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The emission of greenhouse gases is one of several ecosystem processes to consider when managing, restoring, or conserving wetlands. Greenhouse gas management is challenging because wetlands tend to simultaneously act as CO₂ sinks and CH₄ or N₂O sources. Management decisions based solely on greenhouse gas emissions have the potential to create perverse incentives leading to degraded ecosystem function. However, there are many opportunities to reduce greenhouse gas emissions as one goal of overall ecosystem management because wetland greenhouse gas emissions typically increase in response to land use/land cover change (Fig. 3.5; Tan et al., 2020).

Tan et al. (2020) performed a meta‐analysis of the greenhouse gas consequences of land use/land cover change (LULCC) on coastal wetlands, riparian wetlands, and peatlands and found that anthropogenic disturbances increase radiative forcing by 65–2,949% compared to their natural state (Fig. 3.5), amounting to 0.96 ± 0.22 Gt CO₂‐eq/yr, which is equivalent to ~8–10% of annual global emissions due to LULCC. Changing emissions of CO₂ contributed to radiative forcing because ecosystem respiration increased more than did gross primary production, reflecting the fact that wetland LULCC frequently involves drainage. The direction of LULCC on CH₄ emissions is typically opposite that of CO₂, with systems changing from net sources of CH₄ to smaller net sources (or sinks) due to increased O₂ flux (Knox et al., 2015). Radiative forcing from N₂O occurs when LULCC activities are accompanied by nitrogen loading from fertilizer or manure. Reducing fertilizer applications and managing runoff from agricultural fields that drain to wetlands is one option for managing N₂O emissions (Verhoeven et al., 2006)

Coastal wetlands have the potential to sequester carbon at relatively high rates while emitting CH₄ at low rates (Poffenbarger et al., 2011), making them attractive for ecosystem management and carbon financing projects (Needelman et al. 2018, Moomaw et al. 2018). Hydrologic restoration and management of degraded sites tends to increase soil carbon sequestration, achieving rates similar to natural sites after two decades in many cases (Craft et al., 2003; O’Connor et al., 2020). However, the increase in carbon sequestration can be accompanied by an increase in CH₄ emissions resulting in net radiative forcing (O’Connor et al., 2020). Uncertainty in spatiotemporal variation in CH₄ emissions and the factors that regulate this variation are a significant barrier to wetland management for greenhouse gas reduction (Holmquist et al., 2018).

The global potential to manage wetlands for greenhouse gas reductions is limited by their area and the biogeochemically imposed trade‐off between CO₂ preservation and CH₄ emissions. Yet, wetland management can make a significant contribution to nature‐based climate solutions (Fargione et al., 2018). For example, at least 27% of U.S. coastal marshes have been freshened due to tidal restrictions, so the restoration of (saline) tidal rhythms could reduce radiative forcing by 12 Tg CO₂‐eq/yr by reducing CH₄ emissions (Fargione et al., 2018; Kroeger et al., 2017). Reconnecting wetlands to (freshwater) rivers through the construction of large‐scale river diversions could also suppress CH₄ emissions by supplying NO₃^–, Fe(III) oxides, and SO₄^2–, although these effects may be limited to the immediate vicinity of the diversions (Holm et al., 2016). In the US, the radiative balance of CO₂ and CH₄ fluxes is favorable for restoring peatlands and seagrass meadows (9 and 6 Mg CO₂‐eq/ha/yr, respectively), and for avoided losses of seagrass (7 Mg CO₂‐eq/ha/yr; Fargione et al., 2018). In cases 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).