Higher CH4 Fluxes Research Articles

Methane fluxes from a small boreal lake measured with the eddy covariance method

AbstractFluxes of methane, CH4, were measured with the eddy covariance (EC) method at a small boreal lake in Sweden. The mean CH4 flux during the growing season of 2013 was 20.1 nmol m−2 s−1 and the median flux was 16 nmol m−2 s−1 (corresponding to 1.7 mmol m−2 d−1 and 1.4 mmol m−2 d−1). Monthly mean values of CH4 flux measured with the EC method were compared with fluxes measured with floating chambers (FC) and were in average 62% higher over the whole study period. The difference was greatest in April partly because EC, but not FC, accounted for fluxes due to ice melt and a subsequent lake mixing event. A footprint analysis revealed that the EC footprint included primarily the shallow side of the lake with a major inlet. This inlet harbors emergent macrophytes that can mediate high CH4 fluxes. The difference between measured EC and FC fluxes can hence be explained by different footprint areas, where the EC system “sees” the part of the lake presumably releasing higher amounts of CH4. EC also provides more frequent measurements than FC and hence more likely captures ebullition events. This study shows that small lakes have CH4 fluxes that are highly variable in time and space. Based on our findings we suggest to measure CH4 fluxes from lakes as continuously as possible and to aim for covering as much of the lakes surface as possible, independently of the selected measuring technique.

Methane fluxes and rice yields as a function of sulfate fertilizer with incorporated rice stubble

A trial was conducted by applying different sulfate (SO4) rates on methane (CH4) emissions and grain yields (GY) in a field with recently incorporated rice stubble (IRS), 7.5 t ha-1. Ammonium phosphate SO4 fertilizer (42% SO4) was applied at the rates of 0, 50, 100, and 210 kg SO4 ha-1. The whole field was kept flooded with irrigation water. The results showed that the impact of SO4 on CH4 emissions weakened through the stages of rice growth. High daily CH4 fluxes at the reproductive stage governed the quantities of seasonal CH4 emission (SME), and led to a high ratio of SME/IRS. Only the highest rate of 210 kg SO4 ha-1 could reduce SME by 66.9%. The highest GY was 4.08 t ha-1 at 100 kg SO4 ha-1. The whole experiment gave high values of SME/GY and SME/IRS. To reduce CH4 emission without adverse effects on GY, split application of SO4 at 100-155 kg SO4 ha-1 with the last application preferably during the late tillering stage should be tested, along with incorporating rice stubble into the soil immediately after harvest.

Effect of plant species richness on methane fluxes and associated microbial processes in wetland microcosms

Plant species richness influences methane emission from constructed wetlands (CWs), but the underlying mechanisms remain poorly understood. Here, we determined the rate of methane oxidation and flux, the abundance of methanogen 16S rRNA gene and methanotroph pmoA gene (encoding the α-subunit of the particulate methane monooxygenase), and the microbial diversity of methanogens and methanotrophs in the CW microcosm. The microcosms cultivated with a plant species richness level of 1 (monoculture) and 4 (mixture) species were fed with simulated wastewater with a high nitrogen concentration (336mgL−1). The results revealed a methane emission ranging from 0.101 to 0.476mg CH4m−2h−1 in different systems; the planted systems achieved a higher methane flux rate than the unplanted systems. The highest CH4 flux was detected in the mixture system. Qualitative and quantitative analysis of the methanogen and methanotroph diversity revealed a relative stability of the microbial community structures, but the populations varied. Pearson analysis demonstrated a significant correlation between methanogenic and methanotrophic populations and potential methane oxidation activity and CH4 flux, respectively. Redundancy analysis (RDA) showed that the plant biomass exerted an important influence on both methanogenic and methanotrophic communities. The environmental conditions in the mixture systems supported a relatively higher abundance of methanogens and lowest abundance of methanotrophs, which might provide an explanation for the higher CH4 emissions. Rumex japonicus performed better than other plant species in mediating nitrogen removal. The CH4 flux of the monoculture system with Rumex japonicus did not differ from that of the mixture system. Thus, we hypothesize that Rumex japonicus may play an important role in mixed CWs, and its function in the mixture system necessitates further investigation.

Growing season carbon dioxide and methane exchange at a restored peatland on the Western Boreal Plain

Boreal peatlands represent globally important long term sinks of carbon; however, horticultural peat extraction disrupts this carbon sink function, converting these ecosystems to large sources of greenhouse gases. Peatland restoration mitigates these emissions but to date no measurement of greenhouse gas exchange has been conducted on restored peatlands in western Canada, a region where continental climate could impact restoration success. We measured CO2 and CH4 fluxes during the growing season in a restored, cutover peatland in northern Alberta (Boreal Plain Ecozone) and compared these to fluxes measured on a neighboring unrestored area. Restoration resulted in a shift in mean growing season fluxes from 378g CO2C and −0.2g CH4C at the unrestored site to −30g CO2C and 3.7g CH4C at the restored site, where positive values indicate flux of carbon from the peatland to the atmosphere. Carbon dioxide exchange was correlated to vascular vegetation cover that varied depending on local water table position. Water table was also related to CH4 flux, with higher emissions from wet sites. Restoration activities should avoid creating very dry microsites where greenhouse gas emissions will remain high, while very wet sites may accumulate carbon as CO2 but will likely create areas of high CH4 flux.

Diversity and activity of methanotrophs in landfill cover soils with and without landfill gas recovery systems

Aerobic CH4 oxidation plays an important role in mitigating CH4 release from landfills to the atmosphere. Therefore, in this study, oxidation activity and community of methanotrophs were investigated in a subtropical landfill. Among the three sites investigated, the highest CH4 concentration was detected in the landfill cover soil of the site (A) without a landfill gas (LFG) recovery system, although the refuse in the site had been deposited for a longer time (∼14–15 years) compared to the other two sites (∼6–11 years) where a LFG recovery system was applied. In April and September, the higher CH4 flux was detected in site A with 72.4 and 51.7gm−2d−1, respectively, compared to the other sites. The abundance of methanotrophs assessed by quantification of pmoA varied with location and season. A linear relationship was observed between the abundance of methanotrophs and CH4 concentrations in the landfill cover soils (R=0.827, P<0.001). The key factors influencing the methanotrophic diversity in the landfill cover soils were pH, the water content and the CH4 concentration in the soil, of which pH was the most important factor. Type I methanotrophs, including Methylococcus, Methylosarcina, Methylomicrobium and Methylobacter, and type II methanotrophs (Methylocystis) were all detected in the landfill cover soils, with Methylocystis and Methylosarcina being the dominant genera. Methylocystis was abundant in the slightly acidic landfill cover soil, especially in September, and represented more than 89% of the total terminal-restriction fragment abundance. These findings indicated that the LFG recovery system, as well as physical and chemical parameters, affected the diversity and activity of methanotrophs in landfill cover soils.

National-scale estimation of methane emission from paddy fields in Japan: Database construction and upscaling using a process-based biogeochemistry model

We have estimated methane (CH4) emission from total rice (Oryza sativa L.) paddies in Japan by means of a process-based biogeochemistry model, DeNitrification-DeComposition(DNDC)-Rice, combined with a geographic information system (GIS) database of climate, soil and farming practices. In the GIS database, 2 million ha of rice paddies were divided into 17,408 units according to 136 climate areas, 16 soil types, four classes of drainage rate and two classes of groundwater level, to simulate CH4 flux from each of the units applying the DNDC-Rice model. As a result, the national-scale CH4 emission in 1990 was estimated to be 216 Gg carbon (C), 13% lower than a previous inventory estimated by the Tier 2 method. By our Tier 3 approach, a relatively higher CH4 flux was estimated from eastern regions than from western regions of Japan, presumably due to the differences in climate and water management. Sensitivity analysis and uncertainty assessment indicated that it is important to account for the heterogeneity in soil properties such as field water capacity, iron (Fe) concentration and drainage rate, in order to reduce the uncertainty in regional estimates.

Spatial variation in landscape‐level CO2 and CH4 fluxes from arctic coastal tundra: influence from vegetation, wetness, and the thaw lake cycle

Regional quantification of arctic CO2 and CH4 fluxes remains difficult due to high landscape heterogeneity coupled with a sparse measurement network. Most of the arctic coastal tundra near Barrow, Alaska is part of the thaw lake cycle, which includes current thaw lakes and a 5500-year chronosequence of vegetated thaw lake basins. However, spatial variability in carbon fluxes from these features remains grossly understudied. Here, we present an analysis of whole-ecosystem CO2 and CH4 fluxes from 20 thaw lake cycle features during the 2011 growing season. We found that the thaw lake cycle was largely responsible for spatial variation in CO2 flux, mostly due to its control on gross primary productivity (GPP). Current lakes were significant CO2 sources that varied little. Vegetated basins showed declining GPP and CO2 sink with age (R(2) = 67% and 57%, respectively). CH4 fluxes measured from a subset of 12 vegetated basins showed no relationship with age or CO2 flux components. Instead, higher CH4 fluxes were related to greater landscape wetness (R(2) = 57%) and thaw depth (additional R(2) = 28%). Spatial variation in CO2 and CH4 fluxes had good satellite remote sensing indicators, and we estimated the region to be a small CO2 sink of -4.9 ± 2.4 (SE) g C m(-2) between 11 June and 25 August, which was countered by a CH4 source of 2.1 ± 0.2 (SE) g C m(-2) . Results from our scaling exercise showed that developing or validating regional estimates based on single tower sites can result in significant bias, on average by a factor 4 for CO2 flux and 30% for CH4 flux. Although our results are specific to the Arctic Coastal Plain of Alaska, the degree of landscape-scale variability, large-scale controls on carbon exchange, and implications for regional estimation seen here likely have wide relevance to other arctic landscapes.

Effects of plant species on soil microbial processes and CH4 emission from constructed wetlands

Methane (CH4) emission from constructed wetland has raised environmental concern. This study evaluated the influence of mono and polyculture constructed wetland and seasonal variation on CH4 fluxes. Methane emission data showed large temporal variation ranging from 0 to 249.29 mg CH4 m−2 h−1. Results indicated that the highest CH4 flux was obtained in the polyculture system, planted with Phragmites australis, Zizania latifolia and Typha latifolia, reflecting polyculture system could stimulate CH4 emission. FISH analysis showed the higher amount of methanotrophs in the profile of Z. latifolia in both mono and polyculture systems. The highest methanogens amount and relatively lower methanotrophs amount in the profile of polyculture system were obtained. The results support the characteristics of CH4 fluxes. The polyculture constructed wetland has the higher potential of global warming.

Varying atmospheric methane concentrations affect soil methane oxidation rates and methanotroph populations in pasture, an adjacent pine forest, and a landfill

We describe experiments to better understand how CH4 oxidation rates by different methanotroph communities respond to changing CH4 concentrations. We used a novel system of automatically monitored chambers to investigate the response of CH4 oxidation rates in a New Zealand pasture and adjacent pine forest soil exposed to varying atmospheric CH4 concentrations.Type II methanotrophs that dominate CH4 oxidation in the forest soil became progressively saturated as CH4 concentrations rose from ambient (1.8ppmv) to 570ppmv, as shown by a decrease in uptake efficiency from 20% to 2% removal. By contrast, CH4 oxidation in the pasture soil where Type I methanotrophs dominate increased in proportion to the increase in CH4 inlet concentration, oxidising about 2% of the inlet CH4 flux throughout. Modelling based on Michaelis-Menten kinetics revealed that low-affinity (Type I) methanotrophs were solely responsible for CH4 oxidation in pasture soils, whereas high affinity (Type II) methanotrophs only contributed about 10% of the CH4 oxidation in the forest soil. Increased aeration status using a soil–perlite (1:1) mixture doubled CH4 oxidation rates at both ambient (1.8ppmv) and 40ppmv atmospheric CH4. A similar volcanic soil previously exposed for 8 y to high CH4 fluxes from a landfill had removal efficiencies consistently above 95% for atmospheric CH4 concentrations up to 7500ppmv when the CH4 oxidation rate was7000μg CH4 kg−1soil h−1.

Annual variation of CH4 emissions from the middle taiga in West Siberian Lowland (2005–2009): a case of high CH4 flux and precipitation rate in the summer of 2007

ABSTRACTWe described continuous measurements of CH4 and CO2 concentration obtained at two sites placed in the middle taiga, Karasevoe (KRS) and Demyanskoe (DEM), in West Siberian Lowland (WSL) from 2005 to 2009. Although both CH4 and CO2 accumulation (ΔCH4 and ΔCO2) during night-time at KRS in June and July 2007 showed an anomalously high concentration, higher ratios of ΔCH4/ΔCO2 compared with those in other years indicated that a considerably higher CH4 flux occurred relative to the CO2 flux. The daily CH4 flux calculated with the ratio of ΔCH4/ΔCO2 and terrestrial biosphere CO2 flux from an ecosystem model showed a maximum in July at the both sites. Although anomalously high flux was observed in June and July 2007 at KRS, only a small flux variation was observed at DEM. The high regional CH4 flux in June and July 2007 at KRS was reproduced using a process-based ecosystem model, Vegetation Integrative Simulator for Trace gases (VISIT), in response to high water table depth caused by the anomalously high precipitation during the summer of 2007.

Seasonal and spatial variations of methane emissions from montane wetlands in Northeast China

Abstract To evaluate the seasonal and spatial variations of CH4 emissions and understand the controlling factors, we measured CH4 fluxes and their environmental variables from seven natural wetlands in mountainous regions in northeast China using a static chamber technique during a growing season from May to October in 2008. Four sites were significant atmospheric CH4 sources, ranked in order from highest to lowest according to their seasonal mean CH4 release; these sites were a marsh (34.18 mgCH4 m−2 h−1), two deciduous forested swamps (0.83–18.21 mgCH4 m−2 h−1) and a thicket swamp (0.43 mgCH4 m−2 h−1). Coniferous forested swamps, forested fens and bogs are unique wetlands in northeast China and represent large wetland coverage in this zone, but they were observed to be weak sinks of atmospheric CH4 (−0.08 to −0.01 mgCH4 m−2 h−1). Similar seasonal variations can be observed at marsh, thicket swamp and two deciduous forested swamps sites, with peaks were observed during the summer and early autumn (July to early September). However, no seasonal pattern was found at the other three sites. Seasonal variations of CH4 fluxes were primarily affected by the soil temperature. However, spatial variation among wetlands was mainly controlled by the water table, the soil temperature, plant aboveground biomass and potential CH4 production. A high water table and herb-dominant sites had high potential CH4 production rates and thus induced high CH4 fluxes. In contrast, a low water table and tree- or moss-dominant sites had low potential CH4 production rates and induced low CH4 fluxes.

Methane dynamics in different boreal lake types

Abstract. This study explores the variability in concentrations of dissolved CH4 and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from −2.8 to 5.9°C. We examined how lake CH4 dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH4 concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH4 concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH4 concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH4 release from randomly selected lakes was 49 mmol m−2 a−1. The sum CH4 flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH4 release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boreal region small lakes have higher CH4 fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH4 release.

Resistivity‐based monitoring of biogenic gases in peat soils

Biogenic free‐phase gas (FPG) formation was induced in a peat block (dimensions 0.28 × 0.21 × 0.21 m) extracted from a peatland in Maine. Electrical resistivity (ER), surface deformation, and methane (CH4) flux from the peat surface was monitored over a 48‐day period during which the temperature remained constant at 21 ± 1°C. ER measurements were made on 5 vertical electrode arrays, each containing 20 electrodes spaced at 0.01‐m intervals. Surface deformation was monitored using 30 elevation rods equally spaced across the surface of the block, and average CH4 flux from the peat surface estimated by integration of measurements obtained with a portable gas detector over a 20‐min time period. Pore water conductivity was recorded at three depths (0.06, 0.09, and 0.15 m) at a single point in the block. ER measurements were inverted for the ratio resistivity change relative to a background data set and corrected for changes in pore fluid conductivity, permitting an estimate of equivalent change in gas content assuming (1) insignificant surface conduction, (2) porosity changes estimated from peat surface expansion, and (3) an Archie saturation exponent n of 1.3 based on results from a parallel block experiment. The resistivity ratios reveal a pattern of FPG evolution consistent with surface deformation and CH4 flux data. During the first part of the experiment (approximately the first 24 days), a gradual buildup in FPG within a layer 4–6 cm below the peat surface (water table) occurs concurrent with modest surface deformation and low CH4 fluxes. In contrast, during the latter half of the experiment (approximately 25–48 days), a complex pattern of more pronounced gas buildup and release at multiple depths occurs concurrent with large rates of surface deformation and higher CH4 fluxes. The experiment demonstrates that ER monitoring is a viable geophysical technology for imaging and monitoring biogenic gas fluxes in peat soils. Here the resistivity clearly shows that FPG is preferentially generated in layers about 0.04–0.06 m below the peat surface and that the buildup of gas is spatially nonuniform even in a relatively small peat block. Furthermore, the experimental results suggest that factors other than temperature and atmospheric pressure must control biogenic gas accumulation and release. As the method is readily deployable at the field scale, possibly in an autonomous monitoring mode, resistivity measurements may permit significant improvements in understanding of carbon gas generation and release from northern peatlands.

Sedge Succession and Peatland Methane Dynamics: A Potential Feedback to Climate Change

Under the warmer climate, predicted for the future, northern peatlands are expected to become drier. This drying will lower the water table and likely result in reduced emissions of methane (CH4) from these ecosystems. However, the prediction of declining CH4 fluxes does not consider the potential effects of ecological succession, particularly the invasion of sedges into currently wet sites (open water pools, low lawns). The goal of this study was to characterize the relationship between the presence of sedges in peatlands and CH4 efflux under natural conditions and under a climate change simulation (drained peatland). Methane fluxes, gross ecosystem production, and dissolved pore water CH4 concentrations were measured and a vegetation survey was conducted in a natural and drained peatland near St. Charlesde-Bellechasse, Quebec, Canada, in the summer of 2003. Each peatland also had plots where the sedges had been removed by clipping. Sedges were larger, more dominant, and more productive at the drained peatland site. The natural peatland had higher CH4 fluxes than the drained peatland, indicating that drainage was a significant control on CH4 flux. Methane flux was higher from plots with sedges than from plots where sedges had been removed at the natural peatland site, whereas the opposite case was observed at the drained peatland site. These results suggest that CH4 flux was enhanced by sedges at the natural peatland site and attenuated by sedges at the drained peatland site. However, the attenuation of CH4 flux due to sedges at the drained site was reduced in wetter periods. This finding suggests that CH4 flux could be decreased in the event of climate warming due to the greater depth to the water table, and that sedges colonizing these areas could further attenuate CH4 fluxes during dry periods. However, during wet periods, the sedges may cause CH4 fluxes to be higher than is currently predicted for climate change scenarios.

A comparison of methane flux in a boreal landscape between a dry and a wet year

We used field measurements of methane (CH4) flux from upland and wetland soils in the Northern Study Area (NSA) of BOREAS (BOReal Ecosystem‐Atmosphere Study), near Thompson, Manitoba, during the summers of 1994 and 1996 to estimate the overall CH4 emission from a 1350 km2 landscape. June–September 1994 and 1996 were both drier and warmer than normal, but summer 1996 received 68 mm more precipitation than 1994, a 40% increase, and had a mean daily air temperature 0.6°C warmer than 1994. Upland soils consumed CH4 at rates from 0 to 1.0 mg m−2 d−1, with small spatial and temporal variations between years, and a weak dependence on soil temperature. In contrast, wetlands emitted CH4 at seasonal average rates ranging from 10 to 350 mg CH4 m−2 d−1, with high spatial and temporal variability, and increased an average of 60% during the wetter and warmer 1996. We used Landsat imagery, supervised classification, and ground truthing to scale point CH4 fluxes (&lt;1 m2) to the landscape (&gt;1000 km2). We performed a sensitivity analysis for error terms in both areal coverage and CH4 flux, showing that the small areas of high CH4 emission (e.g., small ponds, graminoid fens, and permafrost collapse margins) contribute the largest uncertainty in both flux measurements and mapping. Although wetlands cover less than 30% of the landscape, areally extrapolated CH4 flux for the NSA increased by 61% from 10 to16 mg CH4 m−2 d−1 between years, entirely attributed to the increase in wetland CH4 emission. We conclude that CH4 fluxes will tend to be underestimated in areas where much of the landscape is covered by wetlands. This is due to the large spatial and temporal variability encountered in chamber‐based measurements of wetland CH4 fluxes, strong sensitivity of wetland CH4 emission to small changes in climate, and because most remote sensing images do not adequately identify small areas of high CH4 flux.

CH4 sources estimated from atmospheric observations of CH4 and its 13C/12C isotopic ratios: 1. Inverse modeling of source processes

A time‐dependent inverse modeling approach that estimates the global magnitude of atmospheric methane sources from the observed spatiotemporal distribution of atmospheric CH4, 13C/12C isotopic ratios, and a priori estimates of the source strengths is presented. Relative to the a priori source estimates, the inverse model calls for increased CH4 flux from sources with strong spatial footprints in the tropics and Southern Hemisphere and decreases in sources in the Northern Hemisphere. The CH4 and 13C/12C isotopic ratio observations suggest an unusually high CH4 flux from swamps (∼200 ± 44 Tg CH4/yr) and biomass burning (88 ± 18 Tg CH4/yr) with relatively low estimates of emissions from bogs (∼20 ± 14 Tg CH4/yr), and landfills (35 ± 14 Tg CH4/yr). The model results support the hypothesis that the 1998 CH4 growth rate anomaly was caused in part by a large increase in CH4 production from wetlands, and indicate that wetland sources were about 40 Tg CH4/yr higher in 1998 than 1999.

A geochemical perspective and assessment of leakage potential for a mature carbon dioxide–enhanced oil recovery project and as a prototype for carbon dioxide sequestration; Rangely field, Colorado

Measurements of CO2 and CH4 soil gas concentrations and gas exchange with the atmosphere at a large-scale CO2-enhanced oil recovery (EOR) operation at Rangely, Colorado, United States allowed assessment of the microseepage potential. Shallow and deep soil gas concentrations and direct transport of CO2 and CH4, complemented by carbon isotopes, have demonstrated an estimated microseepage rate to the atmosphere of approximately 400 t CH4/yr from the 78-km2 area of the Rangely field. Preliminary estimates of deep-sourced CO2 losses are in the range of 170 to less than 3800 t/yr. Several holes as much as 9 m deep for nested sampling of gas composition, stable carbon isotopic ratios for CO2 and CH4, and carbon-14 measurements on CO2 indicate that deep-sourced CO2 microseepage loss was detected. Methanotrophic oxidation of microseeping CH4 to CO2 in soils demonstrates significant contribution to soil gas CO2 in high CH4 flux areas, necessitating a revision of the estimated direct CO2 microseepage rate to less than 170 t/yr over the Rangely field.An evaluation of produced water quality from pre-CO2-EOR to 1999 demonstrates an increase in some parameters, particularly of Ca2+ and HCO3, indicating dissolution of ferroan calcite and ferroan dolomite in the Weber Formation. Inverse computer modeling suggested carbonate mineral sequestration was possible within the constraints of evolution of produced water quality. X-ray analyses of well scales, however, do not support the presence of significant mineral sequestration. Instead, modeling indicates that the bulk of the CO2 that has been injected since 1986 is sequestered as dissolved CO2.

Carbon dioxide and methane dynamics in a sub-Arctic peatland in northern Finland

We studied carbon dynamics on various surface parts of a highly patterned fen, typical in northern Finland, to examine the importance of different microsites to the areal carbon fluxes. The studies were carried out in June-September 1995 on a mesotrophic flark fen (an aapa mire) in Kaamanen (69°08?N, 27° 17?E). Wet flarks, moist lawns and dry strings accounted for 60%, 10% and 30% of the surface area, respectively. A static chamber technique was applied to measure the CH4 exchange, the instantaneous net ecosystem exchange (NEE, transparent chamber) and the ecosystem respiration (Rtot? opaque chamber) in several microsites. The static chamber results were compared with those obtained by the eddy covariance technique. The mean daytime areal net ecosystem CO2 exchange rate measurement in conditions where photosynthesis was light saturated (PAR>400 ?mol m-2 s-1) varied during the measurement period from ?59 mg CO2-C m?2h?1 (release) to 250 (uptake). The mean CH4 emission during the measuring period was 78 mg CH4-Cm?2 d?1 on the flarks, 68 mg on the lawn and 6.0 mg on the strings. The strings without shrubs (mainly Betula nana) were in general net sources of CO2, even during the middle of the growing season, whereas the lawns, flarks and also strings growing B. nana showed a daytime net uptake of CO2. Areally integrated chamber results showed lower CO2 and higher CH4 fluxes than predicted from the eddy covariance measurements.

Carbon dioxide and methane dynamics in a sub-Arctic peatland in northern Finland

We studied carbon dynamics on various surface parts of a highly patterned fen, typical in northern Finland, to examine the importance of different microsites to the areal carbon fluxes. The studies were carried out in June-September 1995 on a mesotrophic flark fen (an aapa mire) in Kaamanen (69°08?N, 27° 17?E). Wet flarks, moist lawns and dry strings accounted for 60%, 10% and 30% of the surface area, respectively. A static chamber technique was applied to measure the CH4 exchange, the instantaneous net ecosystem exchange (NEE, transparent chamber) and the ecosystem respiration (Rtot? opaque chamber) in several microsites. The static chamber results were compared with those obtained by the eddy covariance technique. The mean daytime areal net ecosystem CO2 exchange rate measurement in conditions where photosynthesis was light saturated (PAR>400 ?mol m-2 s-1) varied during the measurement period from ?59 mg CO2-C m?2h?1 (release) to 250 (uptake). The mean CH4 emission during the measuring period was 78 mg CH4-Cm?2 d?1 on the flarks, 68 mg on the lawn and 6.0 mg on the strings. The strings without shrubs (mainly Betula nana) were in general net sources of CO2, even during the middle of the growing season, whereas the lawns, flarks and also strings growing B. nana showed a daytime net uptake of CO2. Areally integrated chamber results showed lower CO2 and higher CH4 fluxes than predicted from the eddy covariance measurements.

Simultaneous records of methane and nitrous oxide emissions in rice-based cropping systems under rainfed conditions

Rainfed rice (Oryza sativa L.)-based cropping systems are characterized by alternate wetting and drying cycles as monsoonal rains come and go. The potential for accumulation and denitrification of NO3− is high in these systems as is the production and emission of CH4 during the monsoon rice season. Simultaneous measurements of CH4 and N2O emissions using automated closed chamber methods have been reported in irrigated rice fields but not in rainfed rice systems. In this field study at the International Rice Research Institute, Philippines, simultaneous and continuous measurements of CH4 and N2O were made from the 1994 wet season to the 1996 dry season. During the rice-growing seasons, CH4 fluxes were observed, with the highest emissions being in organic residue-amended plots. Nitrous oxide fluxes, on the other hand, were generally nonexistent, except after fertilization events where low N2O fluxes were observed. Slow-release N fertilizer further reduced the already low N2O emissions compared with prilled urea in the first rice season. During the dry seasons, when the field was planted to the upland crops cowpea [Vigna unguiculata (L.) Walp] and wheat (Triticum aestivum L.), positive CH4 fluxes were low and insignificant except after the imposition of a permanent flood where high CH4 fluxes appeared. Evidences of CH4 uptake were apparent in the first dry season, especially in cowpea plots, indicating that rainfed lowland rice soils can act as sink for CH4 during the upland crop cycle. Large N2O fluxes were observed shortly after rainfall events due to denitrification of accumulated NO3−. Cumulative CH4 and N2O fluxes observed during this study in rainfed conditions were lower compared with previous studies on irrigated rice fields.