[M-IS13] 生物地球化学

[MIS13-12] 泥炭湿地におけるアブラヤシプランテーションの排水路における一酸化二窒素排出と発生メカニズム

*仁科 一哉¹、Melling Lulie²、平田 竜一¹、高橋 善幸¹、豊田 栄³、寺嶋 晃太朗³、伊藤 雅之⁴ (1.国立環境研究所、2.SARAWAK Tropical Peat Research Institute、3.東京工業大学、4.兵庫県立大学)

キーワード：一酸化二窒素、オイルパームプランテーション、窒素同位体

In South-East Asia, oil palm plantation has rapidly expanded in the past few decades even in peatland areas. In the light of nitrogen cycling, this land-use change would result in enrichment on the ecosystem N cycling due to the N fertilization for the maintaining oil palm growth and the wastewater emission from the palm mills in the plantation. Oil palm plantations converted from peat swamp forests require the drainage to keep cultivation and thus has unique hydrological characteristics. In such plantation, drainage water surface dominates up to 5% of plantation area and therefore it is important to evaluate the gas exchange between the drainage water and atmosphere.

In this study, we have conducted spatial sampling for dissolved N₂O concentration (dN₂O) in the oil palm plantation on drained peat soil in July 2018, in Sarawak, Malaysia in order to evaluate the indirect N₂O emission from palm oil plantation. We have intensively sampled the water samples from the drainage in the plantation (ca. 120 km²) and the water samples from Raja river, which is adjacent to the plantation. 90% of samples (N = 84) in the drainage water showed over-saturation in dissolved N₂O concentration (up to 570%), suggesting that the drainage water in the oil palm plantation acts the source of N₂O to the atmosphere. The emission factor for indirect emission (EFr: i.e., dN₂O/nitrate concentration) showed relatively higher than the default EFr in IPCC. However, there were no clear linear relationships between the water qualities (including nitrate) and dN₂O. We have also analyzed the isotopic composition of dN₂O in both drainage and river samples. This analysis suggested the denitrification contributed dN₂O in the drainage water.