Читать книгу Wetland Carbon and Environmental Management - Группа авторов - Страница 65

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 to the soil of O₂ and/or highly reactive organic matter from enhanced root growth, inclusive of root exudates (Bernal et al., 2017; A. A. Wolf et al., 2007). In some peatlands exposed to elevated CO₂, the activity of the extracellular enzymes β‐glucosidase and phenol oxidase decreased (Fenner et al., 2007) or did not change (Kang et al., 2005), perhaps because reactive carbon was not limiting at those sites. Using elevation change as a proxy, elevated CO₂ enhanced belowground productivity and increased soil carbon storage in a brackish tidal marsh (Langley et al., 2009).

The intrusion of saline water into freshwater systems can affect wetland–atmosphere CO₂ exchanges. Net ecosystem production is often depressed by saltwater intrusion (Herbert et al., 2018; Neubauer, 2013) but can be unchanged in some years or in response to transient salinity increases (Herbert et al., 2018). The changes in net ecosystem production reflect salinity‐related declines in plant CO₂ fixation (Neubauer, 2013; Sutter et al., 2014) and variable heterotrophic respiration responses to increased salinity (Herbert et al., 2015). Changes in heterotrophic respiration could reflect a shift from methanogenesis to energetically favorable SO₄^2– reduction (Weston et al., 2011), reduced activity of extracellular enzymes (Jackson & Vallaire, 2009; Neubauer et al., 2013), or indirect effects that are mediated through soil organic matter availability and composition, microbial community structure, soil O₂ availability, and/or nutrient availability (Herbert et al., 2015; Tully et al., 2019).

Fire is an increasingly common feature in many wetlands, especially during drought or periods of seasonal water drawdown (Hope et al., 2005; Turetsky, Kane et al., 2011) and intentional land clearing activities (Marlier et al., 2015). Fire represents a pathway for the abiotic oxidation of wetland biomass and soil organic matter, generating emissions of CO₂ (and much smaller amounts of CH₄; Kuwata et al., 2016). Surface fires cause a short‐term burst of CO₂ emissions as surface vegetation and litter are burned but may promote a decrease in long‐term CO₂ emissions if thermally altered organic matter becomes more resistant to microbial decomposition (Flanagan et al., 2020). Smoldering fires can burn tens of centimeters of soil organic matter, converting hundreds to thousands of years of accumulated carbon back to CO₂ and significantly increasing global CO₂ emissions (Page et al., 2002; Turetsky et al., 2015; Turetsky, Donahue et al., 2011).