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3.1). At Time 1, Wetlands 1 and 2 had a positive radiative balance over a 100‐year period, indicating that the warming due to CH4 emissions was greater than the cooling due to long‐term carbon preservation in each wetland. For Wetland 1, the radiative balance was exactly the same in the two time periods because carbon sequestration and CH4 emission rates did not change. Thus, the radiative forcing of Wetland 1 was zero (Table 3.1) and its contribution to Earth’s energy budget had not changed over time. In contrast, the radiative balance in Wetland 2 was lower in Time 2 than in Time 1 due to a management action. This means that radiative forcing was negative, such that the perturbation (that is, the management action) applied to Wetland 2 had offset some of the climatic warming from fossil fuel combustion and land use changes. In this example, Times 1 and 2 correspond to any pair of years. In the context of the attribution of current climate change, the Intergovernmental Panel on Climate Change (IPCC) reports radiative forcing relative to the year 1750 (i.e., the pre‐Industrial era; Myhre et al., 2013). Determining what the radiative balance of a wetland was more than 250 years ago presents considerable challenges.
Finally, please note that the GWP and SGWP are properties of greenhouse gases, not of an ecosystem. We sometimes see them incorrectly used as a synonym for radiative balance, as in the “global warming potential (GWP) was calculated in CO2 equivalents” or “we observed a significant difference in GWP between aerobic and anaerobic treatments.” We do not wish to single out specific authors, so we have purposely not provided citations for these quotes. Instead, our goal is to illustrate how these terms have been misused in the scientific community.
3.3. FACTORS CONTROLLING CARBON PRESERVATION
Wetlands are global hotspots for the preservation of organic carbon in terms of the total amount of preserved carbon (Sabine et al., 2004), the annual rate of carbon preservation (Mcleod et al., 2011), and the efficiency of carbon preservation (e.g., >5% of ecosystem net primary production stored in peatlands vs. <<1% in ocean sediments; Frolking et al., 2010; Hedges & Keil, 1995). From a climate perspective, organic carbon preserved in a wetland represents CO2 that was fixed by primary producers in the wetland (or elsewhere) and therefore is no longer in the atmosphere acting as a greenhouse gas. The long‐term preservation of organic carbon in wetland soils is the major reason why wetlands can have beneficial climatic effects (Frolking & Roulet, 2007). Below, we discuss factors that contribute to carbon preservation in wetland soils.
3.3.1. Carbon Inputs
The magnitude of carbon inputs to a wetland determines the maximum rate of carbon preservation in that wetland, although the actual rate will be considerably lower due to decomposition of organic carbon and losses of gaseous, dissolved, and particulate carbon from the wetland (Fig. 3.1). Carbon inputs can be autochthonous (originating within the system, e.g., CO2 fixation by wetland plants) or allochthonous (originating from outside the system, e.g., inputs of sediment‐associated carbon and terrestrial detritus). The importance of autochthonous vs. allochthonous inputs varies from one wetland to the next. For example, carbon inputs to ombrotrophic bogs are dominated by autochthonous production by Sphagnum mosses and other plants. In contrast, the ratio of autochthonous to allochthonous carbon inputs can be very different in tidal marshes and other wetlands that are regularly flooded by sediment‐laden waters (e.g., Megonigal & Neubauer, 2019). In order to increase the rate of carbon preservation in a wetland, one could increase the inputs of poorly reactive organic matter to the wetland and/or change the environment to increase the carbon preservation efficiency. Note that changing the inputs of highly reactive organic matter or altering its rate of turnover does not directly affect the long‐term rate of carbon preservation because highly reactive organic matter, by definition, is not preserved. However, inputs of highly reactive organic matter can enhance the decomposition of poorly reactive organic matter through priming effects (Bernal et al., 2017; Mueller, Jensen et al., 2016) and the decomposition process itself can change highly reactive organic matter into material with lower reactivity (Baldock et al., 2004; Jiao et al., 2010). Finally, spatiotemporal changes to the wetland environment can alter the reactivity of organic matter (see Organic Matter Characteristics in Section 3.3.2).
Figure 3.1 Wetland carbon inflows, outflows, and preservation. Only a small fraction of the carbon inputs to a wetland is typically preserved over decades to centuries, with an even smaller fraction preserved for millennia. The sizes of the arrows are illustrative of the relative magnitude of different carbon flows in some wetlands, but the figure does not represent any specific wetland type.
Autochthonous Production
Primary production in wetlands can rival that in other highly productive systems such as tropical rain forests and agricultural systems (Millennium Ecosystem Assessment, 2005). Despite this generalization, there is considerable spatial and temporal variability in rates of primary productivity – both between and within wetlands – that is driven by factors including vegetation type, hydrology, climate, soil properties, and water quality. We focus here on production by higher plants but recognize that algal production can be substantial in some systems (e.g., Tobias & Neubauer, 2019 and references therein). Across a wetland landscape, spatial patchiness in vegetation assemblages can lead to greater temporal evenness in ecosystem carbon inputs compared to a system with more homogeneous vegetation (Korrensalo et al., 2020). Spatial variations in vegetation type can also influence carbon preservation since the chemistry of organic matter added to the soil varies with plant species (Belyea, 1996; Dunn et al., 2016; Kögel‐Knabner, 2002). Regular hydrologic pulsing (e.g., tidal rhythms, seasonal river flooding) enhances productivity versus wetlands with stagnant water or continuous deep flooding (Brinson et al., 1981; Odum et al., 1995). Interannual variations in sea level cause corresponding changes in salt marsh plant productivity (Morris et al., 2002). Vegetation productivity and species composition respond to climate over both short and long periods (e.g., Cavanaugh et al., 2014; Feurdean et al., 2019; Johnson et al., 2005; Mendelssohn & Morris, 2000). Rising atmospheric CO2 levels increase production rates of C3 wetland plants but not C4 plants (Caplan et al., 2015; Curtis et al., 1989; Fenner et al., 2007). This generalization is supported by wetland studies, but it is worth noting that C4 plants can show positive growth responses, albeit smaller responses than are seen in C3 plants (Ainsworth & Long, 2005; B. G. Drake, 2014; Wand et al., 1999). Increasing salinity reduces plant productivity (Sutter et al., 2014), even in plants that are adapted to growing in saline conditions (Mendelssohn & Morris, 2000), although this may be a transient response at the ecosystem scale if the plant assemblage shifts to become dominated by salt‐tolerant plants (Herbert et al., 2015). Although wetlands are efficient at recycling inorganic nutrients (Hopkinson, 1992; Neubauer, Anderson et al., 2005), primary production often increases with allochthonous nutrient inputs (Brantley et al., 2008; Morris et al., 2002; Thormann & Bayley, 1997). Soil pH can influence plant productivity and community composition, especially in highly acidic conditions (Chapin et al., 2004; P. H. Glaser et al., 1990; MacCarthy & Davey, 1976). Interactions between these factors are common (e.g., Erickson et al., 2007; Langley & Megonigal, 2010), but discussing them is beyond the scope of this chapter.
Allochthonous Inputs
Wetlands can be sinks for a variety of allochthonous materials including sediment‐associated carbon (discussed in this section), organic detritus, and atmospheric inputs of dust, ash, and pollen. Organic detritus can take the form of plant material (e.g., leaves, wood) from terrestrial systems (Fetherston et al., 1995; Holgerson et al., 2016) as well as phytoplankton, macroalgae, and seagrass detritus from aquatic environments (Hanley et al.,