[M-IS25] 生物地球化学

[MIS25-13] 大気沈着窒素によって引き起こされる日本海側の河川水中の窒素濃度の上昇と下降

*杉本 亮¹、平井 タケル¹、根岸 晃芸¹、坪井 智子¹、田原 大輔¹、浅井 和由²、山田 誠³、渡邊 裕美子⁴、藤田 素子⁵、サントス アイザック^6,7 (1.福井県立大学海洋生物資源学部、2.地球科学研究所、3.龍谷大学経済学部、4.京都大学大学院理学研究科、5.京都大学東南アジア研究所、6.サザンクロス大学、7.イェーテボリ大学)

キーワード：硝酸イオン、越境大気汚染、長期トレンド、集水域

Large amounts of reactive nitrogen (N) emitted from northeastern Asia are deposited onto forest ecosystems in watersheds draining to the Sea of Japan. Consequently, N concentrations in some Japanese rivers and aquifers have been increased until around 2010. In recent years, this trend has been changed possibly due to the decrease of atmospheric N emissions in China. Here, we hypothesize that Japanese groundwater and river water record the recent atmospheric deposition of N transported from East Asia. We rely on the ¹⁷O anomaly of nitrate and groundwater dating via SF₆ to reveal atmospheric nitrate contributions to the Kita River watershed in Japan in the last three decades. Our latitudinal survey in central Japan revealed that the meteoric water sources were mostly supplied from the Sea of Japan in the northern area and from the Pacific Ocean in the central and southern areas. The mean atmospheric nitrate fraction in surface river water and groundwater in the Kita River watershed located in the northern area were 4.1 ± 1.3% and 5.2 ± 3.0%, respectively. Combining the SF₆-derived groundwater dating with the ¹⁷O anomaly shows an increase in atmospheric nitrate fraction in riverine nitrate until 2010 but a decrease since 2010, consistent with long-range atmospheric transport and deposition.