日本地球惑星科学連合2022年大会

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[J] 口頭発表

セッション記号 M (領域外・複数領域) » M-IS ジョイント

[M-IS14] 生物地球化学

2022年5月26日(木) 13:45 〜 15:15 202 (幕張メッセ国際会議場)

コンビーナ:木庭 啓介(京都大学生態学研究センター)、コンビーナ:柴田 英昭(北海道大学北方生物圏フィールド科学センター)、大河内 直彦(海洋研究開発機構)、コンビーナ:山下 洋平(北海道大学 大学院地球環境科学研究院)、座長:木庭 啓介(京都大学生態学研究センター)、柴田 英昭(北海道大学北方生物圏フィールド科学センター)、山下 洋平(北海道大学 大学院地球環境科学研究院)、大河内 直彦(海洋研究開発機構)

15:00 〜 15:15

[MIS14-06] Stable isotopic evidence for the excess leaching of unprocessed atmospheric nitrate from a forested catchment in Fukuoka, Japan

*丁 瑋天1角皆 潤1中川 書子1、篠塚 賢一3、笠原 玉青2智和 正明2 (1.名古屋大学環境学研究科、2.九州大学農学研究院、3.福岡工業大学情報工学部)


キーワード:窒素飽和、森林渓流水、直接流出率

Forest ecosystems are deficient in nitrogen in general. Excess loading of nitrogen, however, often leads to “nitrogen saturation” (Aber et al., 1989) in forest ecosystems from which significant quantify of nitrogen is eluted as nitrate. The forested stream eluted from Fernow Experimental Forest USA, for instance, showed elevated average nitrate concentration of 60 µM, so that the forest has been classified into stage 3, the highest stage of nitrogen saturation (Rose et al., 2015). Such enrichment of stream nitrate often caused environmental problems such as degradation of water quality, eutrophication, and N2O emission in downstream area.
In recent, Chiwa (2021) reported the average concentrations of the forested streams eluted from FK catchments (FK1 and FK2) in Kasuya Research Forest, Kyushu University (Japan), were more than 90 µM, implying that the forested catchments were under the extraordinary high nitrogen saturation stage. In order to verify that the forested catchments (FK1 and FK2) were under the high nitrogen saturation stage, we determined the export flux of unprocessed atmospheric nitrate relative to the entire deposition flux (Matm/Datm ratio) in the catchments, based on the 17O-excess of nitrate eluted from the catchments, because Nakagawa et al. (2018) proposed this Matm/Datm ratio in each forested catchment as an objective, more reliable index to clarify each nitrogen saturation stage. Specifically, we determined temporal variation in the concentrations and stable isotopic compositions, including Δ17O, of stream nitrate in the FK catchments for more than two years. Besides, the same parameters of the MY forested catchment in Shiiba Research Forest, Kyushu University (Japan) were also monitored during the same period for comparison, where the average stream nitrate concentration was low. While showing the average nitrate concentrations of 109.5, 94.2, and 7.2 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. As a result, the average concentration of unprocessed atmospheric nitrate ([NO3atm]) were estimated to be 10.8, 6.1, and 0.2 µM in FK1, FK2, and MY, respectively, and the Matm/Datm ratio were estimated to be 15.1, 8.5, and 1.1 % in FK1, FK2, and MY, respectively. The Matm/Datm ratio determined in FK1 (15.1 %) were the highest ever reported from temperate forested catchments. Thus, we concluded the nitrogen saturation was responsible for the enrichment of nitrate in the streams of FK catchments, together with the elevated NO3atm leaching from the catchments. Additionally, we found that all the data of Matm/Datm ratios reported for the forested catchments with the annual precipitation more than 1000 mm in the word showed clear normal correlation with the average concentration of stream nitrate, indicating that the Matm/Datm ratio in each forested ecosystem can be a robust index for nitrogen saturation.