pH also exerts strong control on decomposition rates and is negatively correlated with soil carbon preservation. Regulation of extracellular enzyme activity is one mechanism by which pH interferes with decomposition and has been cited as a reason why soil carbon pools sometimes increase in response to drainage or decrease in response to rewetting (Fenner & Freeman, 2011). In northern peatlands, pH exerts indirect control on soil carbon stocks by favoring Sphagnum species that decompose slowly (low pH) or vascular species that decompose relatively quickly (high pH). Thus, pH manipulation to favor one functional plant group over another is one option for altering carbon preservation (e.g., Beltman et al., 2001).
Temperature regulates the rates of all biological, physical, and chemical processes that control organic matter decomposition, and is another physicochemical factor that may cause unexpected soil carbon responses to drainage. For example, short‐term lab and field drainage in wet tussock tundra tends to increase soil organic matter decomposition rates, as expected, but feedbacks operating at larger spatiotemporal scales involving plant community shifts and their effects on snow cover, albedo, and thermal balance have the potential to slow permafrost degradation and preserve soil carbon (Göckede et al., 2019). Feedbacks involving wetland responses to a warming planet include shifting plant distributions, changing estuarine salinity distributions, and altered wetland hydrology, all of which can directly or indirectly impact the preservation of wetland carbon. Incorporating large‐scale feedbacks into wetland management activities is a contemporary challenge.
3.5.4. Managing Greenhouse Gas Emissions
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 SO42–, 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).
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
