(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). The observed increases in DOC export can also be related to the recovery from acidification due to atmospheric deposition (Monteith et al., 2007), driven by the increased solubility of organic matter at higher pH (Evans et al., 2012; Pschenyckyj et al., 2020).
The export of DOC from peatlands is sensitive to water discharge (Dinsmore et al., 2013; Pastor et al., 2003), which can vary due to changes in precipitation, storage within the wetland, and/or losses to evapotranspiration. Since climate change is altering the frequency and severity of precipitation events (Hartmann et al., 2013), this could affect DOC export by changing the water balance or making export more flashy (Holden, 2005). Following large rain events, there are increased inputs of DOC to aquatic systems (Jager et al., 2009; Paerl et al., 2018) that can cause hypoxia and anoxia in downstream aquatic systems (Paerl et al., 1998). In colder climates, changes in the balance between snow and rain, plus earlier melting of the snowpack, can change the timing of DOC export (Billett et al., 2012).
The DOC exported from wetlands is generally “modern” in age (that is, post‐1950), which is consistent with shallow flow paths of water through surface soils (Billett et al., 2012; Evans et al., 2007; S. Moore et al., 2013; Raymond & Hopkinson, 2003). However, the recent origin of exported bulk DOC can mask inputs of smaller amounts of millennial‐aged DOC, which can be mineralized upon entry to the aquatic system (Dean et al., 2019). In aquatic systems, DOC from wetland and terrestrial systems is subject to microbial mineralization, photochemical oxidation, and flocculation in lakes, streams, rivers, and estuaries (Cole et al., 2007). Much of this processing occurs in freshwater lentic and lotic systems. The relatively short transit time from estuaries to the coastal ocean suggests that DOC exported from estuarine wetlands (e.g., salt marshes) is likely not metabolized within estuaries (Cai, 2011). Although the chemical structure of terrestrial DOC should make it resistant to decay – certainly in comparison to phytoplankton‐derived DOC – very little terrestrial DOC is found in the ocean (Blair & Aller, 2012; Cai, 2011; Hedges & Keil, 1995).
Dissolved Inorganic Carbon and Methane
Wetlands can export inorganic carbon as dissolved CH4, dissolved CO2 (plus small amounts of carbonic acid, H2CO3), bicarbonate (HCO3–), and carbonate (CO32–). For consistency with the literature, we use the term dissolved inorganic carbon (DIC) to refer to the sum of dissolved CO2, HCO3–, and CO32–; dissolved CH4 will be mentioned specifically when we are talking about that molecule. Wetland porewaters are often supersaturated with inorganic carbon that can diffuse into overlying water when a wetland is flooded or can be advectively transported out of the wetland into adjacent water bodies. The observed supersaturation of CO2 in both freshwaters (Butman & Raymond, 2011; Regnier et al., 2013) and estuaries (Cai, 2011; Chen et al., 2013) is partially due to DIC exports from wetlands (Cai & Wang, 1998; Neubauer & Anderson, 2003; Richey et al., 2002; Tzortziou et al., 2011).
The export of DIC is a function of porewater DIC concentrations and hydrology. The DIC concentrations are sensitive to the factors that affect rates of soil respiration and the emission of CO2 to the atmosphere (see Carbon Dioxide in Section 3.4.1). In regularly inundated tidal marsh soils, the DIC export to the estuary parallels seasonal patterns in marsh productivity and respiration (Neubauer & Anderson, 2003; Z. A. Wang & Cai, 2004). In contrast, when hydrology is less consistent, water flow has a controlling role on DIC export. For example, precipitation events serve to transfer porewater DIC into adjacent aquatic systems (Butman & Raymond, 2011). Similarly, dissolved gases that accumulate in soil during winter can be flushed out during the spring thaw (Billett & Moore, 2007).
We use DIC flux studies from two wetlands – an acid peat bog in the Central Valley of Scotland and a tidal freshwater marsh in Virginia, USA – to illustrate the importance of water chemistry on CO2 evasion. The hydrologic export of DIC represented a sizeable route of carbon loss from each system (Dinsmore et al., 2010; Neubauer & Anderson, 2003). In the stream draining the peat bog, roughly 90% of the exported DIC was emitted to the atmosphere as CO2 within the local catchment (Dinsmore et al., 2010). In contrast, as water drained from the marsh, only ~2–6% of the exported DIC was emitted to the atmosphere during a single ebb tide (Neubauer & Anderson, 2003). In both sites, CO2 evasion to the atmosphere would continue with additional downstream transport until equilibrium with the atmosphere was achieved. The lower atmospheric evasion of wetland‐derived DIC in the marsh compared to the peatland reflects the effects of pH on DIC partitioning. The low pH of stream water at the peatland (annual pH means of 4.5–4.8; Billett et al., 2004)