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Wetlands can export inorganic carbon as dissolved CH₄, dissolved CO₂ (plus small amounts of carbonic acid, H₂CO₃), bicarbonate (HCO₃^–), and carbonate (CO₃^2–). For consistency with the literature, we use the term dissolved inorganic carbon (DIC) to refer to the sum of dissolved CO₂, HCO₃^–, and CO₃^2–; 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 CO₃^2– (Neubauer & Anderson, 2003). Because carbonate alkalinity does not change due to CO₂ evasion (Frankignoulle, 1994), the 81% of the DIC exported as HCO₃^– and CO₃^2– 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).