fluxes between ecosystems and the atmosphere, it is the better metric to use when looking at radiative balances in wetlands (Neubauer & Megonigal, 2015).
Table 3.1 Radiative balance and radiative forcing for two hypothetical wetlands at two time periods
| Long‐term carbon preservation rate | CH4 emission rate | Radiative balance | Radiative forcing | |||
|---|---|---|---|---|---|---|
| Wetland | Time | (g CO2 m–2 yr–1) | (g CH4 m–2 yr–1) | (g CO2‐eq m–2 yr–1) | (g CO2‐eq m–2 yr–1) | (g CO2‐eq m–2 yr–1) |
| Wetland 1 | Time 1 | 75 | 10 | 450 | 375 | 0 |
| Time 2 | 75 | 10 | 450 | 375 | ||
| Wetland 2 | Time 1 | 150 | 40 | 1800 | 1650 | –1080 |
| Time 2 | 150 | 16 | 720 | 570 |
For Wetland 1, we assume there is no change in rates of carbon preservation or CH4 emission over time. For Wetland 2, we assume that a management action lowered CH4 emissions but did not affect long‐term carbon preservation. Note that the carbon preservation and CH4 emission rates are mass fluxes (e.g., g CH4 per area per time, not g C or mol C per area per time). The CH4 mass flux is converted to a CO2‐equivalent (CO2‐eq) flux by multiplying the mass flux by the 100‐year SGWP value of 45 (Neubauer & Megonigal, 2015). The radiative balance of a site is the difference between the warming due to CH4 emissions and the cooling due to carbon preservation, with a positive radiative balance indicating that the wetland has a net warming effect over a 100‐year period. Radiative forcing is the difference in the radiative balance between the two time periods, with negative radiative forcing indicating that a wetland is having a smaller warming effect (or a greater cooling effect) in Time 2 vs. Time 1.
We have used the SGWP to calculate the radiative balance and radiative forcing for two hypothetical wetlands (Table 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).
