Preprints
https://doi.org/10.5194/bg-2022-30
https://doi.org/10.5194/bg-2022-30
 
01 Mar 2022
01 Mar 2022
Status: this preprint is currently under review for the journal BG.

Tracing the source of nitrate enriched in a forested stream during storm events

Weitian Ding1, Urumu Tsunogai1, Fumiko Nakagawa1, Takashi Sambuichi1, Hiroyuki Sase2, Masayuki Morohashi2, and Hiroki Yotsuyanagi2 Weitian Ding et al.
  • 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8601, Japan
  • 2Asia Center for Air Pollution Research, 1182 Sowa, Nishi-ku, Niigata-shi, Niigata 950-2144, Japan

Abstract. To clarify the source of nitrate increased during storm events in temperate forested streams, we monitored temporal variation in the concentrations and stable isotopic compositions including Δ17O of stream nitrate in a forested catchment (KJ catchment, Japan) during three storm events I, II, and III. The stream showed significant temporal variation in nitrate concentration, from 24.7 µM to 122.6 µM, from 28.7 µM to 134.1 µM, and from 46.6 µM to 114.5 µM during the storm events I, II, and III, respectively. On the other hand, the isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate showed a decrease in accordance with the increase in the stream nitrate concentration, from +2.5 ‰ to −0.1 ‰, from +3.0 ‰ to −0.5 ‰, and from +3.5 ‰ to −0.1 ‰ for δ15N, from +3.1 ‰ to −3.4 ‰, from +2.9 ‰ to −2.5 ‰, and from +2.1 ‰ to −2.3 ‰ for δ18O, and from +1.6 ‰ to +0.3 ‰, from +1.4 ‰ to +0.3 ‰, and from +1.2 ‰ to +0.5 ‰ for Δ17O during the storm events I, II, and III, respectively. Besides, we found strong linear relationships between the isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate and the reciprocal of stream nitrate concentrations during each storm event, implying that the temporal variation in the stream nitrate can be explained by simple mixing between two distinctive endmembers of nitrate having different isotopic compositions. Furthermore, we found that both concentrations and the isotopic compositions of soil nitrate obtained in the riparian zone of the stream were plotted on the nitrate-enriched extension of the linear relationship. We conclude that the soil nitrate in the riparian zone was responsible for the increase in stream nitrate during the storm events. In addition, we found that the concentration of unprocessed atmospheric nitrate in the stream was stable at 1.6 ± 0.4 µM, 1.8 ± 0.4 µM, and 2.1 ± 0.4 µM during the storm events I, II, and III, respectively, irrespective to the significant variations in the total nitrate concentration. We conclude that the storm events have little impacts on the concentration of unprocessed atmospheric nitrate in the stream and thus the annual export flux of unprocessed atmospheric nitrate relative to the annual deposition flux can be a robust index to evaluate nitrogen saturation in forested catchments, irrespective to the variation in the number of storm events and/or the variation in the elapsed time from storm events to sampling.

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 bg-2022-30', Anonymous Referee #1, 21 Mar 2022
    • AC1: 'Reply on RC1', Weitian Ding, 24 Apr 2022
  • RC2: 'Comment on bg-2022-30', Anonymous Referee #2, 28 Mar 2022
    • AC2: 'Reply on RC2', Weitian Ding, 24 Apr 2022

Weitian Ding et al.

Weitian Ding et al.

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Short summary
Excessive leaching of nitrate from forested catchments during storm events degrades water quality and causes eutrophication in downstream areas. Thus, tracing the source of nitrate increase during storm events in forested streams is important for sustainable forest management. On the basis of the isotopic compositions of stream nitrate, including Δ17O, this study clarified that the source of stream nitrate increase during storm events is soil nitrate in the riparian zone.
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