Tomoko Nakano

Methane consumption and soil respiration by a birch forest soil in West Siberia

Methane and carbon dioxide fluxes were measured in a birch forest soil in West Siberia, in August 1999, June 2000 and September 2000. The study site had a very thick organic horizon that was subject to drought during the observation periods. The soils always took up CH4, while CO2 was released from the surface to the atmosphere. CH4 consumption and CO2 emission rates ranged from 0.092 to 0.28 mg C m−2 h−1 and from 110 to 400 mg C m−2 h−1 respectively. The CH4 consumption rate and soil temperatures showed significant relationships for individual measurements. The soil respiration rate was weakly correlated with individual soil temperatures. This study examined the effect of current and lagged soil temperatures at a depth of 5 cm on CH4 consumption and soil respiration. The variation in the correlation coefficient between CH4 consumption and lagged soil temperature was greatest at a 4-h lag, whereas that for soil respiration showed a gentle peak at lags from several hours to half a day. This difference in the temperature-related lag effect between CH4 consumption and soil respiration results from differences in the exchange processes. Neither flux showed any correlation with soil moisture. The limited variation in soil moisture during our observation period may account for the lack of correlation. However, the droughty soil conditions resulted in high gas diffusion and, consequently, high CH4 consumption.

Temporal variations in soil-atmosphere methane exchange after fire in a peat swamp forest in West Siberia

Abstract Temporal variations in methane (CH4) exchange between the soil and the atmosphere during a period of 3 years after a forest fire were estimated by combining field measurements of CH4 flux with an analysis of satellite images. The study area was located in a boreal peat swamp forest in the West Siberian plain that experienced a severe fire in the summer of 1998. The surface of the burned area was classified into bare soil, open water and recovered vegetation. In the summers of 1999 and 2000, CH4 fluxes, using a closed-chamber method, and environmental variables, such as soil temperature and soil water content, for each of the three surface types in the burned area and in the unburned area were measured. In general, CH4 fluxes were controlled by the surface moisture in the burned area and by the temperature in the unburned forest. Temporal changes in the areal coverage of soil, water and vegetation in the burned area were investigated using NOAA AVHRR (Advanced Very High Resolution Radiometer) data with subpixel land-cover characterization. Based on the satellite information, temporal changes in the moisture conditions of the burned surface were estimated and temporal variations in the CH4 fluxes for the entire burned area were calculated. The cumulative CH4 emission rates from the entire burned area during the summer (from June to August) were estimated to range from 0.39 to 0.48 g C m−2 during the period from 1999 to 2001. In contrast, unburned forest soils were consistently net CH4 consumers. The cumulative consumption rate during the summer was calculated to be approximately 0.4 g C m−2 based on a reanalysis air temperature dataset. As the surface soil had become extremely wet since the fire, the soil had become a net emitter of CH4 after the fire disturbance, although CH4 oxidation predominated in the unburned forest.

Response of ecosystem respiration to soil water and plant biomass in a semiarid grassland.

Abstract The Mongolian steppe zone constitutes a major part of East Asian grasslands. The objective of this study was to evaluate the quantitative dependence of ecosystem respiration (Reco) on the environmental variables of soil water and plant biomass in a semiarid grassland ecosystem. We determined Reco using opaque, closed chambers in a Mongolian grassland dominated by graminaceous perennial grasses during six periods: July 2004, May 2005, July 2005, September 2005, June 2006, and August 2009. Using the data collected when soil water content and aboveground biomass were relatively constant, values of Reco were fitted to an exponential temperature function, and the standardized rate of Reco at 20°C (R20) and temperature sensitivity (Q10) of Reco were calculated for each measurement plot and period. The results indicate that aboveground biomass significantly affected the variation in R20, and the relationship was expressed with a linear model. The R20 residuals of the linear biomass model were highly correlated with soil water content by a quadratic function. The Q10 values showed a weak positive relationship with soil water content. Temporal and spatial variations in Reco were well predicted by the exponential temperature model with R20, which relates to aboveground biomass and soil water content, and with Q10, which relates to soil water content.

Modeling gross primary production and ecosystem respiration in a semiarid grassland of Mongolia

Abstract We constructed an empirical model in which the rates of gross primary production (GPP) and ecosystem respiration (Reco) were calculated using meteorological, soil and plant variables, based on our previous measurements of carbon dioxide (CO2) fluxes using a closed-chamber technique in Mongolian grasslands. In the previous studies, we determined GPP and Reco using transparent and opaque chambers and simultaneously measured the environmental variables inside and beside the chambers. These measurements allowed us to quantify direct and accurate relationships between the rates of GPP and Reco and their controlling factors. The GPP, Reco and their difference net ecosystem CO2 exchange (NEE) were computed in a semiarid grassland site of Mongolia for the growing seasons of 2010 and 2011 using observed values of photosynthetically active radiation, air and soil temperatures, vapor pressure deficit, soil water content and estimated values of aboveground biomass from a remotely sensed vegetation index. Model performance was validated by comparing the modeled and observed NEE values using an eddy covariance technique. Results showed that the model successfully reproduced the magnitude and seasonal variations of the observed NEE, and regression analysis showed reasonable agreement in both years. Cumulative rates of GPP and Reco during the plant-growing season (May − September) were 200.3 and 210.5 g carbon (C) m−2 in the dry year (2010) and 342.3 and 300.1 g C m−2 in the wet year (2011), respectively. This result indicates that drought was more effective in reducing CO2 uptake by the plant than in reducing ecosystem respiration. Consequently, the grassland ecosystem was a net carbon source during the growing season in 2010 and a net carbon sink in 2011.

Estimation of Methane Emission from West Siberian Lowland with Subpixel Land Cover Characterization Between MODIS and ASTER

Wetlands are one of the most important ecosystems in the world. The West Siberian Lowland (WSL) is the world’s largest high-latitude wetland covering nearly two-thirds of western Siberia; thus, remote sensing techniques play an important role in monitoring them. At least half of this area consists of peatlands, which sequester atmospheric carbon in the form of slightly decomposed plant matter. WSL is considered a net source of methane gas, which is one of the most important greenhouse gases.

Estimation of methane emission from West Siberian Lowland with sub-pixel land cover characterization

The West Siberian Lowland (WSL) is the worlds largest high-latitude wetland covering nearly 2/3 of western Siberia. At least half of this area consists of peatlands, which sequester atmospheric carbon in the form of undecomposed plant matter and it is presumed to be a source of methane gas. In this paper, firstly, an ASTER image near Noyabrsk mire was used to map six wetland ecosystems (red pine, white birch, bog, palsa, open water and bare soil) supplemented by field observation. Then spectral linear mixture analysis was performed between MODIS and ASTER data acquired on the same day. Secondly, field observations were scaled up with these different spatial resolution satellite data. Each of the wetland ecosystem coverage ratio in sub-pixel level was provided by the spectral linear mixture analysis. Field observation shows that the mean rate of CH/sub 4/ emission from bog and open water averaged 5.246 and 1.081 (mg CH/sub 4/ m/sup -2/ h/sup -1/) respectively. The methane emission from the area was estimated by multiplying these average methane emission rate and the area percentage of bog and open water in each pixel. Finally, the mean methane emission over MODIS coverage was estimated to be 1.864/spl times/10/sup 9/ g CH/sub 4/ day/sup -1/.

Modeling of the Interannual Variation in Ecosystem Respiration of a Semiarid Grassland

The Mongolian steppe zone comprises a major part of the East Asian grasslands. The regional climate is typically continental and semiarid, with low precipitation and large diurnal and annual temperature fluctuations. Rainfall normally occurs during the warm season (i.e., summer), but large interannual variability in the amount and pattern of rain has been observed and the area has often suffered from droughts. Respiratory CO2 efflux from terrestrial ecosystems (ecosystem respiration, Reco) is an important component of the global carbon cycle, and since Reco includes soil respiration (both root respiration and heterotrophic microbial decomposition) as well as aboveground respiration, rates of Reco depend on complex interactions between biotic and abiotic factors. In general, temperature has been recognized as the major regulator of Reco (Fang & Moncrieff, 2001; Lloyd & Taylor, 1994). Although many temperature models have been proposed, the simplest is the so-called exponential Q10 relationship (van’t Hoff model), whereby the parameter Q10 represents the factor by which the respiration rate increases with a 10°C temperature increase. Soil water is also an important variable controlling Reco, especially in semiarid and arid regions (e.g., Davidson et al., 1998; Hunt et al., 2002; Wen et al., 2006; Xu & Qi, 2001). Studies have shown that both very low and very high water content reduces ecosystem and soil respiration via the direct inhibition of biological activity or the inhibition of oxygen diffusion, respectively (Davidson et al., 1998; Gaumont-Guay et al., 2006; Wen et al., 2006). An increasing number of studies have indicated that Reco rates are positively correlated with features of plant productivity such as the photosynthetic rate, leaf area index, and dry weight of green biomass (e.g., Aires et al., 2008; Crain et al., 1999; Jassal et al., 2008; Sampson et al., 2007). Grassland ecosystems in semiarid regions demonstrate a sensitive response to climate changes, especially to changes in precipitation (Miyazaki et al., 2004; Ni, 2003; Shinoda et al., 2010), suggesting that interannual climate variability can lead to major changes in the respiratory CO2 release from the ecosystem. Studies have demonstrated that the ecosystem switches from a net carbon sink during non-drought years to a net carbon source in drought years (Aires et al., 2008; Meyers, 2001). We conducted field measurements of CO2 fluxes and environmental parameters in a semiarid grassland area of Mongolia since 2004 and have quantitatively documented the