comparable to the total CH4 emissions from freshwater wetlands (Segarra et al., 2015). Rates of anaerobic oxidation of CH4 in wetlands and wet soils are correlated with rates of CH4 production (Blazewicz et al., 2012; Segarra et al., 2015). The anaerobic oxidation of CH4 can be coupled with the reduction of NO3– or nitrite (NO2–) (Hu et al., 2014; Raghoebarsing et al., 2006), Mn(III, IV) and Fe(III) (Beal et al., 2009; Egger et al., 2015), humic acids (Smemo & Yavitt, 2011; Valenzuela et al., 2017), or SO42– (Egger et al., 2015; Knittel & Boetius, 2009). It is not always straightforward to identify which electron acceptors drive the oxidation of CH4 (Gupta et al., 2013; Segarra et al., 2013), but the electron acceptor likely varies in freshwater vs. saline wetlands, organic vs. mineral soils, and oligotrophic vs. eutrophic sites, as is the case for terminal metabolism (see Anaerobic metabolism in Section 3.3.2).
Nitrous Oxide (N2O)
Although this chapter is focused on carbon, we will briefly discuss the emissions of N2O. Recent global wetland emissions of N2O are “negligible” (Anderson et al., 2010), but management activities and environmental changes have the potential to increase emissions of this powerful greenhouse gas. The production of N2O, which is a byproduct of both denitrification and nitrification, is largely controlled by nitrogen availability and soil redox status (Davidson et al., 2000). Nitrous oxide emissions are greatly enhanced in wetlands exposed to high nutrient loading (Hefting et al., 2003; Moseman‐Valtierra et al., 2011) and inversely related to soil C:N ratios (Klemedtsson et al., 2005). Further, peatlands that experience drought or anthropogenic lowering of the water table have higher N2O emissions than those with a high water table (Pärn et al., 2018; Prananto et al., 2020). The production of N2O is also affected by the availability of electron acceptors and electron donors, concentrations of hydrogen sulfide, temperature, and pH (Cornwell et al., 1999; Joye & Hollibaugh, 1995; Megonigal et al., 2004; Pärn et al., 2018; Parton et al., 1996).
Emission Pathways
There are three major pathways by which gases produced in wetland soils can be emitted to the atmosphere: diffusion, transport through plants, and ebullition. The rate of diffusion of gases out of a wetland soil is a function of the concentration gradient between soil pore spaces and the overlying water column or atmosphere, the wetness of the soil, and the amount of atmospheric/water column turbulence (Lai, 2009; Le Mer & Roger, 2001). Because molecular diffusion is a relatively slow process, rates of CH4 oxidation can be more important when diffusion is the major route of export from the wetland (Bridgham et al., 2013). However, while a low water table increases the distance CH4 has to diffuse through oxidized soils and therefore provides more opportunities for the aerobic oxidation of CH4 (Roslev & King, 1996), this can occur at the radiative expense of higher rates of N2O production (Pärn et al., 2018).
The aerenchyma tissues that allow vascular wetland plants to transport O2 to their roots permit gases produced in soils to be efficiently vented through plants by passive diffusion or (faster) convective gas flows (Colmer, 2003). Gas transport through both herbaceous and woody plants can account for a substantial portion of total wetland CH4 emissions (Covey & Megonigal, 2019; Gauci et al., 2010; Neubauer et al., 2000; Pangala et al., 2017; Whiting & Chanton, 1992). Methane that is transported through plants spends less time in oxidized surface soils and therefore is less susceptible to being oxidized to CO2 (Joabsson et al., 1999), although CH4 oxidation can be enhanced in the rhizosphere due to root O2 loss (van Bodegom et al., 2001). There is a temporal coupling between CH4 production and emission in vegetated wetlands, but this relationship breaks down in unvegetated sediments because the lack of vegetation reduces CH4 emissions and promotes transient CH4 storage (Reid et al., 2013) that leads to enhanced ebullition.
Ebullition (bubbling) occurs when the local hydrostatic pressure decreases due to changes in temperature, air pressure, and water levels (Chanton et al., 1989; Männistö et al., 2019; Tokida et al., 2007), allowing gas bubbles to rise. As with plant‐mediated gas transport, the rapid vertical movement of gas bubbles allows CH4 to quickly transit active CH4 oxidation regions (Lai, 2009). Rates of ebullition are spatially patchy and temporally variable but can be the major route of CH4 transport from some wetlands (Devol et al., 1988; Goodrich et al., 2011; Walter et al., 2006). The importance of ebullition can be substantially lower for CO2 and N2O due to their higher solubility (McNicol et al., 2017). Because gas transport through plants helps prevent the accumulation of gases in soil pore spaces (Reid et al., 2013), ebullition is likely to be most important in unvegetated wetlands or those with few vascular plants (Stanley et al., 2019).
3.4.2. Export of Dissolved Organic and Inorganic Carbon
Wetland soils contain high concentrations of dissolved organic and inorganic carbon that can be exported to adjacent surface water and groundwater systems. Quantifying the export of dissolved forms of carbon requires accurate measures of water flow, which is especially challenging where flows are bidirectional (e.g., in tidal wetlands) or diffuse (that is, not in defined channels). The issue is further complicated by the fact that some – but not all – of the carbon exported from wetlands will end up in the atmosphere as CO2 or CH4. Therefore, accurately describing the climatic impacts of a wetland requires the accurate quantification of how much dissolved carbon is exported from the wetland and the ultimate fate of that carbon (that is, emissions to atmosphere vs. long‐term preservation) in downstream aquatic systems.
Dissolved Organic Carbon
Wetlands are a major source of DOC to streams, lakes, rivers, and estuaries (Childers et al., 2000; Kristensen et al., 2008; Mulholland & Kuenzler, 1979). DOC export rates depend on DOC concentrations in soil pore spaces, leaching that occurs directly into the water column (e.g., of plant litter), and flows of water through the wetland (Dinsmore et al., 2013; Jager et al., 2009; K. C. Petrone et al., 2007). The DOC concentrations in streams draining peat‐dominated catchments have been increasing (Freeman, Evans et al., 2001) as have DOC concentrations in many rivers and lakes (Evans et al., 2005; Monteith et al., 2007; Skjelkvåle et al., 2005). The DOC exported from tidal wetlands has distinctive optical properties such as high DOC‐specific absorption, low spectral slope, and high fluorescence that reflect its relatively high molecular weight and aromatic‐rich structure compared to estuarine‐derived DOC (Tzortziou et al., 2008), a property that can be used to observe DOC sourced from tidal wetlands using remote sensing (Cao et al., 2018).
Climate change and alterations in atmospheric chemistry have the potential to increase rates of wetland DOC export. Rising air temperatures increase wetland DOC concentrations and cause DOC to become enriched in phenolic compounds (Freeman, Evans, et al., 2001), thereby inhibiting DOC degradation in receiving systems (Freeman et al., 1990). Similarly, there is generally greater DOC export from tropical vs. boreal peatlands (Drösler et al., 2014). In boreal and alpine regions, melting permafrost is leading to higher DOC export from wetlands to aquatic systems (Frey & Smith, 2005), with evidence that this DOC is rapidly consumed by heterotrophic bacteria or degraded through photochemical mechanisms (T. W. Drake et al., 2015; Selvam et al., 2017). Rising atmospheric CO2 concentrations increase plant productivity in peatlands and enhance DOC exudation from plants, contributing to increased rates of DOC export (Freeman, Fenner, et al., 2004). Similarly, salt marshes respond to elevated CO2 with higher porewater DOC concentrations, but only in the plant communities that exhibit CO2‐related increases in growth (C3 but not C4 plants; Keller, Wolf, et al., 2009; Marsh et al., 2005). There can be synergies between elevated CO2 and warming that further increase DOC export (Fenner et al., 2007).
