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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.