7.9.2: Carbon Accumulation and GHG Emissions from Freshwater Wetlands (Including Permafrost) In a Changing Climate

Wetland conservation has important implications for atmospheric C cycles, since a substantial portion of the soil C pool is stored in wetlands. Northern high latitude and tropical peatlands store more than 600 PgC (Gorham 1991; Hugelius et al. 2014), which is among the largest reserves in the world (Köchy et al. 2015). This amount is more than two-thirds as much as is stored in the atmosphere and comparable to the amount stored in global forest biomass (Pan et al. 2011). Wetland conditions are critical for C accumulation and storage since decomposition in these systems is limited by a lack of oxygen due to water saturation (Brinson et al. 1981). Therefore, when plant productivity exceeds decomposition there is a net accumulation of soil C. This process eventually develops deep peat deposits, which may accumulate for thousands of years. In high latitudes of the Northern Hemisphere, the accumulation process is further intensified by the presence of permafrost, which can have contrasting effects on hydrology, leading to either wetland formation or loss (Sannel and Kuhry 2008). The negative climate feedback (i.e. net cooling effect) that results from increased plant productivity and the long-term C accumulation and storage by wetlands is, in part, offset by CH₄ emissions from freshwater wetlands (Turetsky et al. 2014). Freshwater wetlands represent the largest natural source of CH₄, releasing approximately 180 – 220 Pg CH₄ yr^-1 (Mikaloff Fletcher et al. 2004, Kirschke et al. 2013). However, wetlands that accumulate peat account for less than a quarter of all wetland CH₄ emissions (Turetsky et al. 2014 and references therein).

The influence of future climate on wetland soils C will depend upon the same factors that facilitated C accumulation in these systems: water saturated soils and minimal modification of wetlands through land-use change, and in the case of high latitude peat lands, low temperature. Globally, temperature, low oxygen (due to soil saturation), and the chemical and physical form of the organic matter, are the primary factors limiting decomposition in wetlands. Changes in precipitation and evapotranspiration patterns, which alter the water balance of wetland ecosystems, will substantially influence wetland C cycling. However, the magnitude, directionality, and seasonality of projected hydrologic changes are regionally variable (Collins et al. 2013), and therefore, the fate of soil C stored in wetlands will depend on local conditions. In contrast, changes in the global energy balance, usually manifested by an increase in temperature, are most likely to accelerate the decomposition rate of wetland organic C stored at the soil surface. Deeper C pools may be unaffected unless there are associated changes in hydrology (van Groenigen et al 2016). These potential losses of below ground C may also be partially offset by increased primary productivity.

The greenhouse gas dynamics of permafrost regions differ in important ways from liquid water wetlands. The microbial metabolism of soil carbon is greatly reduced when the soils are frozen for long periods. Thawing changes the availability of oxygen and liquid water, and activates bacterial metabolism, which leads to a relatively abrupt increase in emissions of either or both carbon dioxide and methane. In addition, the low solubility of methane in water causes the accumulation of this gas in bubbles under the permafrost layer. Thawing releases these bubbles, which substantially contributes to this abrupt emission increase. In permafrost regions, increased temperature will have both direct and indirect effects on wetland C storage; permafrost thaw can dramatically affect hydrology in the Arctic, but the C consequences of that change are dependent upon landscape conditions (Olefeldt et al. 2016). Permafrost thaw can lead to wetland drainage because permafrost restricts vertical water flow. As the permafrost thaws to deeper soil layers or is completely thawed, the perched water table may be lowered, resulting in drier surface soils. Permafrost-mediated wetland drainage can lead to substantial C losses because of higher rates of aerobic bacterial metabolism. However, permafrost thaw can also result in ground collapse that can cause wetland formation and substantially increase CH₄ emissions from permafrost ecosystems (Christensen et al. 2004; Natali et al. 2015; Schuur et al. 2015).

The effects of climate changes on wetland C storage will be determined largely by the extent to which the wetlands have been modified through land-use change (Petrescu. et al 2015). Altering wetlands can increase the vulnerability of the organic C pool by weakening the self-regulating feedbacks that exist in many peatland systems (Frolking et al. 2010). Land use change that affects wetland hydrology has had substantial impacts on wetland structure and function. Draining wetlands decreases CO₂ uptake and increases rates of microbial decomposition and CO₂ release (Mietten et al 2017). Soil C is also lost by peat extraction, drainage and other disturbance (Laıne et al. 2014; Evans et al. 2015; Strain, et al 2017). The hydrologic changes can be so large that they result in massive losses of C to the atmosphere, such as occurred during the fires in tropical peatlands in Southeast Asia (Page et al. 2002).

While the drainage of natural wetlands for conversion to agricultural land results in net losses of soil organic C, radiative forcing from wetland conversion depends on relative changes in the direction and magnitude of two major GHGs: CO₂ and CH₄ (Petrescu et al. 2015). Despite a decline in CH₄ emissions following wetland drainage, wetland conversion to cropland results in a significant net increase in atmospheric radiative forcing (heat trapping) (Petrescu et al. 2015). On the other hand, land use changes that cause flooding and creation of wetlands can alter C pools through the saturation and burial of organic C (Knoll et al. 2014). Despite the potential for C sequestration, reservoir formation leads to increased GHG emissions, primarily because of CH₄ emissions from ponded water and highly fluctuating water levels in reservoirs compared to natural lakes (Deemer et al. 2016; Hayes et al. 2017).

Increased atmospheric CO₂ is projected to almost double current freshwater wetland CH₄ emissions, primarily due to warmer temperatures as well as enhanced precipitation (Shindell et al. 2004). The increase in CH₄ emissions under high CO₂ concentrations will primarily result from increased emission rates from tropical wetlands and from wetland expansion in northern high latitudes (Shindell et al. 2004; van Groenigen et al. 2011). The response of wetlands to future climate scenarios will also vary across wetland systems. For example, Wu and Roulet (2014) suggest that ombrotrophic (rain-fed) peatlands will maintain structure and function, but fen-like systems that rely on terrestrial water inputs are much more vulnerable to climate change. Land use and climate-mediated changes in CH₄ emissions from freshwater wetlands can produce a large increase in radiative forcing (heat trapping) in decades to several centuries, but in the long-term (century-millennia), C sequestration by wetlands represents, at present, a net cooling effect (Frolking and Roulet 2007; Neubauer and Megonigal 2015). However, land use, land use change, and fire can cause abrupt changes in soil C storage in wetlands, switching these long-term C sinks to sources of C to the atmosphere (Joosten et al. 2016)

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Excerpted from:

Moomaw, W.R., Chmura, G.L., Davies, G.T. et al. Wetlands In a Changing Climate: Science, Policy and Management. Wetlands 38, 183–205 (2018). Accessed December 2023 https://doi.org/10.1007/s13157-018-1023-8 CC-BY