2019). Similarly, differences in the abundance of Fe(III) explain why rates of Fe(III) reduction are trivial in peatland soils (Keller & Bridgham, 2007) but can account for the majority of anaerobic carbon turnover in soils with more mineral matter (Kostka et al., 2002; Neubauer, Givler et al., 2005; Roden & Wetzel, 1996; Yao et al., 1999). Likewise, SO42– limitation causes methanogenesis to be more important than SO42– reduction in freshwater wetlands (Neubauer, Givler et al., 2005; Weston et al., 2014), with the relative importance of these processes switching in brackish and saline wetlands as SO42– availability increases (Poffenbarger et al., 2011).
The resupply and regeneration of electron acceptors is necessary to maintain rates of soil metabolism. Soluble electron acceptors such as NO3– and SO42– can diffuse into the anaerobic zone or be resupplied by the advective movement of water through the soil. In contrast, solid‐phase electron acceptors such as Mn(III, IV) oxides and Fe(III) (oxyhydr)oxides are effectively regenerated in situ at aerobic–anaerobic interfaces, including the rhizosphere and walls of infaunal burrows (Gribsholt et al., 2003; Luo et al., 2018; Neubauer, Givler et al., 2005) or where moving water delivers O2 to subsurface soils (Roychoudhury et al., 2003). Lastly, electron acceptors including Fe(III) and SO42– can be regenerated in wetlands through anaerobic chemoautotrophic reactions where the oxidation of Fe2+ is coupled with the reduction of NO3– (Straub et al., 2001) and the oxidation of reduced sulfur compounds proceeds with NO3–, MnO2, or Fe(III) serving as the oxidant (Schippers & Jørgensen, 2002). The contribution of these chemoautotrophic reactions to anaerobic carbon cycling is largely unknown (Burgin & Hamilton, 2008; Carey & Taillefert, 2005; but see Schippers & Jørgensen, 2002).
The supply of electron donors is as important as the resupply/regeneration of electron acceptors in regulating anaerobic metabolism. The energetic potential of an electron donor (that is, its ability to give up electrons to an electron acceptor) can be summarized in thermodynamic concepts such as the nominal oxidation state of carbon (NOSC: LaRowe & Van Cappellen, 2011) and the oxidation state of organic carbon (Cox; Masiello et al., 2008). For uncharged molecules, the difference between NOSC and Cox values is negligible (Hockaday et al., 2009) and we treat these terms as synonymous. Thermodynamic calculations and experimental culture work indicate that aerobic microbes can use a wide range of organic carbon molecules as electron donors, but anaerobic decomposers can use fewer substrates due to thermodynamic limitations (Keiluweit et al., 2016; LaRowe & Van Cappellen, 2011). As a group, NO3– and metal reducers can use many organic molecules as electron donors, including amino acids, short‐ and long‐chain fatty acids, some aromatic compounds, the monomers (e.g., glucose) resulting from extracellular enzymatic hydrolysis of polymers, and products of fermentation such as H2, acetate, lactate, and pyruvate (Küsel et al., 1999; Megonigal et al., 2004; Reddy & DeLaune, 2008). Sulfate reducers are able to use many of the same electron donors (Christensen, 1984; Parkes et al., 1989; Sørensen et al., 1981), but some cannot use glucose and other monomers and thus are largely dependent on the activities of fermenters for electron donors (Reddy & DeLaune, 2008). The denitrifiers, metal reducers, and SO42– reducers can oxidize electron donors all the way to CO2 (or to H2O, when H2 is the electron donor) or they can ferment larger molecules to acetate (Megonigal et al., 2004; Reddy & DeLaune, 2008). The thermodynamics of using CO2 as an electron acceptor means that methanogens can use the smallest number of electron donors. Hydrogenotrophic methanogens use H2 (and sometimes formate) as the electron donor while acetoclastic methanogens use acetate as both electron acceptor and electron donor, with some also able to use methanol, methylated amines, and methylated sulfur compounds (Bridgham et al., 2013).
Decomposer communities.
The redox environment affects the community of bacterial and fungal decomposers, with fungal activity being greatly reduced under anaerobic conditions. Because fungi are capable of degrading poorly reactive molecules such as lignin and cellulose (Thormann, 2006), the suppression of fungal activity in anaerobic soils likely enhances carbon preservation. Fungi are often more important than bacteria in the initial decomposition of litter from wetland and riparian plants (Hieber & Gessner, 2002; Kuehn et al., 2000; Verma et al., 2003). Fungal abundances decline with depth from surface litter layers to wetland soil horizons, reflecting the lower O2 availability in the soil (Ipsilantis & Sylvia, 2007). Fungal abundances in bulk anaerobic soil can be orders of magnitude lower than those of bacteria and archaea (Dang et al., 2019). Given their redox sensitivity, it is perhaps not surprising that fungal community composition, extracellular enzyme activities, and soil respiration rates respond to water level changes (Jassey et al., 2018). However, fungi may be able to transport O2 into anaerobic soils, facilitating their own aerobic metabolism (Padgett & Celio, 1990), and obligately anaerobic fungi have been found in the deep biosphere and the guts of ruminants (H. Drake & Ivarsson, 2018 and references therein), raising questions about the true role of fungi as decomposers in anaerobic wetland soils.
Organic Matter Characteristics
The chemical composition and structure of organic molecules influences their reactivity and ultimate fate (mineralization vs. preservation) in wetlands. Organic matter has often been referred to as recalcitrant, meaning highly resistant to degradation, or labile, meaning highly susceptible to degradation. However, the reactivity of organic matter depends on the chemical composition of the organic molecule itself and the physicochemical environment. Therefore, we will avoid using the terms recalcitrant and labile and will instead talk about the reactivity of molecules, with the recognition that reactivity can vary between different environments (after LaRowe et al., 2020).
The chemistry of wetland organic matter depends, in part, on its source. For example, lignin makes up ~15–30% of woody tissue biomass, <10% of the biomass of vascular plants, and is absent in mosses (Benner et al., 1987; van Breemen, 1995). The concentration of the phenol sphagnum acid, which is only found in Sphagnum mosses, varies by an order of magnitude between different species (Rudolph & Samland, 1985). Phytoplankton and benthic microalgae have lower concentrations of structural carbohydrates (e.g., cellulose) than herbaceous or woody plants and, therefore, have lower ratios of carbon to nitrogen (N) (Sterner & Elser, 2002). Differences such as these can influence the preservation of various autochthonous and allochthonous carbon inputs.
Carbon quality.
Major classes of organic matter include carbohydrates, proteins and amino acids, lipids, lignin, and tannins. The reactivity (or “quality”) of organic carbon varies as a function of factors including its elemental stoichiometry, bond structure, and the degree of oxidation (e.g., NOSC or Cox). For example, lignin and tannins are phenolic compounds that contain aromatic ring structures that are difficult to cleave. Proteins and amino acids are rich in nitrogen whereas carbohydrates lack nitrogen entirely. Carbohydrates range from simple sugars (e.g., glucose) to large polysaccharides (e.g., cellulose, hemicellulose). Lipids are partially or completely hydrophobic and can have linear, branching, and ring structures.
Table 3.3 Nominal oxidation state of carbon for major classes of organic matter
| Compound | NOSC |
|---|---|
| CO2 | + 4 |
| tannins | + 0.64 |
| carbohydrates | + 0.03 |
| lignin | – 0.27 |
| protein | – 0.82 |