Nigel T. Roulet

Interannual variability in the peatland-atmosphere carbon dioxide exchange at an ombrotrophic bog

This loss was equivalent to between 30 and 70% of the net CO2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (260 g CO2 m 2 yr 1 ). In the fourth year, with the dry summer, however, annual NEE was only 34 g CO2 m 2 yr 1 , 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. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1890 Hydrology: Wetlands; KEYWORDS: peatland, bog, net ecosystem exchange, eddy covariance, carbon dioxide

Modeling Northern Peatland Decomposition and Peat Accumulation

To test the hypothesis that long-term peat accumulation is related to contemporary carbon flux dynamics, we present the Peat Decomposition Model (PDM), a new model of long-term peat accumulation. Decomposition rates of the deeper peat are directly related to observable decomposition rates of fresh vegetation litter. Plant root effects (subsurface oxygenation and fresh litter inputs) are included. PDM considers two vegetation types, vascular and nonvascular, with different decomposition rates and aboveground and belowground litter input rates. We used PDM to investigate the sensitivities of peat accumulation in bogs and fens to productivity, root:shoot ratio, tissue decomposability, root and water table depths, and climate. Warmer and wetter conditions are more conducive to peat accumulation. Bogs are more sensitive than fens to climate conditions. Cooler and drier conditions lead to the lowest peat accumulation when productivity is more temperature sensitive than decomposition rates. We also compare peat age–depth profiles to field data. With a very general parameterization, PDM fen and bog age–depth profiles were similar to data from the the most recent 5000 years at three bog cores and a fen core in eastern Canada, but they overestimated accumulation at three other bog cores in that region. The model cannot reliably predict the amount of fen peat remaining from the first few millennia of a peatlands development. This discrepancy may relate to nonanalogue, early postglacial climatic and nutrient conditions for rich-fen peat accumulation and to the fate of this fen peat material, which is overlain by a bog as the peatland evolves, a common hydroseral succession in northern peatlands. Because PDM sensitivity tests point to these possible factors, we conclude that the static model represents a framework that shows a consistent relationship between contemporary productivity and fresh-tissue decomposition rates and observed long-term peat accumulation.

PEATLANDS, CARBON STORAGE, GREENHOUSE GASES, AND THE KYOTO PROTOCOL: PROSPECTS AND SIGNIFICANCE FOR CANADA

The Kyoto Protocol accepts terrestrial sinks for greenhouse gases (GHGs) as offsets for fossil fuel emissions. Only carbon sequestered in living biomass from re- and afforestation is presently considered, but the Protocol contains a provision for the possible future inclusion of other land uses and soils. As a result, the possibility of sequestration of carbon in wetlands, and particularly peatlands, is being discussed. Natural peatlands are presently a relatively small sink for CO2 and a large source of CH4: globally, they store between 400 and 500 Gt C. There are large variations among peatlands, but when the “global warming potential” of CH4 is factored in, many peatlands are neither sinks nor sources of GHGs. Some land-use changes may result in peatlands acting as net sinks for GHGs by reducing CH4 emissions and/or increasing CO2 sequestration (e.g., forest drainage), while other land uses may result in large losses of CO2, CH4, and N2O (e.g., agriculture on organic soils, flooding for hydroelectric generation). Other land uses, such as peatland creation and restoration, produce no net change if they are replacing or restoring a previous level of GHG exchange. These are analogous to reforestation of deforested areas. On closer examination, the inclusion of peatlands in a national greenhouse gas strategy as sinks, despite their large role in the terrestrial carbon cycle, may not significantly reduce net greenhouse gas emissons. If the sinks are to be considered, it is reasonable that terrestrial sources associated with all land uses on peatlands also should be considered. If peatlands are not considered explicitly, but soils in forest and agriculture systems are included in the Kyoto Protocol in the future, then those peatlands impacted by these land uses will be incorporated implicitly.

Relationship Between Ecosystem Productivity and Photosynthetically Active Radiation for Northern Peatlands

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 = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. 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.

Groundwater flow patterns in a large peatland

Abstract Groundwater flow patterns and geochemistry were studied in the Mer Bleue bog, near Ottawa, Ontario, Canada. Groundwater flow patterns alternated between recharge, i.e. head gradients producing flow from the surface of the peatland to the deeper peat, and discharge, i.e. head gradients indicating flow from the deeper peat towards the surface of the peatland, during the summer of 1998. The patterns were controlled by changes in precipitation, evapotranspiration and the differential head response of catotelm peat (lower layer) to changes in water table elevation. Above-average rainfall in the spring created recharge patterns of groundwater flow in the peatland. Evapotranspiration exceeded precipitation for a three-week period in mid-summer decreasing head at the water table and reversing flow from recharge to discharge. A sustained moisture deficit maintained a flow reversal for 32xa0days, until a 46xa0mm rainfall raised the water-table, reversing the vertical hydraulic gradients and restoring recharge flow. The mixing of meteoric water and deeper groundwater controls geochemical profiles. Diffusion modeling shows that the peatland is a long-term recharge system. However, electrical conductivity and cation concentrations in peat pore-waters increase (up to 60%) during a flow reversal, and then decrease when recharge conditions are re-established. The redistribution of substrates from reversed flow is likely important to peatland biogeochemical function.

Nitrogen deposition and increased carbon accumulation in ombrotrophic peatlands in eastern Canada

[1]xa0Recent and long-term accumulation rates of carbon (C), using 210Pb- and 14C-dating, were examined in 23 ombrotrophic peatlands in eastern Canada, where average 1990–1996 atmospheric wet nitrogen (N) deposition ranged from 0.3 to 0.8 g N m−2 yr−1. The average recent rate of C accumulation (RERCA) over the past 150 years was 73 ± 17 (SD) g C m−2 yr−1, ranging from 40 to 117 g C m−2 yr−1. The difference in RERCA between hummocks (78 g C m−2 yr−1) and hollows (65 g C m−2 yr−1) was significant. Increased RERCA over the past 50 years was found in hummocks and hollows in regions of higher N deposition and related to both elevated N deposition and growing degree-days above +5°C. There was a statistically significant positive relationship between N deposition alone and present-day C accumulation in both hummocks and hollows (R2 = 0.28 and 0.38, respectively). Recent N accumulation was significantly larger in high N deposition regions. The total average aboveground vegetation biomass of hollows and hummocks did not differ significantly with N deposition. However, a significantly larger vascular plant leaf biomass was found in both hollows and hummocks of the high N deposition class than in the low N deposition class (>0.6 and <0.4 g m−2 yr−1, respectively). The average long-term apparent rate of C accumulation (LORCA) at 15 sites was 19 ± 8 (SD) g C m−2 yr−1, with no significant difference due to age of peat inception, latitude, or continentality.

Greenhouse Gas Emissions from Canadian Peat Extraction, 1990-2000: A Life-cycle Analysis

This study uses life-cycle analysis to examine the net greenhouse gas (GHG) emissions from the Canadian peat industry for the period 1990-2000. GHG exchange is estimated for land-use change, peat extraction and processing, transport to market, and the in situ decomposition of extracted peat. The estimates, based on an additive GHG accounting model, show that the peat extraction life cycle emitted 0.54 x 10(6) t of GHG in 1990, increasing to 0.89 x 10(6) t in 2000 (expressed as CO2 equivalents using a 100-y time horizon). Peat decomposition associated with end use was the largest source of GHGs, comprising 71% of total emissions during this 11-y period. Land use change resulted in a switch of the peatlands from a GHG sink to a source and contributed an additional 15%. Peat transportation was responsible for 10% of total GHG emissions, and extraction and processing contributed 4%. It would take approximately 2000 y to restore the carbon pool to its original size if peatland restoration is successful and the cutover peatland once again becomes a net carbon sink.

Mercury cycling in boreal ecosystems: The long-term effect of acid rain constituents on peatland pore water methylmercury concentrations

Sulphate-reducing bacteria have been identified as primary methylators of mercury (Hg) in the laboratory and in field investigations. However, no studies have investigated the effect of long-term deposition of sulphate on methylmercury (MeHg) dynamics in peatlands, which are known to be significant sources of MeHg to downstream waters in the boreal forest zone. As an ancillary experiment to a larger project investigating the effects of acid rain constituents on peatland carbon dynamics, the influence of experimentally elevated Na2SO4 and/or NH4NO3 deposition on peat pore water MeHg concentrations was determined using a simple mesocosm experimental design. After three years, additions of S in amounts equivalent to the 1980s dry and wet deposition in Southern Sweden resulted in peat pore water MeHg concentrations up to six times above background levels. Elevated N loads had no effect on pore water MeHg concentrations.

Seasonal contribution of CO2 fluxes in the annual C budget of a northern bog

[1]xa0Peatlands are sinks for carbon dioxide (CO2) because net primary production exceeds decomposition. The contribution of non-growing-season fluxes to the annual C budget of a peatland is, to date, little studied. We therefore measured the changes in the pattern of carbon exchange with seasons in a bog located in the cool temperate climate region. The growing season CO2-C uptake was of −113 g m−2. During the non-growing season, 36 g C m−2 was lost to the atmosphere, resulting in an estimated net ecosystem production of −76 g C m−2. Despite the non-growing-season loss equaling 33 to 40% of the summer uptake, the net annual accumulation of was 3 times the long-term average net accumulation rate usually cited in the literature. The high rate of non-growing-season efflux could be supported directly by temporal concurrent respiration and the release of stored CO2 from prior production. These results indicate the need to revise current models to address peat thermal properties inducing CO2 production at lower temperature ranges.

Spatial and temporal dynamics of mercury in Precambrian Shield upland runoff

Methylated and total Hg, and TOC concentrations were measured in precipitation and runoff in a first order Precambrian Shield watershed, and in precipitation, throughfall, shallow groundwater and runoff in a zero Precambrian Shield watershed. Plots dominated by open lichen-covered bedrock and another containing small patches of conifer forest and thin discontinuous surficial deposits were monitored within the zero order catchment. Methyl (3–10 fold) and non-methyl (1.4–2.8 fold) Hg concentrations changed irregularly during rainfall and snowmelt runoff events in all catchments. Temporal patterns of Hg concentration in runoff included flushing and subsequent dilution as well as peak concentrations coinciding with peak or recession flow. Mercury export was highest from lichen-covered bedrock surfaces as a result of high runoff yields and minimal opportunity for physical retention and in the case of MeHg demethylation. Forest canopy and lichen/bedrock surfaces were often net sources for Hg while forest soils were mostly sinks. However, upland soils undergoing periodic reducing conditions appear to be sites for the in situ production of MeHg.