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. 3.2). This is related to multiple factors including a decline in the volume of soil located in the aerobic zone, decreases in organic matter quality as the most reactive compounds are preferentially lost, and enrichment in soil mineral content as the organic fraction is decomposed (Bhadha et al., 2009).
The global impact of drainage on soil carbon stocks was originally estimated by Armentano and Menges (1986) at 239–319 Mt CO2/yr in 1980. Subsequent estimates are larger by a factor of 4 or more with rates of 1,298 Mt CO2/yr in 2008 (Joosten, 2010). The increase reflects continuing carbon losses from historical drainage (Drexler et al., 2009; Hooijer et al., 2012) and extensive new drainage activity in tropical peatlands, where peat loss to microbial respiration can be comparable to CO2 emissions from instantaneous oxidation due to fire (Couwenberg et al., 2010; Hergoualc’h & Verchot, 2014). Vast areas of tidal marshes have been drained and “reclaimed” for agriculture in China (Ma et al., 2014) and mangrove forests are excavated for shrimp and salt ponds, releasing large amounts of soil carbon (Kauffman et al., 2014). The global impact of land use/land cover change on coastal wetlands, riparian wetlands, and peatlands is to decrease net CO2 uptake by 70–457% compared to their natural state (Tan et al., 2020). The sole exception to this pattern is the creation of relatively fresh wetlands from saline coastal wetlands, which perhaps increases NPP by relieving salt stress, although (from a radiative forcing perspective) this may be offset by increased CH4 emissions.
Hydrologic restoration to wetland vegetation (Knox et al., 2015) or rice agriculture (Deverel et al., 2016) can dramatically slow the rate of soil organic carbon loss but recovery of soil carbon stocks requires decades to centuries (Craft et al., 2003; O’Connor et al., 2020; Sasmito et al., 2019). Rewetting tends to reduce CO2 emissions (Wilson et al., 2016; Xu et al., 2019) with the magnitude of change varying with factors such as climate, site nutrient status, antecedent water table depth, and chemical composition of soil organic matter.
Counterintuitively, rewetting can increase CO2 emissions in some circumstances (R. M. Petrone et al., 2003; Waddington et al., 2010). There are instances where wetland responses to drainage or drought do not follow the expected pattern of increased CO2 emissions and soil carbon loss (Laiho, 2006; H. Wang et al., 2015), results that run counter to expectations based solely on redox control of decomposition rates and reflect regulation by other factors. For example, poor substrate quality prevented an increase in soil respiration in response to three years of experimentally imposed drought in a minerotrophic fen (Muhr et al., 2011); rewetting increased decomposition in a peatland because a preceding drought triggered an increase in enzyme activity (Bonnett et al., 2017); and drought or drainage can suppress decomposition rates indirectly through plant community composition changes that favor species with phenolic‐rich tissue (H. Wang et al., 2015).
3.5.2. Managing Organic Matter Characteristics
Chemical composition and the structure of chemical bonds strongly influence the preservation of organic matter. The wide variety of methods used to quantify chemical composition reflects the chemical complexity of organic matter including
