Wetland Carbon and Environmental Management. Группа авторов

where wetland restoration or creation would cause greenhouse gas emissions to increase (O’Connor et al., 2020), techniques such as transplanting intact soils and plants can minimize these impacts by avoiding soil disturbances that otherwise favor greenhouse gas emissions (Moomaw et al., 2018, and references therein). Methane emissions often vary between patches of different vegetation types (Kao‐Kniffin et al., 2010; Mueller, Hager, et al., 2016; Villa et al., 2020) due to a variety of plant traits that affect the production, oxidation, and transport of CH₄ (Moor et al., 2017; Section 3.4.1). This suggests that greenhouse gas emissions could be managed in restoration projects through careful selection of plant species composition. To do so, it is important to realize that the influence of different plant traits on CH₄ emissions cannot be entirely understood from short‐term flux measurements because they fail to capture ebullition and hydrologic export. For example, Bansal et al. (2020) reported that short‐term CH₄ fluxes were five times higher from planted vs. plant‐free sediments, but when they accounted for pulses of CH₄ release, total emissions were equal between sites. Thus, the influence of plant species must account for the full CH₄ budget and not rely entirely on inferences based on diffusive flux rates. One challenge to implementing wetland activities in carbon financing systems is projecting how the greenhouse gas balance will change over a century timescale (Needelman, Emmer, Oreska et al., 2018).

Figure 3.5 Contributions of CO₂, CH₄, and N₂O to radiative forcing due to land use/land cover change. Colored bar segments show the radiative forcing from each individual gas. Black circles show the overall radiative forcing from all three gases combined.

      Source: Data from Tan et al. (2020).

      3.5.5. Managing Dissolved Organic Carbon Export

      Wetland management can alter rates of wetland DOC export, with implications for both climate and water quality. Wetland‐derived DOC affects the color of aquatic systems, which can be seen by the casual observer as the tea‐colored water draining from swamps and organic‐rich soils. This colored DOC reduces the penetration of visible and ultraviolet light through the water column, can alter temperature gradients and vertical stratification, and affects primary production and food web structure (Schindler et al., 1996; Wetzel, 1992; Williamson et al., 1999, 2015). In aquatic systems, DOC also alters acid–base interactions, often by reducing the acid‐neutralizing capacity (Driscoll et al., 1994) and can alter the bioavailability of metals including aluminum, copper, and lead (Brooks et al., 2007; Landre et al., 2009; McAvoy, 1988).

      Anthropogenic disturbances including drainage, deforestation, and fire can substantially change DOC dynamics and the chemical composition of the exported DOC (S. Moore et al., 2013; Rixen et al., 2016; Strack et al., 2008; Urbanová et al., 2011). The drainage of wetlands increases DOC export (Drösler et al., 2014; Kreutzweiser et al., 2008; S. Moore et al., 2013; Rixen et al., 2016). The rewetting of wetlands can return DOC export rates to pre‐drainage levels, although there may be a short‐term DOC pulse during the initial stages of rewetting (Blain et al., 2014). Further, disturbances such as drainage and deforestation cause an increasing fraction of the DOC to be derived from preserved soil carbon rather than recent plant production (Gandois et al., 2013; S. Moore et al., 2013). Over time, the depletion of soil carbon due to disturbance can reduce the export of DOC (Sippo et al., 2019). Fires in peatlands can cause a short‐term increase in DOC concentrations and export (Clay et al., 2009; Olivares et al., 2019; Zhao et al., 2012) but a decrease over the longer‐term (1–10 years post‐fire; Shibata et al., 2003; Worrall et al., 2007). The effects of fire on DOC export may be less important than the effects of climate change in northern peat‐dominated catchments (Burd et al., 2018). Lastly, wetlands export substantially more DOC per unit area to aquatic systems than do other land use types (Raymond & Hopkinson, 2003) so where wetlands have been lost, there likely has been a substantial reduction in the amount of DOC export (Kristensen et al., 2008; Raymond et al., 2004).

3.6. CONCLUSIONS

      The redox environment, organic matter characteristics, and physicochemical factors are well understood to be the fundamental attributes that determine the capacity of wetlands to capture, preserve, and release carbon. Just one of these – the redox environment – has been the focus of most management‐informed research and management activities. Yet even this relatively rich body of knowledge has proven insufficient to accurately predict counterintuitive responses that have been observed in response to drainage or impoundment. Developing a more robust predictive capacity for carbon‐focused management activities in wetlands requires a nuanced application of the biogeochemical processes discussed in this chapter. Examples include the responses of extracellular enzymes to water table manipulation and the influence of plant traits related to O₂ transport on rates of organic matter decomposition, CH₄ production, and CH₄ oxidation. Advances in wetland carbon biogeochemistry can be incorporated into management plans to enhance carbon preservation, prevent the destabilization of accumulated soil carbon, and reduce greenhouse gas emissions, thus maintaining the role of wetlands as regulators of global climate. Given the present limits on our ability to optimize wetland creation and restoration for specific carbon and greenhouse gas emission goals, it is wise to prioritize conservation of existing wetland carbon stocks over restoration and management (Moomaw et al., 2018; Neubauer & Verhoeven, 2019). In addition to the biogeochemical considerations we have discussed in this chapter, the cost effectiveness of various restoration and management actions (e.g., Taillardat et al., 2020) has real‐world implications for how wetlands are managed. Managing wetlands for climate regulation should be one facet of a comprehensive plan that also considers valuable co‐benefits of wetlands including water quality improvement, wildlife support, water storage, and cultural services.

ACKNOWLEDGMENTS

      During the preparation of this chapter, SCN was supported by Virginia Commonwealth University and JPM was supported by the Smithsonian Institution.

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