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

Ken’ichi Shinozuka,Kyoichi Otsuki,Chikage Yoshimizu,Ichiro Tayasu,Masaaki Chiwa,Atsushi Kume

doi:10.3390/hydrology4030043

Ken’ichi Shinozuka, Kyoichi Otsuki + Show 4 more

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https://doi.org/10.3390/hydrology4030043

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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.

Highlights

It is vital to identify the factors regulating stream nitrogen (N) eutrophication in a mixed land use river basin [1,2]
Non-point source (NPS) pollution from agricultural and urban areas is the main source of downstream eutrophication [3], while denitrification and assimilation reduces nitrate (NO3 − )
Upland forests have been commonly regarded as having negligible NPSs for downstream eutrophication [6,7]

Summary

Introduction

It is vital to identify the factors regulating stream nitrogen (N) eutrophication in a mixed land use river basin [1,2]. Non-point source (NPS) pollution from agricultural and urban areas is the main source of downstream eutrophication [3], while denitrification and assimilation reduces nitrate (NO3 − ). In stream water, leading to reducing N eutrophication in stream water [4,5]. These factors should be evaluated carefully to improve our understanding of downstream N eutrophication. Upland forests have been commonly regarded as having negligible NPSs for downstream eutrophication [6,7]. N-saturated upland forests can act as an influential NPS, as can agricultural and urban areas [10,11] Elevated atmospheric N deposition resulting from human activities may potentially cause N saturation in forested ecosystems, defined as an excess of biotic demand [8], which causes high levels of NO3 − leaching from upland forested ecosystems to be observed [8,9].

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