6:15 PM - 7:30 PM
[ACG30-P02] Methane dynamics in a temperate forest revealed by plot-scale and ecosystem-scale flux measurements
Soils play important roles as CH4 sources and sinks. CH4 is produced in anoxic environments, including submerged soils, by methanogenic bacteria. On the contrary, CH4 is oxidized by methanotrophic bacteria in upland soils. In general, forest soils are recognized as the efficient sinks for atmospheric CH4, because of their CH4 oxidation capacity in water-unsaturated soil (Le Mer and Roger, 2001). However, we hypothesized that forest ecosystems, especially in wet warm climates such as Asian monsoon climate, are not always CH4 sink. In this study, we examined the CH4 dynamics in a temperate Asian monsoon forest (Kiryu Experimental Watershed: 35oN, 136oE), which included wet areas along riparian zones within the watershed. In order to reveal the spatio-temporal variations of CH4 fluxes, we combined multi-point plot-scale CH4 flux measurements using chamber methods and ecosystem-scale CH4 flux measurements using a micrometeorological method, relaxed eddy accumulation (REA) method (Businger and Oncley, 1990; Hamotani et al., 1996, 2001).
Intensive manual chamber measurements of CH4 fluxes at 60 points in the wet areas and within the water-unsaturated forest floor, respectively, showed that the wet areas had a greater spatial and temporal variability of CH4 fluxes than the forest floor. This indicates that accurate consideration of CH4 fluxes from any wet areas is important in order to evaluate the CH4 budget within the forests. From biweekly continuous manual chamber measurements of CH4 fluxes at 9 points in the wet areas and the forest floor, respectively, hotspots of CH4 emissions were observed during summer and fall immediately after intensive precipitation in the wet areas. On the other hand, in the forest floor, seasonal variations of CH4 fluxes were not simply associated with temperature variations. In contrast, CH4 absorption increased at some measurement plots in spring before intensive summer rainfall. In addition to the manual chamber measurements, we observed the environmental responses of CH4 fluxes at a half-hourly time resolution, by using automated chamber measurements at three plots on the water-unsaturated forest floor. We found that the CH4 absorption flux was greatly weakened by summer intensive rainfall, but recovered and peaked after rainfall as the soil water content decreased. The responses of CH4 fluxes to rainfall were different for each plot. In a dry soil plot with a thick humus layer, CH4 fluxes decreased abruptly at the peak of rainfall intensity, and it increased gradually after rainfall. In a wet soil plot and a dry soil plot with a thinner humus layer, such abrupt decreases in CH4 fluxes were not observed, and CH4 fluxes gradually switched from a sink to neutral following rainfall. Simultaneous measurements of CO2 fluxes provided useful information when considering the controlling factors affecting complex CH4 fluxes in terms of gas diffusivity and microbial activity.
The ecosystem-scale CH4 flux measurements revealed that the Japanese cypress forest switched seasonally between being a sink and source of CH4, and the pattern differed year by year. CH4 fluxes tended to be a source during summer and fall, and switched to a sink during dry period. At hourly to daily timescales, the CH4 fluxes were sensitive to rainfall; rain events increased CH4 emission, decreased CH4 absorption, or shifted CH4 absorption to CH4 emission. The results show that the temperate forest containing riparian zone acted as a CH4 source seasonally, through the increased CH4 emission in the wet areas and/or the decreased CH4 absorption on the water-unsaturated forest floor in response to changing soil temperatures and/or the soil water status. The Asian monsoon rainfall was found to strongly influence temporal variations in CH4 fluxes at both plot-scale and ecosystem-scales.
Intensive manual chamber measurements of CH4 fluxes at 60 points in the wet areas and within the water-unsaturated forest floor, respectively, showed that the wet areas had a greater spatial and temporal variability of CH4 fluxes than the forest floor. This indicates that accurate consideration of CH4 fluxes from any wet areas is important in order to evaluate the CH4 budget within the forests. From biweekly continuous manual chamber measurements of CH4 fluxes at 9 points in the wet areas and the forest floor, respectively, hotspots of CH4 emissions were observed during summer and fall immediately after intensive precipitation in the wet areas. On the other hand, in the forest floor, seasonal variations of CH4 fluxes were not simply associated with temperature variations. In contrast, CH4 absorption increased at some measurement plots in spring before intensive summer rainfall. In addition to the manual chamber measurements, we observed the environmental responses of CH4 fluxes at a half-hourly time resolution, by using automated chamber measurements at three plots on the water-unsaturated forest floor. We found that the CH4 absorption flux was greatly weakened by summer intensive rainfall, but recovered and peaked after rainfall as the soil water content decreased. The responses of CH4 fluxes to rainfall were different for each plot. In a dry soil plot with a thick humus layer, CH4 fluxes decreased abruptly at the peak of rainfall intensity, and it increased gradually after rainfall. In a wet soil plot and a dry soil plot with a thinner humus layer, such abrupt decreases in CH4 fluxes were not observed, and CH4 fluxes gradually switched from a sink to neutral following rainfall. Simultaneous measurements of CO2 fluxes provided useful information when considering the controlling factors affecting complex CH4 fluxes in terms of gas diffusivity and microbial activity.
The ecosystem-scale CH4 flux measurements revealed that the Japanese cypress forest switched seasonally between being a sink and source of CH4, and the pattern differed year by year. CH4 fluxes tended to be a source during summer and fall, and switched to a sink during dry period. At hourly to daily timescales, the CH4 fluxes were sensitive to rainfall; rain events increased CH4 emission, decreased CH4 absorption, or shifted CH4 absorption to CH4 emission. The results show that the temperate forest containing riparian zone acted as a CH4 source seasonally, through the increased CH4 emission in the wet areas and/or the decreased CH4 absorption on the water-unsaturated forest floor in response to changing soil temperatures and/or the soil water status. The Asian monsoon rainfall was found to strongly influence temporal variations in CH4 fluxes at both plot-scale and ecosystem-scales.