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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.