JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS25] Biogeochemistry

convener:Keisuke Koba(Center for Ecological Research, Kyoto University), Hideaki Shibata(Field Science Center fot Northern Biosphere, Hokkaido University), Naohiko Ohkouchi(Japan Agency for Marine-Earth Science and Technology), Youhei Yamashita(Faculty of Environmental Earth Science, Hokkaido University)

[MIS25-13] Rise and fall of nitrogen concentration in a river draining to the Sea of Japan driven by atmospheric deposition

*Ryo Sugimoto1, Takeru Hirai1, Kouki Negishi1, Tomoko Tsuboi1, Daisuke Tahara1, Kazuyoshi Asai2, Makoto Yamada3, Yumiko Watanabe4, Motoko Fujita5, Isaac Santos6,7 (1.Faculty of Marine Biosciences, Fukui Prefectural University, 2.Geo-Science Laboratory Co. Ltd., 3.Faculty of Economics, Ryukoku University, 4.Graduate School of Science, Kyoto University, 5.Center for Southeast Asian Studies, Kyoto University, 6.Southern Cross University, 7.University of Gothenburg)

Keywords:Nitrate, Transboundary air pollution, Long-term trend, Watershed

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.