inorganic carbon (Freeman et al., 2001; Kadlec & Reddy, 2001; Miller et al., 2001; Neubauer & Anderson, 2003; Segers, 1998; Treat et al., 2014). Rates of peat decomposition are negligible at temperatures below 0°C and increase sharply as the liquid water content increases in warmed permafrost soils (Dioumaeva et al., 2003). Secondly, temperature sensitivities can vary within the consortium of decomposers, with subsequent effects on the efficiency of carbon mineralization. Terminal metabolizers (e.g., SO42– reducers) can be more sensitive to temperature than are fermenters, thus leading to the accumulation of fermentation products (e.g., acetate) at lower temperatures and the limitation of terminal metabolism by the (low) abundance of these compounds at higher temperatures (e.g., Fey & Conrad, 2003; Weston & Joye, 2005). Indeed, in some high‐latitude wetlands, acetate is the terminal end product of anaerobic decomposition (Duddleston et al., 2002; M. E. Hines et al., 2001). Thirdly, changing temperatures can result in vegetation shifts that change the nature of organic matter inputs to the soil. Along a 40‐year progression of permafrost thaw, rates of potential CO2 and CH4 production were highest in the sites that had been thawed the longest, a difference mediated by the indirect role of temperature in changing vegetation assemblages and, therefore, the chemistry of organic matter inputs to the soil (Hodgkins et al., 2014). While cold temperatures contribute to wetland carbon preservation, the existence of tropical peatlands is strong evidence that temperature is not the only driver (Hodgkins et al., 2018).
3.4. GREENHOUSE GAS EMISSIONS AND OTHER LOSSES
Wetlands are fundamentally open ecosystems that exchange gases, dissolved compounds, and particulate matter with the atmosphere, surrounding terrestrial ecosystems, and aquatic environments. A simple mass balance perspective illustrates that whatever autochthonous and allochthonous carbon is exported from a wetland is, necessarily, not preserved within the wetland (Fig. 3.1). Management actions can manipulate the factors that cause carbon loss in order to reduce carbon export or change the form of exported carbon to a more climatically benign form.
3.4.1. Greenhouse Gas Emissions
Carbon Dioxide (CO2)
On a mass basis, CO2 almost always accounts for the majority of wetland greenhouse gas emissions. Growth and maintenance respiration by autotrophs produce CO2, with rates of autotrophic respiration typically returning ~40–50% of gross primary production to the atmosphere (Dai & Wiegert, 1996). The mineralization of dissolved and particulate organic carbon within wetland soils also produces CO2 that is emitted directly to the atmosphere or dissolved into wetland porewaters. Because CO2 is an end product of most terminal metabolic pathways, the same factors that enhance carbon preservation (Section 3.3.2) will tend to reduce rates of CO2 production, emission, and export.
Wetland CO2 emissions are affected by a variety of climate‐related disturbances. Drought increases soil O2 levels and can remove the enzymic latch that inhibits extracellular enzyme activities in moss‐dominated peatlands (Freeman, Ostle, et al., 2001) but not necessarily in tree/shrub‐dominated wetlands due to differences in the quantity and types of phenolic compounds produced by the different vegetation types (H. Wang et al., 2015). The drying and warming of wetland soils can stimulate root productivity, especially in shrubs (Malhotra et al., 2020). With increasing atmospheric CO2 levels, enhanced plant productivity and shifts in species composition (Caplan et al., 2015; Erickson et al., 2007) have the potential to prime the decomposition of soil carbon through inputs to the soil of O2 and/or highly reactive organic matter from enhanced root growth, inclusive of root exudates (Bernal et al., 2017; A. A. Wolf et al., 2007). In some peatlands exposed to elevated CO2, the activity of the extracellular enzymes β‐glucosidase and phenol oxidase decreased (Fenner et al., 2007) or did not change (Kang et al., 2005), perhaps because reactive carbon was not limiting at those sites. Using elevation change as a proxy, elevated CO2 enhanced belowground productivity and increased soil carbon storage in a brackish tidal marsh (Langley et al., 2009).
The intrusion of saline water into freshwater systems can affect wetland–atmosphere CO2 exchanges. Net ecosystem production is often depressed by saltwater intrusion (Herbert et al., 2018; Neubauer, 2013) but can be unchanged in some years or in response to transient salinity increases (Herbert et al., 2018). The changes in net ecosystem production reflect salinity‐related declines in plant CO2 fixation (Neubauer, 2013; Sutter et al., 2014) and variable heterotrophic respiration responses to increased salinity (Herbert et al., 2015). Changes in heterotrophic respiration could reflect a shift from methanogenesis to energetically favorable SO42– reduction (Weston et al., 2011), reduced activity of extracellular enzymes (Jackson & Vallaire, 2009; Neubauer et al., 2013), or indirect effects that are mediated through soil organic matter availability and composition, microbial community structure, soil O2 availability, and/or nutrient availability (Herbert et al., 2015; Tully et al., 2019).
Fire is an increasingly common feature in many wetlands, especially during drought or periods of seasonal water drawdown (Hope et al., 2005; Turetsky, Kane et al., 2011) and intentional land clearing activities (Marlier et al., 2015). Fire represents a pathway for the abiotic oxidation of wetland biomass and soil organic matter, generating emissions of CO2 (and much smaller amounts of CH4; Kuwata et al., 2016). Surface fires cause a short‐term burst of CO2 emissions as surface vegetation and litter are burned but may promote a decrease in long‐term CO2 emissions if thermally altered organic matter becomes more resistant to microbial decomposition (Flanagan et al., 2020). Smoldering fires can burn tens of centimeters of soil organic matter, converting hundreds to thousands of years of accumulated carbon back to CO2 and significantly increasing global CO2 emissions (Page et al., 2002; Turetsky et al., 2015; Turetsky, Donahue et al., 2011).
Methane (CH4)
Under anaerobic conditions, the final step of the mineralization of organic carbon results in the production of CH4, which is carried out by a subset of the Archaea called methanogens (Bridgham et al., 2013; Megonigal et al., 2004). Methane emissions to the atmosphere reflect the balance between rates of CH4 production (methanogenesis) and CH4 oxidation (methanotrophy). Methane also can be produced abiotically by the burning of vegetation and peat, which can be especially important in years when large peatland fires occur (Kuwata et al., 2016). The last fifteen years have seen reports of aerobic CH4 production by plants (Bruhn et al., 2012; Keppler et al., 2006), fungi (Lenhart et al., 2012), soil macrofauna (Kammann et al., 2009), and in the water column (Damm et al., 2010; Grossart et al., 2011); the importance of these pathways in wetlands is unknown. Globally, wetlands are the largest source of CH4 to the atmosphere, with natural wetlands accounting for 30% of all CH4 emissions (natural + anthropogenic) and paddies associated with rice cultivation adding another 5% to the total (Saunois et al., 2016). Although CH4 is a powerful greenhouse gas, wetland CH4 emissions are not contributing to recent climate change, except to the extent that these emissions have changed in the last ~250 years (Section 3.2).
Methanogenesis has the lowest yield of the terminal metabolic pathways so it tends to be most important when other terminal metabolic pathways are limited by low rates of electron acceptor resupply/regeneration and/or when supply rates of acetate, H2, and other suitable electron donors are high enough to relieve competition with other anaerobic decomposers. The production of CH4 typically requires anaerobic conditions, such that rates of CH4 emissions are inversely related to soil O2 levels (Smyth et al., 2019) and rates of methanogenesis drop sharply in response to decreases in wetland water levels (MacDonald et al., 1998; T. R. Moore & Knowles, 1989). Oxygen inputs can stimulate aerobic respiration (Mueller, Jensen et al., 2016; A. A. Wolf et al.,