[A-CG33] 陸海相互作用－沿岸生態系に果たす水・物質循環の役割－

[ACG33-13] 日本海側の流域圏で進行する越境汚染ストレス：硝酸の三酸素同位体組成を用いた大気沈着窒素の影響評価

*杉本 亮¹、根岸 晃芸¹、浅井 和由²、宮下 雄次³、田原 大輔¹ (1.福井県立大学、2.株式会社　地球科学研究所、3.神奈川県温泉地学研究所)

キーワード：越境汚染, 大気沈着窒素, 三酸素同位体, 窒素飽和, レトロスペクティブ解析, 日本海

Anthropogenic emissions of reactive nitrogen (N) due to fossil fuel combustion and modern agriculture practices have dramatically increased in global scale. In the Japanese watershed along the Sea of Japan, large amounts of reactive N emitted from northeastern Asia have deposited onto forest ecosystems, and thus N concentrations in rivers and groundwaters have been increased year to year. Remotely modified N concentrations of terrestrial waters in local watersheds would affect coastal ecosystems. However, quantitative influence of N deposition on the watershed along the Sea of Japan is still unclear. In this study, we assessed the influence of atmospheric reactive N deposition on the watershed in Wakasa Bay by means of the ¹⁷O anomaly (⊿¹⁷O) in nitrate, which is an alternative tracer for atmospheric nitrate (NO₃^-_atm). As a result, fractions of NO₃^-_atm in stream waters of the Kita River observed in 2014 changed from >10% in winter to 6% in summer. This seasonal variation corresponded to monthly changes in reactive N (= NO₃^-, NH₄⁺ and others) deposition rates onto the watershed. However, most of NO₃^- in stream water was derived from remineralized NO₃^- by nitrification (NO₃^-_re) within the watershed. Apparent nitrification rates estimated by ⊿¹⁷O were significantly higher than total N deposition rates. Although fractions of NO₃^-_atm in groundwater and spring water (mean±SD = 4.3±0.3%), which was dated from 2001 to 2009 by SF₆, were significantly lower than that in stream water, yearly variation in NO₃^-_atm fractions showed exponential increasing trend in the recent decade. These results imply that atmospheric reactive N deposition would affect severe influence on not only the forest ecosystem but also the coastal ecosystems.