Peat Temperature Research Articles

Soil surface CO2 flux in a Minnesota peatland

Soil surface CO2 flux was measured in hollow and hummock microhabitats in a peatland in north central Minnesota from June to October in 1991. We used a closed infrared gas exchange system to measure soil CO2 flux. The rates of CO2 evolution from hummocks (9.8 ± 3.5 g m−2 d−1, [mean ± SE]) were consistently higher than those from hollows (5.4 ± 2.9 g m−2 d−1) (the hummock values included the contribution of moss dark respiration, which may account for 10–20% of the total measured flux). The soil CO2 flux was strongly temperature-dependent (Q10 ≈ 3.7) and appeared to be linearly related to changes in water table depth. An empirical multiplicative model, using peat temperature and water table depth as independent variables, explained about 81% of the variance in the CO2 flux data. Using the empirical model with measurements of peat temperature and estimates of hollow/hummock microtopographic distribution (relative to water table elevation), daily rates of “site-averaged” CO2 evolution were calculated. For the six-month period (May–October), the total soil CO2 released from this ecosystem was estimated to be about 1340 g CO2 m−2.

Northern fens: methane flux and climatic change

Methane flux from northern peatlands is believed to be an important contribution to the global methane budget. High latitude regions are predicted to experience significant changes in surface temperature and precipitation associated with the 2 × CO2 climate scenarios, but the effects of these changes on methane emission are poorly understood. A peatland hydrologic model predicted June - August decreases in water storage of between 82 and 144 mm, using as inputs increases in temperatures of 3 °C and rainfall of 1 mm d-1. These changes translate into a water table drop, relative to the peat surface, of between 14 and 22 cm, depending on whether the fen has a floating or non-floating surface. The 3 °C air temperature increase was predicted to raise peat temperature at 10 cm depth by 0.8 °C. These changes were then applied to relationships derived at a subarctic fen for water table: methane flux: temperature at 10 cm depth. Increased temperatures raise the methane flux by between 5 and 40%, but the lowered water table decreases methane flux by 74 and 81%, at the floating and nonfloating fen sites, respectively. These results suggest that methane emissions from northern peatlands are more sensitive to changes in moisture regime than temperature within the range of changes predicted for 2 × CO2 scenarios.

Winter fluxes of methane from Minnesota peatlands

Winter fluxes of methane were investigated in northern Minnesota during 1988–89 and 1989–90. Two bogs and a fen emitted methane throughout the snow-covered season (November through March). Fluxes decreased to a low level of 3–16 mg CH4 m−2 d−1 in late March, reflecting decreasing peat temperatures and (in 1989–90) increasing depth of frost in the peat. Winter fluxes calculated by integration for an open poor fen, an open bog, a forested bog hollow, and a hummock site in the forested bog averaged 49, 12, 13, and 5 mg m−2 d−1, respectively, in 1989–1990 (the year most measurements were made). These comprised 11%, 4%, 15%, and 21% of total annual flux.

Spatial and temporal variations of methane flux from subarctic/northern boreal fens

Emissions of CH4 were measured by a static chamber technique (five chambers/site) at 23 sites in four fens near Schefferville, subarctic Quebec. At three intensively‐monitored sites along a transect from fen margin to central and flooded sites, mean CH4 fluxes from June to August, 1989 were 65, 125, and 36 mg m−2 d−1, respectively. Pore water CH4 concentrations in the peat profiles to a depth of l m averaged 125 to 200 μM, with lower concentrations (generally <50 μM) at 0.1 m. Total, depth‐integrated storage of CH4 in the peat profiles ranged from 3.5 to 4.3 g m−2. Although CH4 flux was only weakly correlated with either peat temperature at 0.1 m or water table position within each site, there was a strong association of flux and these variables among the three sites, indicating the value of ecological attributes in identifying patterns of CH4 flux. A pulse of CH4 was recorded at two of the three sites in mid‐August, associated with a degassing of the peat profile, based on pore water CH4 concentrations. This pulse appeared to be initiated by the lowering of the water table by between 5 and 10 cm during a 3‐week period of low rainfall and is estimated to have contributed 18 to 65% of the seasonal CH4 emission, depending on the location of the site. An estimate of the regional CH4 flux from 130 km2 of the Schefferville area was based on CH4 flux measurements at 23 sites in the area, stratified by fen type (forested margin, margin, central, flooded, ridge, and pool). June to August regional CH4 flux was 18 mg m−2 d−1, for an area in which fen coverage was 29%. When extrapolated to the global scale, these results indicate that northern fens may contribute about 14 Tg yr−1, somewhat lower than other recent estimates.

Methane flux from Minnesota Peatlands

Northern (>40°N) wetlands have been suggested as the largest natural source of methane (CH4) to the troposphere. To refine our estimates of source strengths from this region and to investigate climatic controls on the process, fluxes were measured from a variety of Minnesota peatlands during May, June, and August 1986. Sites included forested and unforested ombrotrophic bogs and minerotrophic fens in and near the U.S. Department of Agriculture Marcell Experimental Forest and the Red Lake peatlands. Late spring and summer fluxes ranged from 11 to 866 mg CH4 m−2 d−1, averaging 207 mg CH4 m−2 d−1 overall. At Marcell Forest, forested bogs and fen sites had lower fluxes (averages of 77 ± 21 mg CH4 m−2 d−1 and 142 ± 19 mg CH4 m−2 d−1) than open bogs (average of 294 ± 30 mg CH4 m−2 d−1). In the Red Lake peatland, circumneutral fens, with standing water above the peat surface, produced more methane than acid bog sites in which the water table was beneath the moss surface (325 ± 31 and 102 ± 13 mg CH4 m−2 d−1, respectively). Peat temperature was an important control. Methane flux increased in response to increasing soil temperature. For example, the open bog in the Marcell Forest with the highest CH4 flux exhibited a 74‐fold increase in flux over a three‐fold increase in temperature. We estimate that the methane flux from all peatlands north of 40° may be on the order of 70 to 90 Tg/yr though estimates of this sort are plagued by uncertainties in the areal extent of peatlands, length of the CH4 producing season, and the spatial and temporal variability of the flux.

METHANE AND CARBON DIOXIDE EVOLUTION FROM SUBARCTIC FENS

Rates of net methane and carbon dioxide evolution from four subarctic fens over one summer ranged from 0 to 7 mmol CH4 m−2 d−1 and from 2 to 29 mmol CO2 m2 d−1. Average molar ratios of carbon dioxide to methane ranged from 3 to 10. Partially because of the high spatial variability in evolution rates, the temperature dependence of carbon dioxide was weak, but stronger for methane, with significant (P < 0.05) positive correlations at two sites, especially with peat temperatures. Annual flux of methane is estimated to be 0.1–0.6 g C m−2 which, although low compared to other wetlands, becomes a substantial atmospheric contribution when the large area occupied by subarctic peatlands is taken into account. Key words: Methane, carbon dioxide, peatlands, fens

Atmospheric methane sources: Alaskan tundra bogs, an alpine fen, and a subarctic boreal marsh

Methane (CH 4 ) flux measurements from Alaskan tundra bogs, an alpine fen, and a subarctic boreal marsh were obtained at field sites ranging from Prudhoe Bay on the coast of the Arctic Ocean to the Alaskan Range south of Fairbanks during August 1984. In the tundra, average CH 4 emission rates varied from 4.9 mg CH 4 m -2 d -1 (moist tundra) to 119 mg CH 4 m -2 d -1 (waterlogged tundra). Fluxes averaged 40 mg CH 4 m -2 d -1 from wet tussock meadows in the Brooks Range and 289 mg CH 4 m -2 d -1 from an alpine fen in the Alaskan Range. The boreal marsh had an average CH 4 emission rate of 106 mg CH 4 m -2 d -1 . Significant emissions were detected in tundra areas where peat temperatures were as low as 4°C and permafrost was only 25 cm below the ground surface. Emission rates from the 17 sites sampled were found to be logarithmically related to water levels at the sites. Extrapolation of our data to an estimate of the total annual CH 4 emission from all arctic and boreal wetlands suggests that these ecosystems are a major source of atmospheric CH 4 and could account for up to 23% of global CH 4 emissions from wetlands. DOI: 10.1111/j.1600-0889.1986.tb00083.x

Peat Temperature Regime of a Minnesota Bog and the Effect of Canopy Removal

Peat bogs occupy approximately 6 000 000 ha in the cool sub-humid northern Lake States of the U.S.A. These peatlands are intimately associated with the regional water resources, often serving as the source of streamfiow. They also represent unique wetland ecosystems. Temperature is an important characteristic both of the bog environment and the quality of streamflow water. Alteration of peat temperature could result in changes in the surface vegetation and changes in microbial activity, so affecting the amount of organic matter accumulated (Clymo 1965; Lather & Cragg 1967; Moore & Bellamy 1974) and the quality of streamfiow water. Therefore, it is necessary to determine the temperature regimes of peat bogs in both undisturbed and disturbed situations prior to any large-scale alterations of management and energy input. Accordingly, temperature regimes were determined in a strip-cut black spruce bog to evaluate the effect of canopy removal. METHODS The study bog, located in northern Minnesota on the Marcell Experimental Forest (c. 47032'N, 93028'W), is approximately 560 m long and 140 m wide with the long axis oriented north and south. The peat profile of this lake-filled, forested bog has been described by Boelter (1964). The bog is isolated from the regional groundwater system. Precipitation and run-off from the adjacent uplands comprise the water input to the bog. Outflow is primarily as intermittent surface streamflow from the south end of the bog. In the winter of 1968-69, 30-m-wide strips were clear-cut, in an east-west direction, leaving 45-m-wide strips of sixty-two-year-old black spruce (Picea mariana (Mill.) B.S.P.). The crown closure of the forested strips was& approximately 7500. The strips were logged when snow cover was ample, so that disturbance of the surface vegetation was negligible, and a luxuriant sedge cover rapidly established in the following summer (Brown 1972). Eight temperature stations were located at the south end of the bog and eight at the north end. Two stations were located in the centre of clear-cut strips, two were in the centre of the forested strips and four were on the boundaries between strips. Peat depth at the temperature stations ranged from 2 to 2-8 m. Each temperature station consisted of a series of copper-constantan thermocouples, located at depths of 0-025, 0 3, 1 and 2 m below the interface of the living sphagnum moss and the organic soil material. The thermocouples were inserted through a wooden stake 2-5 x 2-5 cm and 2 m long; the stake was pushed into the peat to the desired depth. Air temperatures were measured at two locations in the clear-cut strips and two locations in the non-cut strips by shaded thermocouples 1 m above the bog surface. Temperatures were read with a portable potentiometer. A battery-operated resistance heating unit was