Wetland Carbon and Environmental Management. Группа авторов
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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 (B. Glaser & Birk, 2012).
Soil 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