soil organic matter (Hall et al., 2016). At redox interfaces like the wetland plant rhizosphere, there is dynamic redox cycling of Fe (e.g., Weiss et al., 2004) where the microbial and chemical dissolution of Fe(III) can release sorbed carbon into solution (Chin et al., 1998; Knorr, 2013). However, many wetlands contain solid‐phase Fe(III) as a coating on vascular plant roots, in shallow soils where atmospheric O2 penetration occurs, and as Fe‐rich concretions (R. M. Chambers & Odum, 1990; Duan et al., 1996; Emerson et al., 1999; Mendelssohn et al., 1995). While there is an overall decline in Fe(III) with increasing soil depth (Cutter & Velinsky, 1988; Griffin et al., 1989), oxidized iron can persist under anaerobic conditions over geologically relevant timescales (Haese et al., 1997). We have focused here on the preservation of organic carbon, but wetlands can contain measurable amounts of inorganic carbon in the form of siderite (FeCO3) (Duan et al., 1996; Hansel et al., 2001; T. Wang & Peverly, 1999).
Lastly, proteins and amino acids can become encapsulated in humic acids and protected from hydrolysis. In soils and sediments, humic acid fractions can be hundreds or thousands of years old yet have high concentrations of amide and amino nitrogen, forms of organic matter which are often highly reactive (e.g., Hedges & Keil, 1995; Knicker et al., 1996; Mahieu et al., 2002; Zang et al., 2000). The humic acids may be forming a micelle‐like structure that traps reactive organic molecules within the hydrophobic interior of the structure (Zang et al., 2000), which is consistent with observations that hydrophobic organic contaminants also have a high affinity for humic acids (De Paolis & Kukkonen, 1997). This protective mechanism may be most important at low pH where humic acids form structures with a lower surface–volume ratio (versus a chainlike structure at higher pH), which enhances the ability of the humic materials to physically trap organic matter (Myneni et al., 1999). Given the low pH of many peatlands and their general paucity of mineral matter, the encapsulation of organic matter by humic acids in peat is likely to be more important than interactions with aluminosilicate clays or iron minerals.
pH.
Wetland soils span a wide range of pH values, from bogs and pocosins with pH values of ~4 or less to riparian floodplains and other wetlands where the pH can exceed 7.5 (e.g., Jacob et al., 2013; Richardson, 2003). We focus here on low pH wetlands since that is where pH has the largest inhibitory effect on carbon mineralization. Rates of CH4 production and emission are low in acidic wetlands and increase when pH is experimentally increased (Dunfield et al., 1993; Ye et al., 2012). The suppression of CH4 emissions by low pH occurs through direct inhibitory effects on the hydrogenotrophic and acetoclastic methanogenic pathways as well as interference with the fermentative processes that generate the substrates used by methanogens (Ye et al., 2012). Atmospheric acid deposition also depresses CH4 emission rates, although this effect is mediated by the competitive suppression of methanogenesis by NO3– and/or SO42– rather than a direct pH effect (Gauci et al., 2004; Watson & Nedwell, 1998). Rates of soil carbon mineralization to CO2 are also limited by low pH due to the inhibitory effects of pH on fermentation (Ye et al., 2012), the suppression of phenol oxidase activity (Williams et al., 2000; Xiang et al., 2013), a microbial community characterized by slow‐growing bacteria (Hartman et al., 2008), and/or the encapsulation of reactive organic matter by humic acids (see Physical protection in Section 3.3.2). Experimental increases of soil pH in the lab often lead to higher rates of CO2 production (e.g., Ye et al., 2012) although a multi‐year field experiment found a decrease in soil CO2 production rates in response to increased pH, perhaps because the native microbial community was well adapted to the original low pH environment (Keller et al., 2005).
Temperature.
Temperature affects the efficiency of carbon preservation through several related mechanisms. Firstly, biological processes such as decomposition generally slow down at cooler temperatures, as demonstrated for multiple indices of decomposition including litter decay, soil enzyme activities, biological oxygen demand, CO2 and CH4 production and emissions to the atmosphere, and the hydrologic export of dissolved organic and inorganic carbon (Freeman et al., 2001; Kadlec & Reddy, 2001; Miller et al., 2001; Neubauer & Anderson, 2003; Segers, 1998; Treat et al., 2014). Rates of peat decomposition are negligible at temperatures below 0°C and increase sharply as the liquid water content increases in warmed permafrost soils (Dioumaeva et al., 2003). Secondly, temperature sensitivities can vary within the consortium of decomposers, with subsequent effects on the efficiency of carbon mineralization. Terminal metabolizers (e.g., SO42– reducers) can be more sensitive to temperature than are fermenters, thus leading to the accumulation of fermentation products (e.g., acetate) at lower temperatures and the limitation of terminal metabolism by the (low) abundance of these compounds at higher temperatures (e.g., Fey & Conrad, 2003; Weston & Joye, 2005). Indeed, in some high‐latitude wetlands, acetate is the terminal end product of anaerobic decomposition (Duddleston et al., 2002; M. E. Hines et al., 2001). Thirdly, changing temperatures can result in vegetation shifts that change the nature of organic matter inputs to the soil. Along a 40‐year progression of permafrost thaw, rates of potential CO2 and CH4 production were highest in the sites that had been thawed the longest, a difference mediated by the indirect role of temperature in changing vegetation assemblages and, therefore, the chemistry of organic matter inputs to the soil (Hodgkins et al., 2014). While cold temperatures contribute to wetland carbon preservation, the existence of tropical peatlands is strong evidence that temperature is not the only driver (Hodgkins et al., 2018).
3.4. GREENHOUSE GAS EMISSIONS AND OTHER LOSSES
Wetlands are fundamentally open ecosystems that exchange gases, dissolved compounds, and particulate matter with the atmosphere, surrounding terrestrial ecosystems, and aquatic environments. A simple mass balance perspective illustrates that whatever autochthonous and allochthonous carbon is exported from a wetland is, necessarily, not preserved within the wetland (Fig. 3.1). Management actions can manipulate the factors that cause carbon loss in order to reduce carbon export or change the form of exported carbon to a more climatically benign form.
3.4.1. Greenhouse Gas Emissions
Carbon Dioxide (CO2)
On a mass basis, CO2 almost always accounts for the majority of wetland greenhouse gas emissions. Growth and maintenance respiration by autotrophs produce CO2, with rates of autotrophic respiration typically returning ~40–50% of gross primary production to the atmosphere (Dai & Wiegert, 1996). The mineralization of dissolved and particulate organic carbon within wetland soils also produces CO2 that is emitted directly to the atmosphere or dissolved into wetland porewaters. Because CO2 is an end product of most terminal metabolic pathways, the same factors that enhance carbon preservation (Section 3.3.2) will tend to reduce rates of CO2 production, emission, and export.
Wetland CO2 emissions are affected by a variety of climate‐related disturbances. Drought increases soil O2 levels and can remove the enzymic latch that inhibits extracellular enzyme activities in moss‐dominated peatlands (Freeman, Ostle, et al., 2001) but not necessarily in tree/shrub‐dominated wetlands due to differences in the quantity and types of phenolic compounds produced by the different vegetation types (H. Wang et al., 2015). The drying and warming of wetland soils can stimulate root productivity, especially in shrubs (Malhotra et al., 2020). With increasing atmospheric CO2 levels, enhanced plant productivity and shifts in species composition (Caplan et al., 2015; Erickson et al., 2007) have the potential to prime the decomposition of soil carbon through inputs
