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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.