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., 2017; Kon et al., 2012). Treatment 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).
