.; Tayasu, I.; Yoshimizu, C.; Otsuki, K.; Kume, A. Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin. Hydrology 2017, 4, 43.

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