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.

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.

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.

Abstract. Peatland carbon and water cycling are tightly coupled, so dynamic modeling of peat accumulation over decades to millennia should account for carbon-water feedbacks. We present initial results from a new simulation model of long-term peat accumulation, evaluated at a wellstudied temperate bog in Ontario, Canada. The Holocene Peat Model (HPM) determines vegetation community composition dynamics and annual net primary productivity based on peat depth (as a proxy for nutrients and acidity) and water table depth. Annual peat (carbon) accumulation is the net balance above-and below-ground productivity and litter/peat decomposition -a function of peat hydrology (controlling depth to and degree of anoxia). Peat bulk density is simulated as a function of degree of humification, and affects the water balance through its influence on both the growth rate of the peat column and on peat hydraulic conductivity and the capacity to shed water. HPM output includes both time series of annual carbon and water fluxes, peat height, and water table depth, as well as a final peat profile that can be "cored" and compared to field observations of peat age and macrofossil composition. A stochastic 8500-yr, annual precipitation time series was constrained by a published Holocene climate reconstruction for southern Québec. HPM simulated 5.4 m of peat accumulation (310 kg C m −2 ) over 8500 years, 6.5% of total NPP over the period. Vascular plant functional types accounted for 65% of total NPP over 8500 years but only 35% of the final (contemporary) peat mass. Simulated age-depth Correspondence to: S. Frolking (steve.frolking@unh.edu) and carbon accumulation profiles were compared to a radiocarbon dated 5.8 m, c.9000-yr core. The simulated core was younger than observations at most depths, but had a similar overall trajectory; carbon accumulation rates were generally higher in the simulation and were somewhat more variable than observations. HPM results were sensitive to centuryscale anomalies in precipitation, with extended drier periods (precipitation reduced ∼10%) causing the peat profile to lose carbon (and height), despite relatively small changes in NPP.

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We measured net ecosystem CO 2 exchange (NEE), plant biomass and growth, species composition, peat microclimate, and litter decomposition in a fertilization experiment at Mer Bleue Bog, Ottawa, Ontario. The bog is located in the zone with the highest atmospheric nitrogen deposition for Canada, estimated at 0.8-1.2 g N m À2 yr À1 (wet deposition as NH 4 and NO 3 ). To establish the effect of nutrient addition on this ecosystem, we fertilized the bog with six treatments involving the application of 1.6-6 g N m À2 yr À1 (as NH 4 NO 3 ), with and without P and K, in triplicate 3 m  3 m plots. The initial 5-6 years have shown a loss of first Sphagnum, then Polytrichum mosses, and an increase in vascular plant biomass and leaf area index. Analyses of NEE, measured in situ with climate-controlled chambers, indicate that contrary to expectations, the treatments with the highest levels of nutrient addition showed lower rates of maximum NEE and gross photosynthesis, but little change in ecosystem respiration after 5 years. Although shrub biomass and leaf area increased in the high nutrient plots, loss of moss photosynthesis owing to nutrient toxicity, increased vascular plant shading and greater litter accumulation contributed to the lower levels of CO 2 uptake. Our study highlights the importance of long-term experiments as we did not observe lower NEE until the fifth year of the experiment. However, this may be a transient response as the treatment plots continue to change. Higher levels of nutrients may cause changes in plant composition and productivity and decrease the ability of peatlands to sequester CO 2 from the atmosphere.

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