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Arctic regions are experiencing rapid warming, leading to permafrost thaw and formation of numerous water bodies. Although small ponds in particular are considered hot spots for methane (CH4) relea ...

Northern lakes are important sources of the climate forcing trace gases methane (CH4) and carbon dioxide (CO2). A substantial portion of lakes' annual emissions can take place immediately after ice melt in spring. The drivers of these fluxes are neither well constrained nor fully understood. We present a detailed carbon gas budget for three subarctic lakes, using 6 years of eddy covariance and 9 years of manual flux measurements. We combine measurements of temperature, dissolved oxygen, and CH4 stable isotopologues to quantify functional relationships between carbon gas production and conversion, energy inputs, and the redox regime. Spring emissions were regulated by the availability of oxygen in winter, rather than temperature as during ice‐free conditions. Under‐ice storage increased predictably with ice‐cover duration, and CH4 accumulation rates (25 ± 2 mg CH4‐C·m−2·day−1) exceeded summer emissions (19 ± 1 mg CH4‐C·m−2·day−1). The seasonally ice‐covered lakes emitted 26–59% of the annual CH4 flux and 15–30% of the annual CO2 flux at ice‐off. Reduced spring emissions were associated with winter snowmelt events, which can transport water downstream and oxygenate the water column. Stable isotopes indicate that 64–96% of accumulated CH4 escaped oxidation, implying that a considerable portion of the dissolved gases produced over winter may evade to the atmosphere.

The release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH₄) clumped isotope (Δ₁₈) and ¹⁴C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ₁₈ values are negatively correlated with CH₄ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH₄ Δ₁₈ and the fraction modern carbon. These correlations imply that CH₄ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ₁₈ values, are not positively correlated with CH₄ flux estimates, highlighting the likely importance of environmental variables other than CH₄ production rates in controlling ebullition fluxes.

Ebullition (bubbling) from climate-sensitive northern lakes remains an unconstrained source of atmospheric methane (CH4). Although the focus of many recent studies, ebullition is rarely linked to t ...

U.S. Department of Energy [DE-SC0004632, DE-SC0010580]; U.S. National Science Foundation (MacroSystems Biology) [1241937]; Northern Ecosystems Research for undergraduates REU Site (NSF EAR) [1063037]; Vetenskapradet (DR) [2007-4547, 2013-5562]

The Laptev and East Siberian Seas have been proposed as a substantial source of methane (CH4) to the atmosphere. During summer 2014, we made unique high-resolution simultaneous measurements of CH4 ...

Sediments deposited along continental margins of the Arctic Ocean presumably host large amounts of methane (CH4) in gas hydrates. Here we apply numerical simulations to assess the potential of gas ...

The ice-cover season and subsequent spring thaw are thought to be of particular importance for the biogeochemical cycle of northern lakes and wetlands. Yet the magnitude of their methane emissions ...

Strong correlations between seasonal energy input and methane (CH4) bubbling (ebullition) in northern lakes suggest that energy proxies might provide a constraint on the magnitude of future CH4 emi ...

Emission of methane (CH4) from surface waters is often dominated by ebullition (bubbling), a transport mode with high-spatiotemporal variability. Based on new and extensive CH4 ebullition data, we ...

Ebullition (bubbling) from small lakes and ponds at high latitudes is an important yet unconstrained source of atmospheric methane (CH4). Small water bodies are most abundant in permanently frozen ...

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%).

Nitric oxide (NO), nitrous oxide (N2O), and methane (CH4) are naturally produced and consumed by soil biogeochemical processes. Naturally high variation between trace gas fluxes may temporarily increase due to agricultural management. We studied spatial and temporal variability of fluxes in the context of a 3-year field experiment established to identify and quantify N2O fluxes and controlling factors using automated field measurements. We measured trace gas fluxes, soil temperature, and moisture from fertilized and unfertilized balsa (Ochroma lagopus) plantations. Combining spatial and temporal sampling we evaluate if automatically measured time series of N2O emissions are representative of overall mean fluxes from fertilized loam under balsa. Soil trace gas fluxes were measured manually at 36 randomly distributed sampling locations per plot. Mean plot emissions were evaluated against fluxes measured by seven chambers commonly used for routine bimonthly manual measurements and against N2O emissions measured by two automated chambers at 4.6-hour sampling intervals. Trace gas fluxes were highly variable over 40 × 40 m plots. Nitrogen oxide fluxes were mainly spatially independent. Fertilization increased nitrogen oxide emissions but did not introduce spatial dependency of flux data. Within about 6 weeks fluxes approached pre-fertilization level again. Given high spatial variation of nitrogen oxide fluxes we find that automatically measured N2O fluxes represent the nature of the flux response well and are in the range of fluxes indicated by spatial sampling. When soils were relatively dry fertilization inhibited CH4 uptake.

Net ecosystem exchange (NEE) of CO2 was measured in four peatlands along plant community, hydrologic, and water chemistry gradients from bog to rich fen in a diverse peatland complex near Thompson, Manitoba, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). A simple model for estimating growing season net ecosystem productivity (NEP) using continuous measurements of photosynthetically active radiation (PAR), and peat temperature was constructed with weekly chamber measurements of NEE from May to October 1996. The model explained 79–83% of the variation in NEE across the four sites. Model estimation and parameter uncertainty calculations were performed using nonlinear regression analyses with a maximum likelihood objective function. The model underestimated maximum NEE and respiration during the midseason when the standard errors for each parameter were greatest. On a daily basis, uncertainty in the midday NEE simulation was higher than at night. Although the magnitude of both photosynthesis and respiration rates followed the trophic gradient bog less than poor fen less than intermediate fen less than rich fen, NEP did not follow the same pattern. NEP in the bog and rich fen was close to zero, while the poor and intermediate fens had higher NEP due to a greater imbalance between uptake and release of CO2. Although all sites had a positive growing season NEP, upper and lower 95% confidence limits showed that the bog and rich fen were either a source or sink of CO2 to the atmosphere, while the sedge-dominated poor and intermediate fens were accumulating approximately 20–100 g CO2 C m−2 over the 5 month period in 1996. Peatland ecosystems may switch from a net sink to a source of carbon on short timescales with small changes in soil temperature or water table position. Since the difference between production and decomposition is small, it is important to understand and quantify the magnitude of uncertainty in these measurements in order to predict the effect of climatic change on peatland carbon exchange.

Published estimates of removal of atmospheric methyl bromide (CH3Br) by agricultural soils are 2.7 Gg yr−1 (Gg = 109 g) [Shorter et al., 1995] and 65.8 Gg yr−1 [Serca et al., 1998]. The Serca et al. estimate, if correct, would suggest that the current value for total removal of atmospheric CH3Br by all sinks of 206 Gg yr−1 (based on Shorter et al., 1995) would be 30% too low. We have calculated a new rate of global agricultural soil uptake of atmospheric CH3Br from a larger sampling of cultivated soils collected from 40 sites located in the United States, Costa Rica, and Germany. First order reaction rates were measured during static laboratory incubations. These data were combined with uptake measurements we reported earlier based on field and laboratory experiments [Shorter et al. 1995]. Tropical (10.2°–10.4°N) and northern (45°–61°N) soils averaged lower reaction rate constants than temperate soils probably due to differing physical and chemical characteristics as well as microbial populations. Our revised global estimate for the uptake of ambient CH3Br by cultivated soils is 7.47±0.63 Gg yr−1, almost three times the value that we reported in 1995.

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July-August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May-June) and fall (September-October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub-dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze-up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.

Rates of methane (CH 4 ) oxidation were measured in three wet sedge communities on Alaska's North Slope in 1993 and 1995 using the selective inhibitor methyl fluoride (CH 3 F). Comparison of CH 4 flux prior to inhibition (net flux) with flux after inhibition (gross flux) enables one to infer CH 4 oxidation rates by difference. Oxidation rates from Franklin Bluffs, Sagavanirktok River floodplain, Toolik Lake Inlet, and Toolik Lake Outlet averaged 24.9, 14.7 (1.6), (17.6), and 25.2 (49.8) mg CH 4 /m 2 /d in 1995 (1993), respectively. Plot level data suggest that oxidation rates vary greatly spatially and temporally down to the meter scale. As a percentage of flux, however, mean CH 4 oxidation rates of 13.4, 21.7 (2.0), (16.5), and 19.4 (38.0)% at Franklin Bluffs, Sagavanirktok River, Toolik Lake Inlet, and Toolik Lake Outlet in 1995 (1993), respectively, were seen to be rather uniform over the region. The effects of temperature on CH 4 production appear to be the dominant control on CH 4 oxidation rates in wet sedge environments as opposed to soil moisture dependency in upland soils. Nutrient fertilization was not found to influence the fraction of CH 4 lost to oxidation. This suggests that CH 4 oxidation, while significant in tundra wetlands, may consume a near-constant fraction of available CH 4 .

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe, NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = alpha PPFD P(sub max)/(alpha PPFD + P(sub max) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = beta PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = -2.0 micro mol m(exp -2) s(exp -1) for bogs and -2.7 micro mol m(exp -2) s(exp -1)) for fens) and lower NEE at moderate and high light levels (P(sub max)= 5.2 micro mol m(exp -2) s(exp -1) for bogs and 10.8 micro mol m(exp -2) s(exp -1) for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = -2.4 micro mol m(exp -2) s(exp -1)) and NEE rates (alpha = 0.020 and P(sub max)= 9.2 micro mol m(exp -2) s(exp -1)) than the upland ecosystems (closed canopy forest, grassland, and cropland). Despite this low productivity, northern peatland soil carbon pools are generally 5-50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.

High-spectral resolution reflectance data were acquired in the laboratory for several species of boreal forest and peatland mosses. Feather mosses and lichens (forests), brown mosses (rich fens), and Sphagnum species (bogs and poor fens) were collected from sites in the northern study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS) near Thompson, Manitoba, and from three peatlands in southeastern New Hampshire. Because mosses are indirect indicators of soil moisture, trace gas flux, and carbon accumulation, these data may enable hyperspectral remote sensing systems, such as the airborne visible and infrared imaging spectrometer and the compact airborne spectrographic imager (AVIRIS) (CASI), as well as future systems such as Lewis and the first New Millennium Project, to infer differences in hydrology and carbon cycling in boreal ecosystems at small spatial scales (< 1–900 m2). Mosses exhibit distinctly different spectral characteristics from vascular plants in the visible, near-infrared (NIR) and short-wave infrared (SWIR) regions. In the visible portion of the spectrum, mosses exhibit typical absorption in the blue and red regions but differ from vascular plants in having a “green” peak reflective of the color (red, brown, or green) of the individual species. The moss reflectance in the NIR spectral region is typically less reflective than the same region in vascular plants and is characterized by strong water absorption features located at approximately 1.00 and 1.20 μm, resulting in pronounced reflectance peaks at approximately 0.85, 1.10, and 1.30 μm (identified as NIR 1, 2, and 3, respectively). The variations in the relative brightness of the three NIR peaks in each of the three groups of mosses (Sphagnum, feather, and brown mosses) are diagnostic and may be indicative of different cellular characteristics. The slope of the NIR plateau as determined by the NIR 3/1 ratio, as well as the shape of the NIR 1 peak and relative dominance of NIR 2, separate the three groups. In addition, the NIR 1 peak in Sphagnum species is marked by a minor absorption feature at 0.85 μm, which is absent in all brown mosses and feather mosses, as well as in vascular plants. Because this absorption feature results in a narrow NIR 1 peak, and the red absorption is also narrow in Sphagnum, the standard normalized difference vegetation index/simple vegetation index ratio (NDVI/VI) methods do not work well in characterizing biomass or greenness. The overall reflectance of mosses in the SWIR region (1.50–2.50 μm) is lower than that of vascular plants because of the higher water content of the moss tissue. In lichens, reflectance in this region is higher than either mosses or vascular plants, due likely to tissue dryness. The lichens included in this study are most dissimilar from mosses and vascular plants in the visible portion of the spectrum.

The surface-atmospheric exchange of CO2 and CH4 was measured continuously using the flux gradient approach from a beaver pond in the northern study area of the Boreal Ecosystem-Atmosphere Study between May 22 and September 19, 1994. The beaver pond was a large source of CO2 and CH4 for the entire study period. The half-hourly mean flux of CO2 and CH4 ranged from −0.498 to 1.135 mg CO2 m−2 s−1 and from −0.805 to 37.5 μg CH4 m−2 s−1, respectively, while the seasonal mean fluxes were 0.072±0.095 mg CO2 m−2 s−1 and 1.26±1.87 μg CH4 m−2 s−1. The beaver pond rarely took up CO2. There was a large flux of both gases during the daytime. This increase is related to the transfer of the gases rather than to specific controls on production. The total efflux of CO2 and CH4 for the 120 days of the study was 678 g CO2 m−2 and 11.3 g CH4 m−2, or 183 and 8.4 g C m−2, respectively. When the measurements ceased, the sediment temperatures were >10°C, so it is reasonable to expect that the fluxes of CO2 and CH4 continued into the late fall. This indicates that the beaver pond released more than 200 g C m−2 yr−1.

Measurements of soil-surface CO2 fluxes are important for characterizing the carbon budget of boreal forests because these fluxes can be the second largest component of the budget. Several methods for measuring soil-surface CO2 fluxes are available: (1) closed-dynamic-chamber systems, (2) closed-static-chamber systems, (3) open-chamber systems, and (4) eddy covariance systems. This paper presents a field comparison of six individual systems for measuring soil-surface CO2 fluxes with each of the four basic system types represented. A single system is used as a reference and compared to each of the other systems individually in black spruce (Picea mariana), jack pine (Pinus banksiana), or aspen (Populus tremuloides) forests. Fluxes vary from 1 to 10 μmol CO2 m−2 s−1. Adjustment factors to bring all of the systems into agreement vary from 0.93 to 1.45 with an uncertainty of about 10–15%.

CH4 and CO2 fluxes were measured in upland boreal forest soils near Thompson, Manitoba, from May 16 to September 16, 1994. Most sites consumed atmospheric CH4, fluxes ranging from +0.6 to −2.6 mg CH4 m−2 d−1, and emitted CO2 at rates between 0.2 and 26.8 g CO2 m−2 d−1. There was some evidence of episodic CH4 emissions after heavy rainfall from soils which normally consumed CH4. There were two distinct groups: sites in which both CO2 and CH4 exchange was strong (mean 5.2 g CO2 m−2 d−1, −1.0 mg CH4 m−2 d−1); and those which had a weak exchange of both gases (mean 2.5 g CO2 m−2 d−1, −0.2 mg CH4 m−2 d−1). The presence of black spruce trees, a Sphagnum spp. ground cover and a thick organic layer (20–50 cm) characterized the weak exchange group. These characteristics were indicative of colder, wetter conditions with slower N cycling and longer path lengths to the zone of CH4 oxidation. The strong exchange group had either aspen, jack pine, or birch trees; a vascular plant cover; and a thin organic layer (1–5 cm). These characteristics were indicative of warmer, drier conditions with faster N cycling and shorter path lengths to the zone of CH4 oxidation. The seasonal average of both CO2 and CH4 flux from 11 sites could be predicted by regressions involving the amount of soil organic matter (0–5 cm depth) and seasonal temperature at a depth of 20 cm (r2=0.69 and 0.82, respectively). Using these vegetation characteristics and a Landsat image of the Boreal Ecosystem-Atmosphere Study (BOREAS) northern study area for areal weighting, net gas fluxes were calculated for the upland soils, estimated to be −0.4 mg CH4 m−2 d−1 and 4 g CO2 m−2 d−1.

Controls on the emission of CH4 and CO2 along moisture gradients in the northern study area of the Boreal Ecosystem-Atmosphere Study (BOREAS), west of Thompson, Manitoba, were examined in the summer of 1994. A transect stretching from three forested upland sites to an open graminoid fen contained within a black spruce (OBS) palsa was compared with a similar transect from upland lichen and Sphagnum moss sites to wetland mire and pond sites adjacent to an active beaver pond (BP). Mean seasonal CH4 (dark CO2) fluxes were −0.51 (7.99), −0.14 (10.3), 0.15 (8.68), and 2.83 (8.22) mg/m2/d (g/m2/d) from the OBS feather moss, lichen, sphagnum moss, and fen sites. Mean seasonal CH4 (dark CO2) fluxes were −1.58 (9.16), 2.03 (7.62), 1144, and 153 mg/m2/d (g/m2/d) from the BP lichen, sphagnum moss, mire, and pond sites. Rates of CH4 oxidation typical of uplands were observed at all sites except for the BP mire and pond. CH4 efflux was observed in all sites. Rates varied by 4 orders of magnitude along the transition from the driest uplands to moist uplands to inundated wetlands. A high water table appears to be necessary for a strong CH4 source. Low soil temperatures inhibit high CH4 flux rates despite inundation. The potential for the conversion of upland sites from net sinks to sources of CH4 by minor changes in the water table was observed. Prediction of CH4 fluxes by plant community analysis appears to require knowledge of the thermal and hydrologic regime of particular sites over the course of the summer period. Dark CO2 fluxes, by contrast, appear more uniform across similar plant community types.

The dynamic carbon balance of a southeastern New Hampshire wetland was constructed for the 1994 growing season using a light-dark box sampling method. Net ecosystem exchange (NEE) (n = 414) and ecosystem respiration (n=234) measurements were made at the 1.7 ha Sphagnum spp. dominated poor fen. The NEE rates ranged from −192 to 492 mg C m−2 h−1 and the ecosystem respiration measurements were between −10 and −365 mg C m−2 h−1. The negative values represent a loss of carbon from the wetland system. NEE and respiration data were used to derive photosynthesis rates of the vegetation at the study site. A simple model, using hourly averages of photosynthetically active radiation, and air and soil temperatures to generate hourly rates of photosynthesis and respiration, was constructed to interpolate the carbon cycling rates at this fen through the entire 1994 growing season. Results of the carbon balance model suggest that the wetland lost an estimated 145 g C m−2 for the 9 month modeling period (April through December). The 1994 climate season was warmer (+1.15°C/month)and drier (−12.3 cm) than the 30 year normals for Durham, New Hampshire, the nearest meteorological station. These data suggest that if future climate change brings about warmer temperatures and lower water tables in peatland soils, positive climatic feedback leading to substantial releases of CO2 from boreal and subarctic peatlands is probable.

Field, laboratory, and computer modeling methods were utilized to quantitatively assess the capability of aerobic microorganisms to oxidize landfill-derived methane (CH4) in cover soils. The investigated municipal landfill, located in Nashua, New Hampshire, was operating without gas controls of any type at the time of sample collection. Soil samples from locations of CH4 flux to the atmosphere were returned to the laboratory and subjected to incubation experiments to quantify the response of oxidation in these soils to temperature, soil moisture, in situ CH4 mixing ratio, soil depth, and oxygen. The mathematical representations of the observed oxidation reponses were combined with measured and predicted soil characteristics in a computer model to predict the rate of CH4 oxidation in the soils at the locations of the measured fluxes described by Czepiel et al. [this issue]. The estimated whole landfill oxidation rate at the time of the flux measurements in October 1994 was 20%. Local air temperature and precipitation data were then used in conjunction with an existing soil climate model to estimate an annual whole landfill oxidation rate in 1994 of 10%.

Methane (CH4) dynamics in pore water, snow pore air, and surface emissions were investigated in a temperate poor fen in New Hampshire over several winters. Total snowfall and average air temperatures during winter months (defined as December, January, and February) were climatologic indicators of significant flux rates from this midlatitude poor fen. Average winter emissions, for the five winters ending in 1994–1995, were 20, 39, 53, 56, and 26 mg m−2 d−1, amounting to 2.0, 5.2, 6.6, 9.2, and 2.0% of the total annual fluxes, respectively. Totaling emissions over 5 years that represent low to average snowfall, winter accounted for 4.3% of emissions to the atmosphere. Winter flux rates were near 55 mg m−2 d−1 for years with average snowfall, and 25 mg m−2 d−1 for years with low snowfall. Concentrations of CH4 sampled in pore water immediately beneath the ice were highly variable (0 to 1.1 mM). The concentration magnitude and standard deviation increased toward the fen center and correlated with spatial variation in hydrology, peat texture, and peat depth. CH4 stores increased in the near-surface pore water as the ice cover formed. Seasonal CH4 buildup in deeper peat began near the end of the growing season, probably due to changing transport mechanisms and temperature effects on solubility. Stored CH4 in the 25- to 75-cm peat layer decreased by 2.7 g m−2 between January and June 1995.

Factors controlling the flux of the radiatively important trace gas methane (CH4) from boreal wetlands were examined at three sites along a moisture gradient from a treed low-shrub bog to an open floating graminoid bog in Fairbanks, Alaska. In the summer of 1992 average static chamber flux measurements were −0.02, 71.5, and 289 mg CH4/m2/d in dry, wet, and floating mat communities, respectively. In contrast, the warmer, drier 1993 field season flux measurements were −0.02, 42.9 and 407 mg CH4/m2/d. The data indicate that despite net oxidation of CH4 in the dry regions of the bog, the wetland is a net source of CH4, with fluxes ranging across three orders of magnitude between different plant communities. Comparison with water levels suggests that CH4 flux is turned on and off by changes in site hydrology. In sites where sufficient moisture is present for methanogenesis to occur, CH4 flux appears to be temperature limited, responding exponentially to soil temperature changes. The combined effects of hydrology and temperature create hot spots of CH4 flux within boreal wetlands. The plant communities within Lemeta Bog respond differently to changes in temperature and moisture availability, creating both positive and negative feedbacks to potential global climate change.

Methane emissions were measured by a static chamber technique in a diverse peatland complex in the Northern Study Area (NSA) of the Boreal Ecosystem Atmosphere Study (BOREAS). Sampling areas represented a wide range of plant community and hydrochemical gradients (pH 3.9–7.0). Emissions were generally larger than those reported from other boreal wetland environments at similar latitude. Seasonal average fluxes from treed peatlands (including palsas) ranged from 0 to 20 mg CH4 m−2 d−1 compared with 92 to 380 mg CH4 m−2 d−1 in open graminoid bogs and fens (with maximum single fluxes up to 1355 mg CH4 m−2 d−1). Permafrost-related collapse scars had similarly high CH4 emissions, particularly in the lag areas where continuous measurements of water table, peat surface elevation, and peat temperature showed that the peat surface adjusted to a falling water table in the abnormally dry 1994 season, maintaining warm, saturated conditions and high CH4 flux later into the season than nonfloating sites. A predictive model for CH4 flux and environmental variables was developed using multiple stepwise regression. A combined variable of mean seasonal peat temperature at the average position of the water table explained most of the spatial variability in log CH4 flux (r2 = 0.64), with height above mean water table (HMWT), water chemistry (Kcorr, pH, Ca), tree cover, and herbaceous plant cover explaining additional variance (r2 = 0.81). Canonical correspondence analysis (CCA) of combined vascular and bryophyte data with environmental variables showed that CH4 flux was negatively correlated with HMWT, the second axis of vegetation variability, and was only weakly correlated with chemistry, the first axis. Sedge and tree cover were correlated with high and low CH4 fluxes, respectively, while shrub cover was of less predictive value. Microtopographic groupings of hummocks and hollows were separated in terms of CH4 flux at the intermediate ranges of the moisture gradient. These data show that multivariate vegetation analyses may provide a useful framework for integrating the complex environmental controls on CH4 flux and extrapolating single point chamber measurements to the landscape scale using remote sensing. (Key words: CH4 flux, peatland, vegetation, and remote sensing.)

Several studies have proposed aerobic methane (CH4) emissions by plants. If confirmed, these findings would further increase the imbalance in the global CH4 budget which today underestimates CH4 sinks. Oxidation by OH-radicals in the troposphere is the major identified sink followed by smaller contribution from stratospheric loss and oxidation by methano- and methylotrophic bacteria in soils. This study directly investigated CH4 exchange by plants in their natural environment. At a forest site in central Sweden, in situ branch chamber measurements were used to study plant ambient CH4 exchange by spruce (Picea abies), birch (Betula pubescens), rowan (Sorbus aucuparia) and pine (Pinus sylvestris). The results show a net uptake of CH4 by all the studied plants, which might be of importance for the methane budget. (Less)

Soil organic matter (SOM) stored in permafrost terrain is a key component in the global carbon cycle, but its composition and lability are largely unknown. We characterize and assess the degree of ...

[1] We have developed a mid-infrared continuous-wave quantum cascade laser direct-absorption spectrometer (QCLS) capable of high frequency (≥1 Hz) measurements of 12CH4 and 13CH4 isotopologues of methane (CH4) with in situ 1-s RMS precision of 1.5 ‰ and Allan-minimum precision of 0.2 ‰. We deployed this QCLS in a well-studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg CH4 m−2 hr−1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 ± 0.8 mgCH4 m−2 hr−1) over the same period. Mean composition of emitted CH4 calculated using EC isoflux methods was −71 ± 8 ‰ (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of −64.5 ± 0.8 ‰. Ebullitive fluxes, representing ∼10% of total CH4 fluxes at this site, were on average 1.2 ‰ enriched in 13C compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process-based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.

Three scales of temporal variability were present in methane (CH 4) flux data collected during a 2.5 year (mid-1990n1992) study at a small, poor fen in southeastern New Hampshire. (1) There was a strong seasonality to the fluxes (high in summer); monthly average fluxes range from 21.4 mg CH 4nm m2nd m1n(February 1992) to 639.0 mg CH 4nm m2d m1n(July 1991). Annual fluxes were 68.8 g CH 4nm m2n(1991) and 69.8 g CH 4nm m2n(1992). (2) There was interannual variability; distribution of flux intensity was very different from 1991 to 1992, particularly the timing and rapidity of the onset of higher fluxes in the spring. (3) There was a high degree of variability in CH 4nflux during the warm season; four successive weekly flux rates in July 1991 were 957, 1044, 170, and 491 mg CH 4nm m2nd m1. Fluxes were correlated with peat temperature (r 2=0.44) but only weakly with depth to water table (r 2n= 0.14 for warm season data). Warm season fluxes appeared to be suppressed by rainstorms. Along with methane flux data we present an analysis of this temporal variability in flux, using a peatland soil climate model developed for this site. The model was driven by daily air temperature, precipitation, and net radiation; it calculated daily soil temperature and moisture profiles, water table location, and ice layer thickness. Temperature profiles were generally in good agreement with field data. Depth to water table simulations were good in 1992, fair in 1990, and poor in the summer of 1991. Using model-simulated peat climate and correlations to methane flux developed from the field data, simulated methane fluxes exhibited the same three modes of temporal variability that were present in the field flux data, though the model underestimated peak fluxes in 1990 and 1991. We conclude that temporal variability in flux is significantly influenced by climate/weather variability at all three scales and that rainfall appears to suppress methane flux for at least several days at this site.

Hydrological changes, particularly alterations in water table level, may largely overshadow the more direct effects of global temperature increase upon carbon cycling in arctic and subarctic wetlands. Frozen cores (n=40) of intact soils and vegetation were collected from a bog near Fairbanks, Alaska, and fluxes of CO{sub 2}, CH{sub 4}, and Co in response to water table variation were studied under controlled conditions in the Duke University phytotron. Core microcosms thawed to a 20-cm depth over 30 days under a 20 hour photoperiod with a day/night temperature regime of 20/10{degrees}C. After 30 days the water table in 20 microcosms was decreased from the soil surface to -15 cm and maintained at the soil surface in 20 control cores. Outward fluxes of CO{sub 2} (9-16 g m{sup -2}d{sup -1}) and CO (3-4 mg m{sup -2}d{sup -1}) were greatest during early thaw and decreased to near zero for both gases before the water table treatment started. Lower water table tripled CO{sub 2} flux to the atmosphere when compared with control cores. Carbon monoxide was emitted at low rates from high water table cores and consumed by low water table cores. Methane fluxes were low (

Bubbles can contribute a significant fraction of methane emissions from wetlands; however the range of reported fractions is very large and accurate characterization of this pathway has proven diff ...

Methane distributions and fluxes were determined in the more southerly of two permanently anoxic basins at the head of the otherwise shallow and tidal Pettaquamscutt River. Fluxes out of the sediment were approximately equal to fluxes across the air-sea interface as determined by flux chamber and were of the order of 3 - 6 mg CH4 m−2 d−1. Ebullition, observed during one set of flux chamber experiments, increased air-sea fluxes by several orders of magnitude. Methane oxidation rates within the waters of the basin were measured by using 14CH4, and a maximum in oxidation rate was observed at the oxic-anoxic interface. Oxidation rates were about a factor of 10 lower than either measured sediment-water or air-sea fluxes. Fluxes across the pycnocline also were lower than sediment-water fluxes and fell at the low end of the range of calculated air-sea fluxes (which were lower than measured air-sea fluxes). Therefore, on the basis of a short term survey, the system appeared to be roughly at steady state. However, in spite of the appearance of steady state, methane profiles measured over a period of several years varied dramatically suggesting that episodic events (ebullition, tidal mixing, upwelling, and intrusion events) must be major factors in the methane budget not obvious from short-term experiments. Episodic events are probably also important for other systems such as productive coastal environments, suboxic estuaries or any other systems in which reduced gases might accumulate within or below a pycnocline. When the pycnocline is tilted or breaks down, substantial amounts of gases may be released to the atmosphere.

Rates of emissions of the biogenic sulfur (S) gases carbonyl sulfide (COS), methyl mercaptan (MSH), dimethyl sulfide (DMS), and carbon disulfide (CS2) were measured in a variety of marine and freshwater wetland habitats in the Florida Everglades during a short duration period in October using dynamic chambers, cryotrapping techniques, and gas chromatography. The most rapid emissions of >500 nmol m−2 h−1 occurred in red mangrove-dominated sites that were adjacent to open seawater and contained numerous crab burrows. Poorly drained red mangrove sites exhibited lower fluxes of ∼60 nmol m−2 h−1 which were similar to fluxes from the black mangrove areas which dominated the marine-influenced wetland sites in the Everglades. DMS was the dominant organo-S gas emitted especially in the freshwater areas. Spectral data from a scene from the Landsat thematic mapper were used to map habitats in the Everglades. Six vegetation categories were delineated using geographical information system software and S gas emissions were extrapolated for the entire Everglades National Park. The black mangrove-dominated areas accounted for the largest portion of S gas emissions to the area. The large area extent of the saw grass communities (42%) accounted for ∼24% of the total S emissions.

Climate change and thawing of permafrost will likely result in increased decomposition of terrestrial organic carbon and subsequent carbon emissions to the atmosphere from terrestrial and aquatic systems. The quantitative importance of mineralization of terrestrial organic carbon in lakes in relation to terrestrial carbon fluxes is poorly understood and a serious drawback for the understanding of carbon budgets. We studied a subarctic lake in an area of discontinuous permafrost to assess the quantitative importance of lake carbon emission for the catchment carbon balance. Estimates of net ecosystem production and stable carbon-isotope composition of dissolved organic carbon in the lake water suggest substantial input and respiration of terrestrial organic carbon in the lake. The lake was a net source of CO2 and CH4 to the atmosphere at ice breakup in spring and during the whole ice-free period. The carbon emission from the lake was similar in magnitude to the terrestrial net release of carbon to the atmosphere. The results indicate that lakes are important sources of catchment carbon emission, potentially increasing the positive feedback from permafrost thawing on global warming.

[1] Soil carbon dioxide efflux (soil respiration, SR) was measured with eight autochambers at two locations along a wetland to upland slope at Harvard Forest over a 4 year period, 2003–2007. SR was consistently higher in the upland plots than at the wetland margin during the late summer/early fall. Seasonal and diel hystereses with respect to soil temperatures were of sufficient magnitude to prevent quantification of the influence of soil moisture, although apparent short-term responses of SR to precipitation occurred. Calculations of annual cumulative SR illustrated a decreasing trend in SR over the 5 year period, which were correlated with decreasing springtime mean soil temperatures. Spring soil temperatures decreased despite rising air temperatures over the same period, possibly as an effect of earlier leaf expansion and shading. The synchronous decrease in spring soil temperatures and SR during regional warming of air temperatures may represent a negative feedback on a warming climate by reducing CO2 production from soils. SR reached a maximum later in the year than total ecosystem respiration (ER) measured at a nearby eddy covariance flux tower, and the seasonality of their temperature response patterns were roughly opposite. SR, particularly in the upland, exceeded ER in the late summer/early fall in each year, suggesting that areas of lower efflux such as the wetland may be significant in the flux tower footprint or that long-term bias in either estimate may create a mismatch. Annual estimates of ER decreased over the same period and were highly correlated with SR.

Although much attention in recent years has been devoted to methane (CH4) emissions from northern wetlands, measurement based data sets providing full annual budgets are still limited in number. Th ...

Methane flux from plots of Peltandra virginica in a bottomland hardwood swamp in southeast Virginia varied from 270 to 670 mg CH4m−2d−1 from May to August and was 6 times greater than the flux from unvegetated adjacent open waters. Variations in chamber temperature, light, and CO2 levels failed to produce systematic changes in CH4 flux rate or its isotopic composition from vegetated plots, suggesting that CH4 flux from Peltandra is independent of stomatal aperture or hourly variations in photosynthetic rate. Comparisons of different chamber techniques for the collection of CH4 for isotopic analysis and for flux measurements from Peltandra suggest that the effect of using simple uncontrolled chambers is small relative to temporal and spatial variations in methane emission and its isotopic composition. These results may be extended to other aquatic macrophytes which also transport gas primarily by molecular diffusion such as Oryza, Carex, Pontederia, Sagittaria, and Cladium. The δ13C of CH4 released from Peltandra varied between −61 and −71 °/oo. Isotopic variations in CH4 associated with Peltandra appeared to be controlled by transport effects. Methane withdrawn from Peltandra stems was considerably 13C enriched relative to sedimentary CH4 and was not an indicator of the isotopic composition of CH4 emitted by these plants. Hence emergent aquatic macrophytes rooted in organic-rich sediments do not appear to be a source of 13C enriched methane to the atmosphere. Emitted CH4 was 13C depleted relative both to the CH4 within stems (−49 to −55 °/oo) and within sediments (-56 to -58 °/oo), suggesting isotopic fractionation associated with the release of CH4 by the plants. The preferential release of 12CH4 by the plants is partly compensated for by the enrichment of 13C in CH4 contained within the plant stem. Although the presence of methanotrophic bacteria within the rhizosphere of Peltandra was demonstrated, the stable isotopic composition of methane emitted from these plants or collected from sedimentary bubbles and plant stems presents no evidence for extensive oxidation of methane within Peltandra or its rhizosphere. Either stable isotopes are not a good indicator of rhizopheric methane oxidation or the microbes' in situ respiration is limited by competition for oxygen with the demands of root respiration, oxidation of complex organic matter, ferrous iron oxidation, and nitrification.

Tropospheric methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) are significant sources of ozone (O 3 ) destroying halogens to the stratosphere. Their O 3 depletion potential (ODP) can be determined from atmospheric lifetimes and therefore their atmospheric budgets, both of which are out of balance with known sink terms larger than identified sources. We have discovered a new source of CH 3 Br and CH 3 Cl emissions to the atmosphere at two wetland sites in the Northeastern United States. We have reason to believe that these compounds are biologically produced in situ. Our measurements indicate that the global annual flux of CH 3 Br and CH 3 Cl from wetlands could be as high as 4.6 Gg yr -1 of CH 3 Br and 48 Gg yr -1 of CH 3 Cl. These are preliminary estimates based on measurements made during the end of the 1998 growing season, a time period of decreased emissions of other trace gases such as methane (CH 4 ).

[1] This study examined daily, seasonal, and interannual variations in CH4 emissions at a temperate peatland over a 5-year period. We measured net ecosystem CO2 exchange (NEE), CH4 flux, water table depth, peat temperature, and meteorological parameters weekly from the summers (1 May to 31 August) of 2000 through 2004 at Sallie's Fen in southeastern New Hampshire, United States. Significant interannual differences, driven by high variability of large individual CH4 fluxes (ranging from 8.7 to 3833.1 mg CH4 m−2 d−1) occurring in the late summer, corresponded with a decline in water table level and an increase in air and peat temperature. Monthly timescale yielded the strongest correlations between CH4 fluxes and peat and air temperature (r2 = 0.78 and 0.74, respectively) and water table depth (WTD) (r2 = 0.53). Compared to daily and seasonal timescales, the monthly timescale was the best timescale to predict CH4 fluxes using a stepwise multiple regression (r2 = 0.81). Species composition affected relationships between CH4 fluxes and measures of plant productivity, with sedge collars showing the strongest relationships between CH4 flux, water table, and temperature. Air temperature was the only variable that was strongly correlated with CH4 flux at all timescales, while WTD had either a positive or negative correlation depending on timescale and vegetation type. The timescale dependence of controls on CH4 fluxes has important implications for modeling.

We sampled air in the canopy layer of undisturbed upland forests during wet and dry seasons at three sites in the Brazilian Amazon region and found that both methane (CH4) and carbon dioxide (CO2) ...

Measurements of CH 3 Br exchange at two New Hampshire peatlands (Sallie's Fen and Angie's Bog) indicate that net flux from these ecosystems is the sum of competing production and consumption processes. Net CH 3 Br fluxes were highly variable and ranged from net emission to net uptake between locations within a single peatland. At Sallie's Fen, net CH 3 Br flux exhibited positive correlations with peat temperature and air temperature during all seasons sampled, but these relationships were not observed at Angie's Bog where flux varied according to microtopography. The major CH 3 Br production process at Sallie's Fen appeared dependent on aerobic conditions within the peat, while CH 3 Br production at Angie's Bog was favored by anaerobic conditions. There was evidence of aerobic microbial consumption of CH 3 Br within the peat at both sites. In a vegetation removal experiment conducted at Sallie's Fen with dynamic chambers, all collars exhibited net consumption of CH 3 Br. Net CH 3 Br flux had a negative correlation with surface temperature and a positive correlation with water level in collars with all vegetation clipped consistent with aerobic microbial consumption. Vegetated collars showed positive correlations between net CH 3 Br flux and air temperature. A positive correlation between net CH 3 Br flux and surface temperature was also observed in collars in which all vegetation except Sphagnum spp. were clipped. These correlations are consistent with seasonal relationships observed in 1998, 1999, and 2000 and suggest that plants and/or fungi are possible sources of CH 3 Br in peatlands. Estimates of production and consumption made on two occasions at Sallie's Fen suggest that peatlands have lower rates of CH 3 Br consumption compared to upland ecosystems, but a close balance between production and consumption rates may allow these wetlands to act as either a net source or sink for this gas.

[1] 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 ( 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.

Ecosystems along the 0degreesC mean annual isotherm are arguably among the most sensitive to changing climate and mires in these regions emit significant amounts of the important greenhouse gas methane (CH4) to the atmosphere. These CH4 emissions are intimately related to temperature and hydrology, and alterations in permafrost coverage, which affect both of those, could have dramatic impacts on the emissions. Using a variety of data and information sources from the same region in subarctic Sweden we show that mire ecosystems are subject to dramatic recent changes in the distribution of permafrost and vegetation. These changes are most likely caused by a warming, which has been observed during recent decades. A detailed study of one mire show that the permafrost and vegetation changes have been associated with increases in landscape scale CH4 emissions in the range of 22-66% over the period 1970 to 2000.

Field enclosure measurements of a temperate forest soil show net uptake of ambient methyl bromide (CH 3 Br), an important trace gas in both tropospheric and stratospheric ozone cycling. The net flux for 1999 was estimated to be - 168 ± 72 μg CH 3 Br m -2 (negative indicates loss from the atmosphere). Individual enclosure flux measurements ranged from -4.0 to +3.3 μg CH 3 Br m -2 d -1 . Soil consumption of CH 3 Br was estimated from laboratory soil incubations. Production of CH 3 Br was calculated as the difference between net flux and predicted consumption. Fungi could be responsible for the production of CH 3 Br in this temperate forest soil.

[1] We conducted an experiment on sand and clay tropical forest soils to test the short-term effect of root mortality on the soil-atmosphere flux of nitrous oxide, nitric oxide, methane, and carbon dioxide. We induced root mortality by isolating blocks of land to 1 m using trenching and root exclusion screening. Gas fluxes were measured weekly for ten weeks following the trenching treatment. For nitrous oxide there was a highly significant increase in soil-atmosphere flux over the ten weeks following treatment for trenched plots compared to control plots. N2O flux averaged 37.5 and 18.5 ng N cm−2 h−1 from clay trenched and control plots and 4.7 and 1.5 ng N cm−2 h−1 from sand trenched and control plots. In contrast, there was no effect for soil-atmosphere flux of nitric oxide, carbon dioxide, or methane.