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The magnitude of carbon inputs to a wetland determines the maximum rate of carbon preservation in that wetland, although the actual rate will be considerably lower due to decomposition of organic carbon and losses of gaseous, dissolved, and particulate carbon from the wetland (Fig. 3.1). Carbon inputs can be autochthonous (originating within the system, e.g., CO₂ fixation by wetland plants) or allochthonous (originating from outside the system, e.g., inputs of sediment‐associated carbon and terrestrial detritus). The importance of autochthonous vs. allochthonous inputs varies from one wetland to the next. For example, carbon inputs to ombrotrophic bogs are dominated by autochthonous production by Sphagnum mosses and other plants. In contrast, the ratio of autochthonous to allochthonous carbon inputs can be very different in tidal marshes and other wetlands that are regularly flooded by sediment‐laden waters (e.g., Megonigal & Neubauer, 2019). In order to increase the rate of carbon preservation in a wetland, one could increase the inputs of poorly reactive organic matter to the wetland and/or change the environment to increase the carbon preservation efficiency. Note that changing the inputs of highly reactive organic matter or altering its rate of turnover does not directly affect the long‐term rate of carbon preservation because highly reactive organic matter, by definition, is not preserved. However, inputs of highly reactive organic matter can enhance the decomposition of poorly reactive organic matter through priming effects (Bernal et al., 2017; Mueller, Jensen et al., 2016) and the decomposition process itself can change highly reactive organic matter into material with lower reactivity (Baldock et al., 2004; Jiao et al., 2010). Finally, spatiotemporal changes to the wetland environment can alter the reactivity of organic matter (see Organic Matter Characteristics in Section 3.3.2).

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.