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https://doi.org/10.5194/bg-2018-258
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-258
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Jul 2018

Research article | 12 Jul 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Biogeosciences (BG) and is expected to appear here in due course.

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: A possible new index for nitrogen saturation

Fumiko Nakagawa1, Urumu Tsunogai1, Yusuke Obata1, Kenta Ando1, Naoyuki Yamashita2,a, Tatsuyoshi Saito2,b, Shigeki Uchiyama2,c, Masayuki Morohashi2, and Hiroyuki Sase2 Fumiko Nakagawa et al.
  • 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
  • 2Asia Center for Air Pollution Research, 1182 Sowa, Nishi-ku, Niigata-shi, Niigata 950-2144 , Japan
  • apresent address: Forestry and Forest Products Research Institute, Tsukuba, Ibaraki 305-8687, Japan
  • bpresent address: Niigata Prefectural Institute of Public Health and Environmental Sciences, 314-1 Sowa, Nishi-ku, Niigata-Shi, Niigata 950-2144, Japan
  • cpresent address: Ministry of Agriculture, Forestry and Fisheries, 1-2-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8950, Japan

Abstract. To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions including 17O-excess (Δ17O) in three forested catchments in Japan (KJ, IJ1, and IJ2) for more than two years. The catchments showed varying flux-weighted average nitrate concentrations: 58.4, 24.4, and 17.1µmolL−1 in KJ, IJ1, and IJ2, respectively. In addition to stream nitrate, nitrate concentrations and stable isotopic compositions in soil water were determined for comparison in the most nitrate-enriched catchment (the KJ site). While 17O-excess of nitrate in soil water showed significant seasonal variation, ranging from +0.1 to +5.7‰, stream nitrate showed small variation, from +0.8 to +2.0‰ in KJ, +0.7 to +2.8‰ in IJ1, and +0.4 to +2.2‰ in IJ2. We concluded that the major source of stream nitrate in each forested catchment was nitrate in groundwater, which buffered the seasonal variations in soil water nitrate. The estimated annual export flux of unprocessed atmospheric nitrate accounted for 9.4±2.6%, 6.5±1.8%, and 2.6±0.6% of the annual deposition flux of atmospheric nitrate in KJ, IJ1, and IJ2, respectively. The export flux of unprocessed atmospheric nitrate relative to the deposition flux showed a clear normal correlation with the flux-weighted average concentration of stream nitrate, indicating that reductions in the biological assimilation rates of nitrate in forested soils, rather than increased nitrification rates, are likely responsible for the enrichment of stream nitrate, probably due to nitrogen saturation. The export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.

Fumiko Nakagawa et al.
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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Fumiko Nakagawa et al.
Fumiko Nakagawa et al.
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To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions in three forested catchments for more than two years. We concluded that the export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.
To clarify the biological processing of nitrate within temperate forested catchments using...
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