Ecosystem‐Scale Methane Emissions From Peatlands of the Hudson Bay Lowlands

Spatial surface patterns of hummocks, hollows, ridges, and pools (microtopography) are common features of many northern peatlands and are particularly distinct within the vast peatlands of the Hudson Bay Lowland (HBL), Canada. Hypotheses and models describe how small-scale feedbacks among vegetation, hydrology, and nutrients cause spatial differences in peat accumulation that enable microforms and surface patterns to develop over time. Empirical tests of the predictions from theoretical models of these proposed feedback mechanisms are limited, particularly in large peatland complexes such as the HBL. We investigate feedbacks controlling peatland structure and function in an ombrogenous bog and a minerogenous fen in the HBL. Our sites represent surface patterns found in many northern peatlands, specifically spatially irregular hummocks and hollows, and parallel ridges and pools that are perpendicular to slope. We found the occurrence of different spatial patterns depends on position within a peat landform, with these differences attributed to the ecohydrological setting. In turn, the ecohydrological setting, with different water table depths, nutrient availability, and species composition, influences the strength and direction of feedback mechanisms at the microform scale. Our data support the prediction of a positive feedback between plant productivity and acrotelm thickness for peat accumulation and hummock growth and that this may be enhanced by water ponding on slopes to form ridge–pool tracks. We did not find evidence to support the proposed feedback among evapotranspiration-driven transport of water and nutrients for the development of hummocks. Our results suggest a combination of mechanisms operating at various temporal and spatial scales is associated with the development of surface patterns in northern peatlands.

Abstract. Peat extraction substantially alters a peatland's surface-atmosphere exchange of carbon (C). The sites are drained, their vegetation is removed, and then the peat is vacuum harvested for use as a horticultural growing medium. Despite this disturbance covering only a small percentage of Canadian peatlands, the shift from being a net sink to a net source of C during the typical 15–40 plus years of active extraction makes it an important system to study. Ours is the first study in Canada to conduct ecosystem scale measurements of carbon dioxide (CO2) and methane (CH4) exchange using eddy covariance from actively extracted peatlands. In order to understand environmental drivers of seasonal and interannual patterns of CO2, and seasonal patterns of CH4 fluxes, daytime ecosystem scale measurements of CO2 and CH4, along with average hourly water table depth (WTD) and soil temperature, were conducted from March to October in 2020, 2021 and 2022 at a Western Site (near Drayton Valley, Alberta), and from May to October in 2020 and 2022 at an Eastern Site (near Rivière-du-Loup, Quebec). In contrast to the positive linear relationship observed in my studies, we observed a unimodal CO2–WTD relationship, with fluxes peaking at WTDs of 47 cm. Water table depth drove interannual variability, suggesting that in deeply drained peatlands, we must consider that insufficient surface moisture conditions can reduce soil respiration. Soil temperature had a significant interaction with WTD with positive relationships during moderate and wet periods (WTD <50 cm) and weakly positive to negative relationships during dry periods (WTD >50 cm) with lower explanatory power. Thus, process-based models using soil temperature alone may overestimate fluxes from drained peatlands during dry periods. The sites were small sources of CH4 (mean May to August fluxes of 7.22 mg C m−2 d−1) compared to natural boreal bogs, though we were not able to capture freeze-thaw periods. After making assumptions for missing nighttime and wintertime data, we estimated an annual CO2-C of 112 to 174 g C m−2 yr−1, which is considerably lower than Canada's current Tier 2 emission factor. This research will aid in updating emission factors for peat extraction in Canada, and will help guide industry site management practices.

Northern peatlands constitute a significant source of atmospheric methane (CH4). However, management of undisturbed peatlands, as well as the restoration of disturbed peatlands, will alter the exchange of CH4 with the atmosphere. The aim of this systematic review and meta‐analysis was to collate and analyze published studies to improve our understanding of the factors that control CH4 emissions and the impacts of management on the gas flux from northern (latitude 40° to 70°N) peatlands. The analysis includes a total of 87 studies reporting measurements of CH4 emissions taken at 186 sites covering different countries, peatland types, and management systems. Results show that CH4 emissions from natural northern peatlands are highly variable with a 95% CI of 7.6–15.7 g C m−2 year−1 for the mean and 3.3–6.3 g C m−2 year−1 for the median. The overall annual average (mean ± SD) is 12 ± 21 g C m−2 year−1 with the highest emissions from fen ecosystems. Methane emissions from natural peatlands are mainly controlled by water table (WT) depth, plant community composition, and soil pH. Although mean annual air temperature is not a good predictor of CH4 emissions by itself, the interaction between temperature, plant community cover, WT depth, and soil pH is important. According to short‐term forecasts of climate change, these complex interactions will be the main determinant of CH4 emissions from northern peatlands. Drainage significantly (p < .05) reduces CH4 emissions to the atmosphere, on average by 84%. Restoration of drained peatlands by rewetting or vegetation/rewetting increases CH4 emissions on average by 46% compared to the original premanagement CH4 fluxes. However, to fully evaluate the net effect of management practice on the greenhouse gas balance from high latitude peatlands, both net ecosystem exchange (NEE) and carbon exports need to be considered.

Wetlands are the largest natural source of methane, yet the roles of source region and paleoclimate in explaining the variability in Holocene atmospheric methane concentrations remain poorly constrained. The Hudson Bay Lowlands (HBL) is one of the world's largest continuous peatland regions and a significant source of methane. We present here, using a novel proxy‐based approach, Holocene methane fluxes for the HBL. Paleo‐methane fluxes were quantified based on water table depth (WTD), inferred from testate amoeba assemblages in nine peat records. WTDs were reconstructed using a North American transfer function and were used to estimate paleo‐methane flux through a linear regression model of contemporary growing season methane fluxes and WTDs from 88 sites across the region. Following HBL peatland initiation in the Middle Holocene, total methane flux is closely related to the increasing area of land emerging from below sea level, controlled by rapid rates of glacial isostatic adjustment. In the Late Holocene, rates of uplift slowed, but methane fluxes remained high due to lower evapotranspiration in a wetter and cooler climate. We estimate that 4.8 ± 1.6 Pg C has been released from HBL peatlands to the atmosphere as CH4 over the last 8,000 years, with an average annual methane emission of 1.1 Tg CH4 yr−1 in the Late Holocene. The values estimated here are broadly consistent with those calculated from other independent methods, on modern and Holocene timescales, demonstrating that testate amoeba records provide an effective approach for scaling local processes to regional paleo‐methane emissions.

Nitrous oxide (N 2 O) emissions were measured by the closed chamber technique from five plots along a transect in a nitrogen‐fertilised grassland, together with soil water content, soil temperature and water table depth, to investigate the effect of water table depth on N 2 O emissions. N 2 O fluxes varied from &lt;1 g N 2 O‐N ha −1 day −1 to peaks of around 500–1200 g N 2 O‐N ha −1 day −1 after N fertiliser applications. There was no significant difference in overall average water table depth between four of the five plots, but significant short‐term temporal variations in water table depth did occur. Rises in the water table were accompanied by exponential increases in N 2 O emissions, through the associated increases in the water‐filled pore space of the topsoil. Modelling predicted that if the water table could be managed such that it was kept to no less than 35 cm below the ground surface, fluxes during the growing season would be reduced by 50%, while lowering to 45 cm would reduce them by over 80%. The strong implication of these results is that draining grasslands, so that the water tables are only rarely nearer to the surface than 35 cm when N is available for denitrification, would substantially reduce N 2 O emissions.

(Table 1) Vegetation coverage of palsa and peat plateaus near the Herchmer and McClintock sites, Hudson Bay Lowlands

Introduction to special section on Synthesis of Recent Terrestrial Methane Emission Studies

Northern peatlands are a globally significant carbon (C) reservoir, yet also function as dynamic methane (CH4) sources to the atmosphere. The fate of peatland C stores and related climate system feedbacks remain uncertain under scenarios of a changing climate and enhanced anthropogenic pressure. Here, we present a synthesis of Holocene peatland C dynamics for the Hudson Bay Lowlands (HBL), Canada, in relation to the past atmospheric CH4 trends, glacial isostatic adjustment, and paleoclimate. We report that peatland age and trophic status, together with paleoclimate, contribute to explaining some of the temporal variation in C accumulation rates (CARs) in the HBL. Our results show that younger, minerotrophic peatlands accumulate C faster, and although detailed paleoclimate data are not available, the results suggest the possibility of higher CARs in association with warmer Holocene climates. Peat initiation rates and CARs were greatest during the mid-Holocene; however, our results reveal that two-thirds of the HBL C pool is stored in peat of late Holocene age, owing to long-term peatland expansion and development. Whereas the HBL has been a net C sink since mid-Holocene peat initiation, the HBL also appears to have been a modest C source, with 85% of the losses occurring during the late Holocene as a consequence of the gradual decay of previously accrued peat. Late Holocene peat decay, under wetter climatic conditions, and from a landscape occupied by an abundance of minerotrophic peatlands, indicates that the HBL may have been a natural terrestrial source of CH4 to the late Holocene atmosphere. While the peatlands of the HBL may continue to function as a globally significant C store, ongoing C losses from the HBL may have important implications for the global C budget and climate system.

Shallow groundwater effects on evaporation and soil temperature in two windblown sands (Eutric Cambisol and Chromic Luvisol) in South Africa

Determination of the hydroperiod of a particular site requires a long period of water table record. On the lower coastal plain hydrology of sites is determined by subtle differences in topography and landscape position. Without vegetational clues, as after plantation establishment, estimation of site hydroperiod becomes even more problematic. Soil classification includes an estimation of drainage class. These classifications are done primarily by noting physical aspects of iron oxide and organic matter. The underlying assumption is that iron oxidation and hydration and organic matter accumulation are a direct indication of long-term average water table depth. A long-term study of water table position on Hobcaw Forest allows examination of the correlation of long- term water table depth to soil drainage class. Hobcaw Forest is located in eastern Georgetown County South Carolina on a Late Pleistocene aged beach ridge with primarily sandy spodosols. Drainage classes of these sandy soils range from excessively well to very poorly. From July 1975 through September 1989 water tables were measured weekly on a grid of 45 shallow wells, distributed across this forest. While all soils displayed a range of water table depths over 150 cm, high temporal correlation allows determination of significant differences in water table depth of wells with averages that differed as little as 5 cm. For the 14 years of measurements, soil drainage class was found to reflect average water table depth. Soil drainage class, combined with taxonomic description, revealed differences of average water table depth that were not significant with less than 4 -5 years of water table measurement.

Drainage of wet soils is a widely used method to enable use of these soils, for example, agriculture. Artificial drainage may, however, cause multiple adverse effects, especially in the case of peat soils where drainage can result in oxidation of peat and associated greenhouse gas emissions, as well as soil subsidence. Shallower water table depths (WTDs), especially in summer, may counter these problems. Here, we assess the effects of two methods aimed at raising the summer WTD using passive or active water infiltration via subsurface drainage systems, on five sites in the coastal plains of the Netherlands. WTDs were monitored for 4 years with high‐frequency measurements in a control and treatment plot, allowing for a statistical comparison of the WTD time series between the two. Using linear mixed effect models, we obtained a combination of six parameters characterising the influence of water infiltration systems (WISs) on the average WTD and on its fluctuations, including the spatial aspect of distance to the subsurface drain. The parameters show that WISs are effective in reducing the WTD fluctuations at four out of five sites, whilst the effect on the average WTD clearly depends on the applied water levels in the WIS. The local site conditions as well as the installation characteristics and management of the WIS are used to explain the (differences in) parameters, highlighting the importance of a correct implementation and management of the WIS.

Predicting shallow water table depth at regional scale from rainfall and soil data

Peatlands, including plateaus, palsas, or polygons, contain a diversity of lichens and bryophytes that dominate northern ecosystems. Studies on the lichen diversity of peatlands in Wapusk National Park on the Hudson Bay Lowlands in Manitoba are rare. The goal of this study was to examine the species diversity of lichen-forming fungi in the northwestern peatlands of Wapusk National Park. Seven locations were sampled in the northwestern portion of the park, including one burned peat polygon, three unburned peat polygons, two peat plateaus, and a peat-covered beach ridge. Eleven species are reported new for the national park. A large number of species had a boreal element, reflecting the presence of the treeline. As expected, the burned peat polygon showed the lowest levels of species similarity with that of other sites. A peat-covered beach ridge also showed low levels of similarity with the other peatlands. The most similar sites were the two peat plateaus. This study also shows species similarity among peatland types. Revegetation of the burned peat polygon was slow and is discussed with reference to park management practices and climate change.

Peatlands help regulate climate by sequestering (net removal) carbon from the atmosphere and storing it in plants and soils. However, as mean annual air temperature (MAAT) increases, peat carbon stocks may decrease. We conducted an in-depth synthesis of current knowledge about ecosystem controls on peatland carbon storage and fluxes to constrain the most influential parameters in probabilistic modelling of peat carbon sinks, such as Bayesian belief networks. Evaluated parameters included climate, carbon flux and mass, land cover, landscape position (defined here as elevation), fire records, and current and future climate scenarios for a 74,300 km2landscape in the Hudson Bay Lowlands, Canada. The Bayesian belief network was constructed with four tiers: 1) exposure, expressed as MAAT, and the state variables of elevation and land cover; 2) sensitivity, expressed as ecosystem conditions relevant to peat carbon mass and its quality for decomposition, peat wetness, and fire; 3) carbon dioxide and methane fluxes and peat combustion; and 4) vulnerability of peat carbon sink strength under warmer MAAT. Simulations were conducted using current (−3.0 to 0.0°C), moderately warmer (0.1–4.0°C), and severely warmer (4.1–9.0°C) climate scenarios. Results from the severely warmer climate scenario projected an overall drying of peat, with approximately 20% reduction in the strong sink categories of net ecosystem exchange and peat carbon sink strength for the severely and, to a lesser degree, the moderately warmer climate scenarios relative to current MAAT. In the warmest temperature simulation, probability of methane emission decreased slightly and the probability of the strong peat carbon sink strength was 27% lower due to peat combustion. Our Bayesian belief network can assist land planners in decision-making for peatland-dominated landscapes, such as identifying high carbon storage areas and those projected to be at greatest risk of carbon loss due to climate change. Such areas may be designated, for example, as protected or reduced management intensity. The Bayesian belief network presented here is built on an in-depth knowledge synthesis to construct conditional probability tables, so is expected to apply to other peatland-dense jurisdictions by changing only elevation, peatland types, and MAAT.

The Hudson Bay Lowlands and adjacent terrain form a vast wetland landscape with low relief and an abundance of organic terrain. Wetland landforms are the main features in the landscape. These include open and forested bogs and peat plateaus; flat fen meadows, and stringed and palsa fens; and swamps, marsh, and open water. Local relief is commonly <2 m. The landscape of the lowlands is the product of its geologic history and present conditions. The area is underlain by Precambrian and Palaeozoic rocks that were peneplaned to a gradient of <2 m/km. During the last glaciation, most of the area was covered by ice flowing out of, or across, the Palaeozoic limestone underlying and adjacent to Hudson Bay, although the western and eastern extremities of the lowlands were influenced, respectively, by Keewatin and Labradorean ice, both of shield provenance. Proglacial lakes Agassiz and Barlow-Ojibway followed the retreating ice sheet northward, deeply inundating the region. The southern margin of the Hudson ice sheet was unstable, and surged into the proglacial lakes. Following the break-up of Hudson ice about 8000 years ago, the lowlands were covered by a high postglacial sea (Tyrrell Sea), which regressed down to present sea level.