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 |
| lipids | – 1.34 |
| CH4 | – 4 |
Average NOSC values for organic matter in sulfidic floodplain sediments are from Figure S4 in Boye et al. (2017). The NOSC values for other systems and sites will vary depending on the identity of the specific molecules that make up each broad class of organic matter. Values for CO2 and CH4 were calculated following LaRowe and Van Cappellen (2011).
These compound classes differ in their potential thermodynamic energy yield, as indicated by their NOSC (Table 3.3). The degree of organic matter oxidation and the physicochemical environment control which molecules are energetically available for microbial degradation and which are preserved (Boye et al., 2017; Pracht et al., 2018). Even molecules with a high potential energy yield can be preserved if they contain bonds that are difficult to cleave (e.g., those in phenolic rings) or if environmental conditions inhibit the activities of extracellular enzymes of the microbial decomposer consortium. This helps explain why, for example, there can be high concentrations of tannins in wetlands and surrounding “blackwater” aquatic systems, even though tannins have the highest NOSC of the major compound classes (Table 3.3). Similarly, the persistence of tropical peats despite warm temperatures is related to high concentrations of aromatic compounds (including phenolics) in low latitude peatlands (Hodgkins et al., 2018).
As organic carbon undergoes decomposition in wetlands, different molecules are preferentially mineralized or preserved, leading to changes in the composition of soil organic matter. The carbon in leaves, stems, and roots of herbaceous plants is more oxidized (higher NOSC) than that in woody plants, which is consistent with higher rates of decay of non‐woody biomass (Randerson et al., 2006). Leaves with higher lignin concentrations decay more slowly than those with less lignin (Day, 1982; J. Hines et al., 2014). During decomposition, cellulose and hemicellulose decay faster than does lignin, as would be predicted by their NOSC values, and leads to changes in organic matter chemistry over time in both litter and soil (Baldock et al., 2004; Benner et al., 1987; Worrall et al., 2017).
The transformation of organic compounds during the decomposition process creates a large pool of soil organic matter of altered reactivity in a process called humification. There is debate as to whether humification generates an amalgamation of small, poorly characterized compounds (Sutton & Sposito, 2005), the synthesis of complex macromolecules with a higher molecular weight than the starting compounds (De Nobili et al., 2020), or if the entire idea of humification should be abandoned entirely (Lehmann & Kleber, 2015). Regardless, it is clear that the chemistry of soil organic matter does change during decomposition. For example, organic matter in deeper peats from bogs, fens, and swamps was more decomposed and less oxidized (lower NOSC) than surface peat, with most of the change happening within the top 50 cm (roughly the last 200 years) (T. R. Moore et al., 2018).
Nutrient availability.
The carbon:nutrient ratio of plants is generally larger than that of soil bacteria and fungi, indicating an imbalance between the supply and demand for nutrients during decomposition (Hessen et al., 2004; Sterner & Elser, 2002). Indeed, litter decomposition studies often show an increase in nutrient concentrations over time, reflecting microbial immobilization of nutrients from the environment (e.g., Conner & Day, 1991). Litter decomposition is sensitive to nutrient availability in plant litter (Enríquez et al., 1993; Webster & Benfield, 1986) and/or the environment (Rejmánková & Houdková, 2006; Song et al., 2011). The degradation of plant litter can be limited by nitrogen availability, as indicated by negative correlations between litter C:N ratios and rates of decomposition (Keuskamp et al., 2015; Lee & Bukaveckas, 2002; Neely & Davis, 1985; Song et al., 2011). A similar pattern is seen with phosphorus (P), where higher litter phosphorus levels can lead to higher decomposition rates (J. Hines et al., 2014). The decomposition of leaf
