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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 – can make the wetland more susceptible to erosion and losses of particulate organic carbon (Deegan et al., 2012; Shuttleworth et al., 2015; Silliman et al., 2012; Walter et al., 2006). Surface soils in wetlands can be mobilized by rain events (Mwamba & Torres, 2002; Tolhurst et al., 2006). Marsh biota can also facilitate erosion, either directly through activities like bioturbation (S. M. Smith & Green, 2013) or indirectly through grazing that removes the stabilizing influence of wetland vegetation (T. J. Smith & Odum, 1981; Visser et al., 1999).
Particulate organic carbon can be exported as water moves across wetland surface or as the biomass of consumers that feed in the wetland. In tidal wetlands, for example, large accumulations of dead plant material (“wrack”) can be redistributed within a wetland or exported to the estuary, especially during spring tides and large storms (Hackney & Bishop, 1981; Hemminga et al., 1990). Aquatic, terrestrial, and avian consumers are able to forage on the wetland surface, consuming organic matter and removing it when they leave the wetland (Fritz & Whiles, 2018; Gurney et al., 2017; Kitti et al., 2009; Klopatek, 1988; Wantzen et al., 2002), but this likely does not impact long‐term carbon preservation.
Lastly, POC can be lost from wetlands through directed anthropogenic activities. The extraction of peat for fuel and horticultural purposes removes the preserved soil carbon and results in the emission of CO2 back to the atmosphere through combustion or decomposition (Cleary et al., 2005). Further, peat extraction typically destroys the living vegetation, resulting in the loss of the wetland carbon sink (Waddington et al., 2010). The logging of forested wetlands can be specifically for harvesting timber (Hutchens et al., 2004) or may be incidental to preparing a site for agriculture or aquaculture (Page et al., 2009; Richards & Friess, 2016). Some wetlands are used directly for grazing of livestock or the plants are harvested for off‐site use (Harrison et al., 2017; Morris & Jensen, 1998; D. C. Smith et al., 1989). Whenever significant amounts of primary production are removed, wetland soil carbon pools and long‐term preservation rates can be affected (Morris & Jensen, 1998).
3.5. MANAGEMENT OF WETLAND CARBON PRESERVATION AND FLUX
Carbon capture, preservation, and flux are foundational processes that govern all facets of wetland ecosystem function, and are thus both the target of, and a response to, management activity. Wetland management and disturbance intentionally or unintentionally affect the biogeochemical mechanisms that preserve organic matter, with consequences for coupled element cycles such as nitrogen mineralization. Here we consider how biogeochemical processes can be manipulated to increase carbon preservation, decrease greenhouse gas emissions, or improve water quality. Our goal is to highlight some common management actions rather than provide a thorough overview of this topic; we leave that to the collective efforts of the other authors in this volume.
3.5.1. Managing the Redox Environment
The redox environment, organic matter characteristics, and physicochemical inhibition are biogeochemical mechanisms that can be manipulated to enhance wetland carbon preservation. From a management perspective, the most important of these is redox, which leverages the large difference in free energy yield of microbial respiration in the presence versus absence of O2. Redox manipulation is the goal of the age‐old technique of raising or lowering water table depth through structures that drain or impound water (McCorvie & Lant, 1993; Rozsa, 1995), and a largely unintended consequence of other activities such as tree thinning in forested wetlands (Jutras et al., 2006) and road construction (Winter, 1988). Draining removes water from soil pore spaces, dramatically increasing the rate of O2 diffusion into the rooting zone. As the rate of O2 flux rises to exceed O2 demand, aerobic respiration becomes the dominant microbial respiration pathway, leading to faster decomposition rates and a decline in soil carbon stocks (Armentano & Menges, 1986).
Subsidence of the soil surface is a nearly universal consequence of prolonged drainage because many wetland soils are carbon‐rich and carbon loss translates into a loss of soil mass and volume (Fig. 3.2). As such, subsidence is a useful metric of soil organic matter stock change in peatlands and other wetlands with organic‐rich soils. The relative contributions of microbial respiration, compaction, fire, and wind erosion to soil elevation change can be modeled to infer that accelerated microbial respiration is a primary driver of elevation loss (Deverel et al., 2016; Ewing & Vepraskas, 2006). As expected of redox‐driven organic matter preservation, subsidence is positively related to water table depth below the soil surface, and rates are higher at sites where the water table is drawn down continuously rather than where it fluctuates seasonally (Carlson et al., 2015; Stephens et al., 1984). Rates of subsidence are fastest during the years immediately following the drawdown of the water table and slow as the soil surface approaches the lowered water table (Fig.