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3 Biogeochemistry of Wetland Carbon Preservation and Flux
Scott C. Neubauer1 and J. Patrick Megonigal2
1 Department of Biology, Virginia Commonwealth University, Richmond, Virginia, USA
2 Smithsonian Environmental Research Center, Edgewater, Maryland, USA
ABSTRACT
The recognition that wetlands play an important role in regulating global climate has led to management actions intended to maintain and enhance the globally significant amounts of carbon preserved in wetland soils while minimizing greenhouse gas emissions. Our goal in this chapter is to review the biogeochemical processes that are relevant to wetland climate regulation, which we do by discussing: (1) the concepts of radiative balance and radiative forcing; (2) the mechanisms for wetland carbon preservation; (3) factors influencing greenhouse gas emissions and other carbon losses; and (4) opportunities for wetland management actions to influence carbon preservation and flux. Wetland carbon preservation, which reflects the accumulation of undecomposed organic material, is a function of the redox environment, organic matter characteristics, and physicochemical factors that inhibit decomposition. However, the conditions that favor carbon preservation often result in increased emissions of methane and nitrous oxide such that there is a biogeochemical tradeoff between carbon preservation and greenhouse gas emissions. The losses of carbon via gaseous and dissolved pathways are sensitive to environmental disturbances and raise challenges about fully accounting for the climatic impacts of wetlands. Wetland management and disturbance intentionally or unintentionally affect biogeochemical processes, such that wise environmental management offers opportunities to enhance wetland carbon preservation, prevent the destabilization of accumulated soil carbon, and reduce greenhouse gas emissions, thus maintaining the role of wetlands as regulators of global climate.
3.1. INTRODUCTION
Climate regulation by wetlands is an important ecosystem service that is increasingly a focus of management and restoration efforts (Erwin, 2009; Moomaw et al., 2018). The basis for these efforts is the observation that wetlands have accumulated globally significant amounts of organic carbon in their soils (Mitra et al., 2005), carbon (C) that is no longer in the atmosphere as the greenhouse gas carbon dioxide (CO2). It is becoming apparent that much of the organic carbon preserved in wetlands is not inherently resistant to decay but instead accumulates because its reactivity is reduced under the environmental conditions in wetland soils (e.g., Spivak et al., 2019). An important corollary of this understanding is that changes to the wetland environment (e.g., as initiated through management decisions or disturbances) could alter those conditions, thus destabilizing the large wetland carbon stores and allowing their export to adjacent aquatic systems as dissolved or particulate carbon or their return to the atmosphere as CO2 and methane (CH4). Further, some of the wetland conditions that promote carbon preservation also lead to the production of the greenhouse gases CH4 and nitrous oxide (N2O), the emissions of which can offset some or all of the climatic benefits provided by wetland carbon preservation.
Environmental management and other human actions, whether purposeful or accidental, can affect the pathways of carbon preservation and removal and therefore have the potential to alter the effects of wetlands on the global climate. In this chapter, we briefly summarize (1) the concepts of radiative balance and radiative forcing as ways of describing how ecosystems and management actions influence the