recent study presented an estimate of northern peatland stocks of 415±150 (min/max) Pg C, of which 185±66 PgC is in permafrost peatlands (calculated for >50°N latitude from Hugelius et al., 2020). This estimate is in line with most earlier studies and is based on harmonized soil maps and >7000 field observations. A fully consistent estimate of northern peatland extent and carbon stocks has long been lacking (Loisel et al., 2017; Yu et al., 2014). The large spread in estimates is due to a lack of unified methods or maps, as well as different definitions used. Studies in recent decades have quantified northern peat C stocks ranging from 270 to 604 PgC, but numbers of 400 to 500 Pg are most commonly used and cited (Yu et al., 2014; Loisel et al., 2017). The estimates are similar in that they all rely on combining data from peat cores (either peat depth or historical peat accumulation) with areal estimates of present day peatland coverage. In most cases however, these studies have not defined a specific lower latitude for the spatial extent, and thus include peat C stored south of >50°N latitude. Most recent studies have used areal estimates for northern peatland extent from Gorham (1991), 3.42 × 106 km2, or from Joostens and Clark (2002), 3.46 × 106 km2. These areal estimates are primarily derived from combining various national inventories of peat resources, which are not available as maps. Global soil or peatland map products that have been used to support broad‐scale modeling, such as WISE30sec or PEATMAP (Batjes, 2016; Xu et al., 2018) show significantly lower coverage of northern peatlands (< 3 ×106 km2). Within the northern permafrost region, more detailed regional soil maps have been used to estimate peatland carbon stocks of ca 300±50 PgC, of which ca 150±30 Pg is in permafrost (Hugelius et al., 2014).
Globally, mineral wetland soils cover an estimated 2.3 × 106 km2 and store ca. 50 PgC (Bridgham et al., 2006). There is no specific estimate for Boreal and Arctic biomes, but spatial analyses of the WISE30sec database (Batjes, 2016) suggest that ca. half of this area and stock is located >50°N. Vegetation C stocks in northern wetlands is usually a small fraction of total ecosystem carbon. In forested northern wetlands, vegetation C is usually 5–10 kg C m–2, but in open wetland systems it rarely exceeds 1–2 kg C/m2 (Alexeyev & Birdsey, 1998). With a total Boreal and Arctic wetland area of 4.5 × 106 km2, of which a quarter is forested, the total vegetation carbon stock is estimated to ca. 10–15 PgC. We estimate that total soil organic carbon stocks in wetlands north of 50° latitude ranges between 400–500 PgC. Defining high‐latitude wetlands to include all permafrost zones (continuous, discontinuous, sporadic) would give an estimate of up to 1672 PgC, which 1466 PgC is in perennially frozen soils and deposits, i.e., Yedoma (Tarnocai et al., 2009).
1.4.5. Temperate Wetlands
Temperate wetlands include those found in the conterminous United States, Europe, India, Asia, Japan, and other temperate regions. These wetlands are generally classified into mineral‐soil based (“freshwater mineral‐soil wetlands”) or peat‐based organic soil wetlands (Mitsch & Gosselink, 2007). Examples of temperate mineral‐soil wetlands include freshwater swamps, freshwater marshes, and riparian forests (Mitsch & Gosselink, 2015). Total global areal estimate for mineral‐soil temperate wetlands is 2,315,000 km2 (Bridgham et al., 2006). Total carbon stock estimates for freshwater mineral‐soil wetlands are 31–46 PgC (Kolka et al., 2018; Bridgham et al., 2006). Peat‐accumulating wetland soils (histosols) occupy 1.3% of the global land area but store approximately 30% of the world’s soil organic carbon (Trettin et al., 2003; Bridgham et al., 2006). Through sequestration and storage, peatlands provide critical ecosystem services that help regulate global climate and mitigate climate change (McLeod et al., 2011). However, these systems are shrinking due to human land‐use modifications and sea level rise (McLeod et al., 2011; Pendleton et al., 2012).
Most peatlands (>50%) occur in northern latitudes; the southern limits of these regions coincide approximately 30–40°N latitude in North America and 50°N latitude in Europe and Asia (Craft et al., 2008; Tarnocai & Stolbovoy, 2006; Yu et al., 2011), where low temperatures are partially responsible for inhibiting organic matter decomposition and limited productivity (Clymo, 1984; Roulet et al., 2007). In the case of wetlands in more temperate or subtropical climates, this temperature effect seems to be more complex and less understood. In a study of temperate freshwater peatlands across a latitudinal gradient in the US, Craft et al. (2008) found that, like terrestrial ecosystems, organic C accumulation in freshwater peatlands is linked to climate through the effects of temperature. Higher C accumulation was measured in cooler and moister climates with lower accumulation in warmer and drier climates. However, other factors, such as species composition (Stricker et al., 2019) also play an important role in the variability in C sequestration and storage. Temperate peatlands in North America cover 861,000 km2 (compared to 3,443,000 km2 peatland coverage globally) (Bridgham et al., 2006; Kolka et al., 2018). In the United States, temperate peatlands cover an area of approximately 93,000 km2 and can be found from Minnesota to Florida and California (Brigham et al., 2006; Craft et al., 2008; Reddy et al., 2015; Drexler et al., 2017). In the southeastern US, temperate peatlands once covered approximately 15,000 km2 (Richardson et al., 2003). In the Southeast US, forested peatlands include peat swamps and shrub‐scrub and pond pine (Pinus serotina)‐dominated bogs referred to as pocosins (Richardson, 1983). In North Carolina alone, peatlands covered 9,079 km2 of the coastal plain, but by 1979 only 2,810 km2 remained (Richardson, 1983). Much of the landscape was deforested at the beginning of the twentieth century, and in the 1970–1980s large canals and ditches were built to facilitate agriculture (Carter, 1975).
In temperate freshwater peatlands, the largest carbon pool occurs in their organic peat soils, with average organic C accumulation ranging from 40–80 g C/m2/yr (Roulet et al., 2007; Frolking et al., 2011). Craft et al. (2008) found accumulation rates ranging from 49±11 (standard error, SE) g C/m2/yr to 86 (maximum) g C/m2/y for temperate freshwater peatlands across a latitudinal gradient in the US. However, C accumulation rates can vary from 7–300 g C/m2/yr (Gorham, 1991; Turunen et al., 2002; Kolka et al., 2011). In these systems, soil accretes slowly, with accretion rates ranging from 0.3 to 10.3 mm/yr. Total biomass of aboveground and belowground components of vegetation in peatlands is variable, depending on the species composition of the vegetation present. In relatively open peatlands, vegetation biomass can be small (e.g., 760 g/m2) but in forested peatlands, the aboveground and belowground components of vegetation biomass can be much higher (e.g., 13,800 g/m2 to 20,000 g/m2)(Gorham, 1991; Brinson & Blum, 1995). The authors assume a carbon content between 0.441 (for herbaceous‐dominated) and 0.501 (for woody‐dominated); this equates to approximately 334 g C/m2 to 10,000 g C/m2 in carbon mass (Martin et al., 2018; Byrd et al., 2018). Increased productivity aboveground, however, does not necessarily imply gains in soil C pool since soil response depends on the interaction of both soil and plant functions, many of which are still understudied (Trettin & Jurgensen, 2003).
For the Northern Hemisphere, estimates of the total C stored in peatlands ranged between 270–604 PgC (Gorham, 1991, Turunen et al., 2002; Frolking et al., 2011; Yu et al., 2014). In North America (including Alaska and Canada), peatlands store approximately 125 PgC (Bridgham et al., 2006). For the conterminous United States, the most recent estimate (Kolka et al., 2018) for total temperate wetland carbon stock from the Second State of the Carbon Cycle Report (SOCCR2) was 13.5 PgC (mainly from peatlands). The China Second Wetlands Survey was used to derive aboveground wetland carbon stocks of 0.22 PgC and soil wetland carbon of 16.65 PgC (Xiao et al., 2019). The SOCCR2 report provided estimates ~1 PgC for Puerto Rico and Mexico. Estimates for other temperate countries were unavailable, i.e., for Europe, India, Japan. Thus, studies are needed to represent these regions and their wetland carbon stocks.
In temperate zones, land‐use activities like water management and drainage are significant contributors to wetland carbon loss (Armentano & Menges, 1986; Richardson et al., 2003; Reddy et al., 2015). Catastrophic wildfires in hydrologically altered temperate peatlands are also common drivers of wetland carbon loss (Page et al., 2009). In one study, emissions during a single fire event measured approximately 3,600 g C/m2 (Turetsky et al., 2011). Many freshwater