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 the content of specific classes of organic compounds (see Carbon Quality in Section 3.3.2); ratios involving specific elements such as C:N ratio or lignin:N ratio; and quantification of functional organic matter moieties such as the O‐alkyl carbon content or syringyl‐to‐vanillyl ratio. Chemical composition is applied to wetland management primarily for understanding decomposition responses to disturbance or manipulation. For example, aerobic microbial respiration rates across a depth sequence of soils from a drained peatland was well explained by the abundance of O‐alkyl carbon (Fig. 3.4), suggesting that soil carbon quality data can be used to improve models of soil carbon loss in response to drainage or drought (Leifeld et al., 2012).
Organic matter composition is rarely the direct target of wetland ecosystem management activities. Perhaps the most common management application for plant chemical composition control of decomposition is in the design of wetlands for wastewater treatment, in which C:N ratios are manipulated to maximize nitrogen removal while minimizing greenhouse gas emissions. A review of constructed wetland designs concluded that a ratio of chemical oxygen demand to nitrogen of 5:1 optimizes nitrogen removal versus N2O in free‐flowing systems, and a C:N ratio of 5:1 minimizes CH4 emissions in vertical subsurface systems (Maucieri et al., 2017). Such ratios can be manipulated through selection of plant species that vary in C:N ratio, lignin content, or other relevant traits (Moor et al., 2017). Similarly, there may be opportunities during wetland restoration projects to select plant species that will promote carbon preservation, while also balancing other project objectives.
Wood has chemical and physical properties that can be leveraged for management or restoration of herb‐dominated wetlands. For example, Fenner and Freeman (2020) proposed that wood amendments preserve soil carbon during drought, a technique that is untested in the field but founded on a well‐developed understanding of the physicochemical inhibition of decomposition by phenolic compounds. Similar considerations suggest that sequestration rates can be improved by encouraging higher woody plant species cover, a process that is occurring unintentionally through climate‐driven invasion of herbaceous‐dominated tidal marshes by woody mangrove species (Doughty et al., 2016). The high lignin content of wood is the basis of adding wood chips to restored wetland soils in order to reduce compaction and therefore the negative effects of restoration construction on plant growth (E. C. Wolf et al., 2019).
Figure 3.4 CO2 production from peat as a function of the concentration of O‐alkyl carbon. Data points represent peat soils at a single site and from different soil depths. OC = organic carbon.
Source: Leifeld et al. (2012).
Plant chemical composition is one of several interacting factors that set the molecular structure of soil organic matter (Kögel‐Knabner, 2002; Schmidt et al., 2011), which is an important control on the soil carbon pool response to disturbance. A history of O2 exposure results in compounds that are resistant to decomposition under aerobic conditions, making the ecosystem less responsive to periodic drought or drainage (Muhr et al., 2011). Carbon mineralization rates in drained wetlands generally decline over time as surficial, reactive carbon pools are lost, a pattern due in part to the increasing age and declining carbon quality of soil organic matter with increasing soil depth (Evans et al., 2014; Leifeld et al., 2012). Lab incubations designed to isolate factors such as chemical composition suggest that the sensitivity of soil organic matter decomposition to O2 availability varies widely among wetland ecosystem types (Table 3.2), as does the potential to produce CH4 under anaerobic conditions (Chapman et al., 2019). Thus, the potential for rewetting to reduce both CO2 emissions and CO2‐equivalent CH4 emissions varies considerably and for reasons that are not well understood.
3.5.3. Managing Physicochemical Inhibition
The availability of O2 regulates carbon preservation through mechanisms other than the often‐cited high free energy yield of aerobic respiration, a thermodynamic constraint on decomposition rates. By contrast, kinetic constraints are imposed by the activity of extracellular enzymes required to break chemical bonds. As discussed earlier (see Phenolic inhibition in Section 3.3.2), the enzymic latch hypothesis states that the absence of O2 triggers a series of events leading to the accumulation of phenolic compounds, which inhibit the hydrolase enzymes that cleave organic bonds (Fig. 3.3; Freeman, Ostle, et al., 2001). The hypothesis has been invoked to explain slow decomposition rates in peatlands and to speculate