wetlands receive allochthonous carbon inputs in sewage (Nag et al., 2019). Carbon inputs associated with dust, ash, and precipitation are not often measured and probably are not important carbon sources in most wetlands.
Allochthonous sediment‐associated carbon can represent a major carbon input to wetlands that experience (semi)regular overbank flooding (González et al., 2014; Hupp et al., 2019; Neubauer et al., 2002). The deposition of allochthonous sediments varies as a function of suspended sediment availability in the water column; the degree of connectivity between the wetland and channel; the frequency, depth, and duration of flooding; and the biomass and physical structure of vegetation (Friedrichs & Perry, 2001; Hupp, 2000). The erosion of sediments from terrestrial landscapes (Wilkinson & McElroy, 2007) has caused increased deposition of allochthonous sediment (and carbon) to some riverine and estuarine wetlands (Khan & Brush, 1994), but others have seen reduced sediment inputs due to reservoirs and levees that restrict sediment movement (Blum & Roberts, 2009; Cabezas et al., 2009). Because wetlands occupy local topographic low spots, they can be sinks for sediment that is eroded from surrounding upland ecosystems (Gleason & Euliss, 1998; McCarty & Ritchie, 2002; S. M. Smith et al., 2001), even in the absence of overbank flooding.
3.3.2. Mechanisms For Carbon Preservation
The preservation of organic carbon occurs because the multi‐stage process of decomposition does not always proceed to completion. The emerging understanding of organic matter decomposition is that the chemical composition of organic matter is important during the early stages of decay, but ecosystem properties drive the overall rates of decomposition (Conant et al., 2011; Lehmann & Kleber, 2015; Schmidt et al., 2011; Spivak et al., 2019). Organic carbon that might be highly resistant to decomposition under one set of environmental conditions may be quickly decomposed under a different set of conditions. By altering the wetland environment, management activities and disturbances have the potential to alter carbon preservation rates and (potentially) destabilize organic carbon that has accumulated over centuries to millennia (e.g., Dorrepaal et al., 2009; Hopple et al., 2020).
In the following sections, we discuss the factors that contribute to efficient preservation of carbon in wetland soils. As an organizational framework, we have classified the controls on wetland carbon preservation into three categories: (1) the redox environment; (2) organic matter characteristics; and (3) physicochemical inhibition of decomposition. Many of these mechanisms are interlinked and could fall into multiple categories.
Redox Environment
The presence of anoxic soils is a characteristic feature of wetlands and one that plays a key role in enhancing carbon preservation by affecting the efficiency of carbon metabolism, the composition of the decomposer community, and the activity of extracellular enzymes (see Phenolic inhibition in Section 3.3.2). The diffusion of O2 slows as soils become water‐saturated, leading to typical O2 penetration depths of millimeters to centimeters at the wetland soil surface and around the roots of vascular plants, where O2 can leak into the soil through the process of root O2 loss (Reddy & DeLaune, 2008). The importance of redox status for carbon preservation is visually apparent in wetlands, where the drainage of organic soils causes noticeable declines in surface elevation from the degradation and loss of soil carbon (Fig. 3.2). Rates of soil carbon mineralization are typically higher under aerobic vs. anaerobic conditions (Table 3.2; Chapman et al., 2019), although the initial decay of the most‐reactive organic compounds may proceed at similar rates regardless of whether O2 or an alternate terminal electron acceptor is used (Kristensen & Holmer, 2001). Bioturbation by crabs and other infauna mixes O2 into the soil, increasing rates of soil carbon mineralization (Guimond et al., 2020). Litter decomposition can be higher in oxygenated hummocks than in low‐oxygen hollows (Courtwright & Findlay, 2011).
Figure 3.2 Subsidence due to peatland drainage in California, Florida, Malaysia, Sumatra, and the United Kingdom. Data for the United Kingdom are from Hutchinson (1980); all other data were extracted from Hooijer et al. (2012). Curves with dashed lines indicate that individual data points were not available for plotting.
Table 3.2 Ratios of aerobic to anaerobic metabolism in different wetland types
Source: Data from Chapman et al. (2019).
| Aerobic: anaerobic ratio | ||
|---|---|---|
| Wetland type | CO2 production | CO2 + CH4 production |
| bog | 3.8 ± 3.7 | 4.3 ± 1.8 |
| fen | 7.0 ± 8.2 | 6.5 ± 7.6 |
| moor | 2.9 ± 1.3 | n.a. |
| swamp | 3.8 ± 1.1 | 5.0 ± 0.7 |
| tropical wetland | 16.0 ± 10.3 | 13.4 ± 9.4 |
| pocosin | 2.2 ± 0.7 | n.a. |
| Overall mean | 7.1 ± 1.2 | 8.2 ± 11.6 |
Values are means ± standard errors. n = 2 to 15 per wetland type. n.a. indicates data were not available for that wetland type.
Anaerobic metabolism.
Where O2 is depleted, a suite of anaerobic pathways can be used by microbes to mineralize organic carbon to CO2 and/or CH4. Thermodynamics dictates that the energy yield from the use of alternate electron acceptors proceeds in the order NO3‐ (denitrification), Mn(III, IV) (manganese reduction), Fe(III) (iron reduction), humic acids (humic acid reduction), SO42– (sulfate reduction), and CO2 (methanogenesis). In an idealized wetland soil, aerobic respiration would occur in surface soils to the depth where O2 becomes depleted, whereupon denitrification would occur in deeper soils where NO3– was available, followed next by the reduction of Mn oxides, and so on, following the thermodynamic order presented above. The same sequence of processes would be expected as one moves laterally away from a wetland plant root. In reality, multiple respiratory pathways can coexist within the same volume of soil due to microscale variations in the availability of electron acceptors and electron donors (Angle et al., 2017; Oremland et al., 1982).
The availability of electron acceptors is important in determining which metabolic pathways are most important in any given wetland. Regardless of the thermodynamics at standard conditions, a reaction will not proceed at appreciable rates if its electron acceptor is present at low concentrations. Thus, low concentrations of NO3– mean that denitrification often accounts for ≤1% of anaerobic carbon mineralization in wetland soils (e.g., Keller & Bridgham, 2007;