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 that the concentration of inhibitory phenolics could be manipulated to suppress decomposition rates in peatlands (Freeman et al., 2012). Raising the water table depth achieves this by limiting O2 availability, but it may be possible to achieve similar results by altering pH, adding reductants, or manipulating plant traits through genetic engineering or plant species composition (Freeman et al., 2012).
The response of extracellular enzymes such as phenol oxidase to management can be complex and generate a wide range of carbon responses ranging from increased carbon preservation to increased carbon mineralization. In an elaboration of the enzymic latch hypothesis, the increase in enzyme activity and decomposition rate triggered by O2 exposure leads to higher nutrient availability and soil pH, which in turn increases decomposition in a positive feedback loop that persists for months to years after the soil has been rewetted (Bonnett et al., 2017; Fenner & Freeman, 2011). Another nuance of the enzymic latch hypothesis is that drainage or drought may inhibit phenol oxidase activity due to low soil water content. Under such conditions, rewetting will increase the activity of the enzyme and stimulate decomposition rates (H. Wang et al., 2015). Management activities based on assumptions about water level controlling decomposition rate should also consider the response of inhibitory phenolic compounds.
Microbial access to organic matter can be physically inhibited by mineral‐carbon interactions that operate in intact wetlands via sorption onto surfaces and coprecipitation of DOC (Hedges & Keil, 1995; Lalonde et al., 2012). Mineral soils tend to be rich in Fe‐ and Al‐oxides that preserve organic matter by forming bonds and physical structures that interfere with microbial degradation (LaCroix et al., 2018), so increasing the availability of minerals could enhance carbon preservation. Dredged sediments from navigation channels are sometimes used to create new wetland islands or are added to tidal marshes to increase elevation (Cornwell et al., 2020; Streever, 2000). The ability of dredge spoils to enhance the preservation of wetland carbon through physical inhibition of decomposition depends on whether their mineral surfaces are already saturated with organic carbon, which is likely to be site specific. Some deltaic sediments tend to have less than a monolayer‐equivalent coating of organic carbon due to enhanced mineralization resulting from O2 exposure during periodic reworking events (Blair et al., 2004), but we do not know the extent to which this applies to river and harbor sediments. Organic‐mineral interactions are promoted in the wetland plant rhizosphere by root O2 loss driving deposition of poorly crystalline iron oxides (Weiss et al., 2005), some of which are stable under anaerobic conditions (Henneberry et al., 2012; Shields et al., 2016). Drainage triggers ferrous iron oxidation and increases mineral protection of organic matter, provided there is sufficient iron in the soil to support this carbon‐stabilizing process (LaCroix et al., 2018). The possibility that iron amendments could be used to stabilize carbon in drained soils has not been investigated to our knowledge. Biochar amendments may enhance wetland carbon preservation by altering microbial assemblages and stabilizing existing organic‐mineral complexes (Zheng et al., 2018); the same mechanism helps explains the high‐organic terra preta soils in the Amazon basin
