Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO 2 -C with the atmosphere of 0.1-0.5 Pg yr À1 and of CH 4 -C of 10-25 Tg yr À1 . Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6-year balance computed from continuous net ecosystem CO 2 exchange (NEE), regular instantaneous measurements of methane (CH 4 ) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6-year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and agedepth profiles from two peat cores. The 6-year mean NEE-C and CH 4 -C exchange, and net DOC loss are À40.2 AE 40.5 (AE 1 SD), 3.7 AE 0.5, and 14.9 AE 3.1 g m À2 yr À1 , giving a 6-year mean balance of À21.5 AE 39.0 g m À2 yr À1 (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH 4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi-maximum limits of the annual C balance between 50 and À105 g m À2 yr À1 . The net C accumulation rate obtained from the two peatland cores for the interval 400-3000 BP (samples from the anoxic layer only) were 21.9 AE 2.8 and 14.0 AE 37.6 g m À2 yr À1 , which are not significantly different from the 6-year mean contemporary exchange.

Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non- Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3, organic matter bulk density averaged 0.11 ± 0.05 g/cm3, and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non- Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu .

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Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.

[1] Eddy covariance measurements of net ecosystem carbon dioxide (CO 2 ) exchange (NEE) were taken at an ombrotrophic bog near Ottawa, Canada from 1 June 1998 to 31 May 2002. Temperatures during this period were above normal except for 2000 and precipitation was near normal in 1998 and 1999, above normal in 2000, and well below normal in 2001. Growing period maximum daytime uptake (À0.45 mg CO 2 m À2 s À1 ) was similar in all years and nighttime maximum respiration was typically near 0.20 mg CO 2 m À2 s À1 , however, larger values were recorded during very dry conditions in the fourth year of study. Winter CO 2 flux was considerably smaller than in summer, but persistent, resulting in significant accumulated losses (119-132 g CO 2 m À2 period À1 ). This loss was equivalent to between 30 and 70% of the net CO 2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO 2 ($À260 g CO 2 m À2 yr À1 ). In the fourth year, with the dry summer, however, annual NEE was only À34 g CO 2 m À2 yr À1, which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower-derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying.

S U M M A R YLaboratory columns (80 cm long, 10 cm diameter) of peat were constructed from samples collected from a subarctic fen, a temperate bog and a temperate swamp. Temperature and water table position were manipulated to establish their influence on emissions of CO, and CH, from the columns. A factorial design experiment revealed significant (P < 0.05) differences in emission of these gases related to peat type, temperature and water table position, as well as an interaction between temperature and water table. Emissions of CO, and CH, at 23°C were an average of 2.4 and 6.6 times larger, respectively, than those at 10°C. Compared to emissions when the columns were saturated, water table at a depth of 40 cm increased CO, fluxes by an average of 4.3 times and decreased CH, emissions by an average of 5.0 times. There were significant temporal variations in gas emissions during the 6-week experiment, presumably related to variations in microbial populations and substrate availability. Using columns with static water table depths of 0, 10,20,40 and 60 cm, CO, emissions showed a positive, linear relation with depth, whereas CH, emissions revealed a negative, logarithmic relation with depth. Lowering and then raising the water table from the peat surface to a depth of 50 cm revealed weak evidence of hysteresis in CO, emissions between the falling and rising water table limbs. Hysteresis (falling > rising limb) was very pronounced for CH, emissions, attributed to a release of CH, stored in pore-water and a lag in the development of anaerobic conditions and methanogenesis on the rising limb. Decreases in atmospheric pressure were correlated with abnormally large emissions of CO, and CH, on the falling limb. Peat slurries incubated in flasks revealed few differences between the three peat types in the rates of CO, production under aerobic and anaerobic conditions. There were, however, major differences between peat types in the rates of CH, consumption under aerobic incubation conditions and CH, production under anaerobic conditions (bog > fen > swamp), which explain the differences in response of the peat types in the column experiment.

Summary1 Above-ground biomass was measured at bog hummock, bog hollow and poor-fen sites in Mer Bleue, a large, raised ombrotrophic bog near Ottawa, Ont., Canada. The average above-ground biomass was 587 g m -2 in the bog, composed mainly of shrubs and Sphagnum capitula. In the poor fen, the average biomass was 317 g m -2, comprising mainly sedges and herbs and Sphagnum capitula. Vascular plant above-ground biomass was greater where the water table was lower, with a similar but weaker relationship for Sphagnum capitula and vascular leaf biomass. 2 Below-ground biomass averaged 2400 g m -2 at the bog hummock site, of which 300 g m -2was fine roots (< 2 mm diameter), compared with 1400 g m -2 in hollows (fine roots 450 g m -2) and 1200 g m -2 at the poor-fen site. 3 Net Ecosystem Exchange (NEE) of CO 2 was measured in chambers and used to derive ecosystem respiration and photosynthesis. Under high light flux (PAR of 1500 µ mol m -2 s -1 ), NEE ranged across sites from 0.08 to 0.22 mg m -2 s -1 (a positive value indicates ecosystem uptake) in the spring and summer, but fell to -0.01 to -0.13 mg m -2 s -1 (i.e. a release of CO 2 ) during a late-summer dry period. 4 There was a general agreement between a combination of literature estimates of photosynthetic capacity for shrubs and mosses and measured biomass and summertime CO 2 uptake determined by the eddy covariance technique within a bog footprint (0.40 and 0.35-0.40 mg m -2 s -1 , respectively). . Root production and decomposition are important parts of the C budget of the bog. Root C production was estimated to be 161-176 g m -2 year -1 , resulting in fractional turnover rates of 0.2 and 1 year -1 for total and fine roots, respectively.

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