Preprints
https://doi.org/10.5194/egusphere-2022-717
https://doi.org/10.5194/egusphere-2022-717
 
09 Aug 2022
09 Aug 2022

Stable isotopic evidence for the excess leaching of unprocessed atmospheric nitrate from forested catchments under high nitrogen saturation

Weitian Ding1, Urumu Tsunogai1, Fumiko Nakagawa1, Takashi Sambuichi1, Masaaki Chiwa2, Tamao Kasahara3, and Kenichi Shinozuka4 Weitian Ding et al.
  • 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
  • 2Kyushu University Forest, Kyushu University
  • 3Faculty of Agriculture, Kyushu University
  • 4River Basin Research Center, Gifu University, 1-1 Yanagido, Gifu, 501-1193, Japan

Abstract. The average concentration of stream nitrate eluted from the FK forested catchments (FK1 and FK2) in Japan was more than 90 µM, implying that these forested catchments were under nitrogen saturation. To verify that these forested catchments were under the nitrogen saturation, we determined the export flux of unprocessed atmospheric nitrate relative to the entire deposition flux (Matm / Datm ratio) in these catchments, because the Matm / Datm ratio has recently been proposed as a reliable index to evaluate nitrogen saturation in forested catchments. Specifically, we determined the temporal variation in the concentrations and stable isotopic compositions, including Δ17O, of stream nitrate in the FK catchments for more than 2 years. In addition, for comparison, the same parameters were also monitored in the MY forested catchment in Japan during the same period, where the average stream nitrate concentration was low, less than 10 µM. While showing the average nitrate concentrations of 109.5, 94.2, and 7.1 µM in FK1, FK2, and MY, respectively, the catchments showed average Δ17O values of +2.6, +1.7, and +0.6 ‰ in FK1, FK2, and MY, respectively. Thus, the average concentration of unprocessed atmospheric nitrate ([NO3atm]) was estimated to be 10.8, 6.1, and 0.2 µM in FK1, FK2, and MY, respectively, and the Matm / Datm ratio was estimated to be 13.9, 7.9, and 1.2 % in FK1, FK2, and MY, respectively. The estimated Matm / Datm ratio in FK1 (13.9 %) was the highest ever reported from temperate forested catchments monitored for more than 1 year. Thus, we concluded that nitrogen saturation was responsible for the enrichment of stream nitrate in the FK catchments, together with the elevated NO3atm leaching from the catchments. While the stream nitrate concentration ([NO3−]) can be affected by the amount of precipitation, the Matm / Datm ratio is independent of the amount of precipitation; thus, the Matm / Datm ratio can be used as a robust index for evaluating nitrogen saturation in forested catchments.

Weitian Ding et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-717', Anonymous Referee #1, 28 Aug 2022
    • AC1: 'Reply on RC1', Weitian Ding, 22 Sep 2022
  • RC2: 'Comment on egusphere-2022-717', Anonymous Referee #2, 04 Sep 2022
    • AC2: 'Reply on RC2', Weitian Ding, 22 Sep 2022

Weitian Ding et al.

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Short summary
By monitoring the concentration and Δ17O of stream nitrate in three forested streams, the new nitrogen saturation index of forested catchments (Matm / Datm ratio) was estimated. We found that (1) the unprocessed atmospheric nitrate in our studied forested stream (FK1 catchment) was the highest ever reported in forested streams, (2) the Matm / Datm ratio can be used as a robust index for evaluating nitrogen saturation in forested catchments as the Matm / Datm ratio is independent of the precipitation.