Emission and oxidation of CH4 and N2O in forest catchment and effect of hydrological process

为了阐明森林流域N_2O和CH_4气体交换的机制，在位于滋贺南部的吉柳实验流域进行了野外实验，重点研究了水文对N_2O和CH_4动态的生态地球化学过程的控制作用。分别测定了河源区河岸带和河岸带CH4和N2O的气体排放速率、土壤气体浓度、土壤和地下沃茨溶解气体浓度。此外，还测定了CH_4中碳的稳定同位素比值。结果表明：1.在相对干燥的土壤中，柏木林全年持续吸收，而在河边湿地则持续排放。特别是在夏季，湿地的CH_4排放速率显著高于坡地的吸收速率。2.湿地CH_4的产生和排放主要受土壤温度和地下水位的控制。CH_4的d13 C的季节变化与地下水中溶解态CH_4浓度的季节变化同步，说明CH_4的主要来源是凋落物提供的有机质分解产生的CO_2 2O主要产生在地下水带边缘，坡耕地土壤DOC供应充足。N_2O的产生主要是由于地下水位以下还原环境中的饱和反硝化作用。在地下水带最深层，N_2O浓度低于环境N_2O浓度，表明N_2O转化为N_2。

To elucidate the mechanisms of N_2O and CH_4 gas exchange in the forested catchment, the field experiments have been conducted at the Kiryu Experimental Watershed located in the southern Shiga prefecture, focusing on the hydrologic controls on biogeochemical processes for N_2O and CH_4 dynamics. Gas emitting rate and concentration in soil gas and dissolved gas concentration in soil and ground waters were measured both for CH_4 and N_2O at the stream side wetland and riparian zones in the headwater (zero-order) catchment. In addition to those observations, the stable isotope ratio of carbon in CH_4 was measured. The results provides following findings.1.Continuous absorption was observed at the relatively dry soil in the Japanese cypress forest all through the year, while emission was constantly observed at the streamside wetland. Especially in the summer, the emission rate from the wetland was remarkably higher than the absorption rate at the slope side in the Japanese cypress stands.2.CH_4 production and emission in the wetland was essentially controlled by soil temperature and groundwater level. Absorption was mainly controlled by the soil moisture content of surface horizon.3.Seasonal variation in the d13C of CH_4 synchronized with that of dissolved CH_4 concentration in groundwater, suggesting that the major origin of CH_4 was the CO_2 produced by the decomposition of the organic matters mainly supplied by litter fall.4.N_2O was mainly produced in the rim of the groundwater zone where the DOC was supply sufficiently from the slope side soils. The N_2O was considered to be produced through denitrification due to saturated condition in reduced environment below the groundwater level. In the deepest horizon in the groundwater zone, N_2O concentration was lower than that of ambient N_2O concentration, suggesting conversion of N_2O to N_2.