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) means that the vast majority of the DIC was exported as dissolved CO2. In contrast, the pH of the marsh tidal creek was 6.4–7.2, such that 19% of the DIC was exported as dissolved CO2 and the remainder as HCO3– and CO32– (Neubauer & Anderson, 2003). Because carbonate alkalinity does not change due to CO2 evasion (Frankignoulle, 1994), the 81% of the DIC exported as HCO3– and CO32– acts as a longer carbon sink and may be exported through the estuary to the ocean. The exported alkalinity also plays a role in buffering pH changes in aquatic systems (Sippo et al., 2016). It is worth noting that high turbulence, as occurs in shallow, fast‐moving streams like the one draining the Scottish peat bog (Dinsmore et al., 2010), can speed the rate of gas evasion but would not change the amount of wetland‐produced CO2 that would ultimately be emitted from the aquatic system to the atmosphere.
Methane can be exported from wetland soils to adjacent water bodies where, because of its low solubility, it will quickly equilibrate with the atmosphere. This can be a substantial pathway of CH4 loss. In a tidal salt marsh, the export of CH4‐supersaturated porewater to a tidal creek, followed by degassing, was as important as CH4 diffusion across the marsh–atmosphere interface (Bartlett et al., 1985). In a temperate freshwater wetland, nearly a third of the annual CH4 emissions were released from the water (Poindexter et al., 2016). In peatlands, the emissions of CH4 from the surface of streams and ponds is on the order of 2–5% of the diffusive soil–atmosphere fluxes (Billett & Moore, 2007; Dinsmore et al., 2010).
3.4.3. Erosion and Losses of Particulate Carbon
Wetlands can export particulate organic carbon (POC) through erosion, hydrologic transport, feeding activities, and direct anthropogenic activities including peat extraction and timber harvesting. Once POC is mobilized, its fate depends on the chemistry of the exported carbon and the environment to which it is transported. In some cases, POC can be redistributed and stored in aquatic sediments or even redeposited back onto the wetland (Hopkinson et al., 2018). However, when POC is solubilized or mineralized to CO2 or CH4, a large fraction is likely to be returned to the atmosphere (e.g., Brown et al., 2019) and the wetland could change from a net carbon sink to a source (Pawson et al., 2012). A related question concerns the fate of soil carbon in coastal wetlands that are drowned by rising sea levels: Will the soil and its preserved carbon stay intact after the vegetation is lost or will it be eroded and dispersed? This is an area of much uncertainty (e.g., DeLaune & White, 2012; Needelman, Emmer, Emmett‐Mattox, et al., 2018; Pendleton et al., 2012).
Erosion of tidal marshes, peatlands, and other wetlands can represent an important vector for the transport of soil carbon into adjacent aquatic systems. The potential importance of POC exports via erosion can be inferred from metrics like the drainage density (that is, km of channel per km2 of wetland) or the extent of wetland edge (Pawson et al., 2012). There is abundant evidence that aboveground plant biomass can reduce erosion by dissipating turbulence and wave energy, even under storm surge conditions (Duarte et al., 2013; Gedan et al., 2011; Möller et al., 2014). Belowground, the network of intact roots and rhizomes helps bind soils, increasing their shear strength and resistance to erosion (Micheli & Kirchner, 2002). Thus, reductions in plant biomass – aboveground or belowground –
