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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).