[A-CG30] 陸域生態系における水・炭素・窒素などの循環に関する研究

★ [ACG30-06] 酸素同位体観測により推定された飛騨高山冷温帯落葉広葉樹林サイトにおける夜間の生態系呼吸に対する土壌呼吸の割合の季節変動

*村山 昌平¹、沢田-高村 近子²、三枝 信子³、石戸谷 重之¹、渡辺 力⁴、伊藤 昭彦³、森本 真司⁵、近藤 裕昭¹、前田 高尚¹、村岡 裕由⁶、宇佐美 哲之¹、青木 周司⁵、中澤 高清⁵ (1.産業技術総合研究所、2.東京大学大気海洋研究所、3.国立環境研究所、4.北海道大学低温科学研究所、5.東北大学大学院理学研究科、6.岐阜大学流域圏科学研究センター)

キーワード：酸素同位体, 森林生態系, 生態系呼吸, 土壌呼吸

Forest ecosystems are one of the important reservoirs in the global carbon cycles. However, environmental factors governing variation in carbon dioxide (CO₂) flux between the atmosphere and the forest ecosystems have not been fully understood, which leads to very large uncertainty in future predictions of response of forest ecosystems to climate change. For more precise prediction of the future global carbon budgets, better understandings of each process in the carbon cycle in the ecosystem are necessary. A stable oxygen isotopic ratio (δ¹⁸O) in CO₂ is a unique tracer reflecting not only the carbon cycle but also the hydrological cycle. Using difference of isotopic fractionation in ¹⁸O between photosynthetic and respiratory processes and between soil and leaf respiratory processes, gross CO₂ fluxes in each of the processes have been estimated in some previous studies at relatively short time scales from diurnal to a few months. In this study, isotopic measurements of not only CO₂ in the atmosphere and soil air but also soil water and atmospheric water vapor were made at a cool-temperate deciduous forest, Takayama (TKY; 36^o08'N, 137^o25'E, 1420 m a.s.l.) in the growing seasons during 2006-2009. From the obtained data, δ¹⁸O values in CO₂ emitted due to soil (R_s) and leaf respirations and total ecosystem respiration (R_ec) during the nighttime were calculated. Then, seasonal variation in relative contribution of R_s to R_ec was estimated form mass balance equations. The obtained result was compared with variation in the ratio of R_s to R_ec estimated from soil chamber and eddy covariance flux measurements. The result from the δ¹⁸O measurements showed that the ratio of R_s to R_ec increased from about 0.5 in late spring to almost 1 in early autumn though the estimated ratios were very scattered. Such a seasonal pattern was similar to that estimated from the flux measurements. In our presentation, factors governing the seasonal variation will also be discussed by comparison with the results simulated for TKY using a process-based ecosystem model (VISIT) and a multi-layer canopy model (MINCER).