Freshwater marshes are prevalent and important stores of carbon. They bury carbon in deeper soils, although reported rates of carbon accumulation are significantly higher over recent (decadal) versus longer (centennial and millennial) timescales. Intrinsic organic matter degradation, long-term climatic and ecological changes, and recent anthropogenic impacts on sediment fluxes and organic matter production may have a role in explaining this discrepancy, yet remain poorly understood for freshwater marshes. We collected a 4-m core from a riverine-influenced marsh in the watershed of Big Creek which drains into Lake Erie in southern Ontario, Canada, and conducted radiometric dating, elemental analyses, and programmed pyrolysis for organic matter characterization. Over the past 5,710 calibrated years, burial of organic (on average 26 ± 34 g C m-2 yr-1) and inorganic (22 ± 25 g C m-2 yr-1) carbon fractions has resulted in high rates of carbon accumulation. We found that elevated recent rates of organic carbon accumulation are driven by fractions that have low thermal stability and are predominantly from aquatic sources. This type of organic carbon is buried intermittently in deeper marsh sediments and corresponds to major hydro-fluvial events (e.g., Nipissing highstands), which coincide with regional marsh development. We deduce that lower fractions of labile carbon in deeper soils reflect long-term degradation, which underscores the notion that high recent rates of carbon accumulation are generally not sustained over centuries and millennia. Our research demonstrates the importance of identifying various carbon fractions in understanding carbon burial in freshwater marsh soils, and informing marsh conservation.

Terrestrial ecosystems of Canada store a large amount of organic carbon (C) in soils, peats and plant materials, yet little is known about the C stock size and distributions, both spatially and in various C pools. As temperature rises, C is becoming available for disturbance, decomposition and eventual release into the atmosphere, which makes the quantification of C stocks in terrestrial ecosystems of Canada of high interest for the assessment of climate change impacts and conservation efforts. We used a large number of field measurements, multisource satellite, climate and topographic data and a machine learning algorithm to produce the first wall-to-wall estimates of C stocks and uncertainties in plants and soils of Canada at 250 m spatial resolution. Our findings show that above and belowground live biomass and detritus store a total of 21.1 Pg C. Whereas the Canadian soils store 384 (±214, 90% confidence interval) Pg organic C in the top 1 m, 92 Pg C of which are stored in peatlands, confirming that the soil organic C dominates terrestrial carbon stocks in Canada. We also find previously under-reported large soil organic C stocks in forested peatlands on the boreal shields of Canada. Given that Canada is warming twice the global average rate and Canadian soils store approximately 25% of world soil C stocks in top 1 m, initiatives to understand their vulnerabilities to climate change and disturbance are indispensable not only for Canada but also for the global C budget and cycle.

The Hudson Bay Lowlands (HBL) is a vast continuous peatland in Northern Canada. The landscape is a mosaic of mostly bogs and fens, with more limited swamp, marsh, forest and open water. Owing to rapid rates of isostatic uplift, younger peats are found closer to the coasts of Hudson and James Bays, with fen-type peatlands somewhat more prevalent on these younger surfaces. More than 30 Pg of carbon have accumulated in the HBL over the Holocene. The rates of Holocene carbon accumulation vary considerably both spatially and temporally, with some sites showing more rapid rates of carbon accumulation in the first 2-3 millennia following peatland initiation. We evaluate here the hypothesis that vegetation changes over the course of the Holocene, including fen-to-bog transitions, partially explain the variability in carbon accumulation. We find that in some cases, more rapid rates of C accumulation in the middle Holocene (5000-8000 yrs before present) are associated with early successional minerotrophic fens with higher carbon densities. Fen-to-bog transitions are recorded in many peat cores collected from present day bogs; however, these transitions are time transgressive, and can depend on the time since initiation, suggesting that climate changes may play a secondary role, relative to hydrological changes and local ecological processes. Fens are highly prevalent in the HBL landscape (covering about 38% of land cover). Cores taken from present day fens and analyzed for carbon accumulation and vegetation change indicate that many fen sites have remained fens since peat initiation. Variability in rates of Holocene carbon accumulation within fen records which have not been subject to any major vegetation change may more closely reflect climate drivers.

Peatland soils are of great interest for study and management because of their high carbon contents and known role in the global carbon cycle. However, carbon stocks have yet to be constrained in many wetland ecosystems. Relative to bogs, fens and saline coastal ecosystems, less is known about carbon stocks in freshwater marsh soils despite their global prevalence, and it is not well understood how disturbance of freshwater marshes may affect carbon-climate dynamics. To better understand the potential for freshwater marshes to be net carbon sinks, we review how freshwater marshes and associated soils are classified, and synthesize available data on short- and long-term rates of carbon accumulation in freshwater marsh soils in temperate North America. Although often described as mineral-based, our findings suggest that freshwater marshes are not restricted to mineral substrates, and that inconsistencies in classification may underestimate presumed carbon stocks. Organic carbon contents and bulk density measurements are highly variable, and can range between 1-45% and 0.04-1.5 g cm-3, respectively. Moreover, rates of carbon accumulation in freshwater marshes are often measured over recent time scales (50-100 years; on average 155 +/- 74 g C m-2 yr-1), while long-term rates (measured over centuries and millennia; on average 51 +/- 38 g C m-2 yr-1) are much more scarce. We suspect that short-term rates are markedly greater than long-term rates of carbon accumulation because they do not account for long-term carbon loss and may reflect large increases in sedimentation since European settlement in North America. However, we also suspect that long-term carbon storage in freshwater marsh soils is underestimated, and that freshwater marshes can have long-term rates of carbon accumulation similar to those reported for temperate peatlands. In this presentation, we will show that variability of rates of carbon accumulation, rates of sediment accretion, bulk density and organic carbon content in freshwater marshes needs to be better constrained in order to accurately quantify their regional and global carbon pools. We will discuss the importance for scientists to specify timeframes over which they are measuring rates of carbon accumulation so that the capacity for wetlands to be net carbon sinks can be correctly understood.

Terrestrial wetlands are a highly significant carbon reservoir in North America. Forested wetlands, or swamps, are an important category of North American wetland and include boreal forested peatlands, swamps dominated by needle-leaved trees including Thuja (cedar), Picea (Spruce), Larix (Tamarack) or Taxodium (bald cypress), swamps dominated by broad-leaved trees or shrubs including Fraxinus (Ash), Ulmus (Elm), or Acer (Maple), as well as mangroves. The Second State of the Carbon Cycle Report estimates that forested wetlands may make up ~55% of the total terrestrial wetland area for North America, although estimates vary considerably due to different mapping conventions and classification systems across national and provincial borders, and also due to the ongoing impacts of land use change. Additionally, that report suggests that forested wetlands contain larger total carbon pools than non-forested wetlands, and that forested wetlands effect 53% of the estimated 123 Tg total wetland annual carbon sink for North America. Uncertainties in the sizes of the forested wetland soil carbon pools continue to be significant due in part to insufficient data on variabilities in carbon densities across diverse swamp types. Further, there are limited data on the rates of vertical accretion of swamp soils and the associated long-term rates of carbon accumulation, needed for better predicting impacts of climate warming on carbon sequestration in swamp soils. We present here a comparative synthesis of swamp soil carbon properties including bulk densities, organic carbon contents, soil thicknesses, rates of vertical accretion and rates of long-term carbon accumulation, from >200 swamp sites. We compare these properties for broad-leaf swamps (including mangroves), needle-leaf swamps, mixed swamps, and shrub-dominated swamps, and also compare across North American Ecoregions. The results show significant variability across peat-forming and mineral swamps, and indicate rates of carbon accumulation in some swamp types similar to those of northern bogs and fens.