2 Wetland Carbon in the United States: Conditions and Changes

       Bergit Uhran1, Zhiliang Zhu2, Lisamarie Windham-Myers3, Benjamin Sleeter4, Nancy Cavallaro5, Kevin D. Kroeger6, and Gyami Shrestha7

       1 U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, Virginia, USA

       2 U.S. Geological Survey, National Climate Adaptation Science Center, Reston, Virginia, USA

       3 U.S. Geological Survey, Water Resources Mission Area, Menlo Park, California, USA

       4 U.S. Geological Survey, Western Geographic Science Center, Tacoma, Washington, USA

       5 U.S. Geological Survey, National Center, Reston, Virginia, USA

       6 U.S. Geological Survey, Woods Hole Coastal and Marine Science Center, Woods Hole, Massachusetts, USA

       7 U.S. Global Change Research Program, U.S. Carbon Cycle Science Program Office, Washington, DC, USA

      ABSTRACT

      Wetlands in the United States are an important terrestrial carbon sink and are impacted by management decisions and land use change. This chapter presents general patterns of wetland carbon management and land use, and identifies the data needs and scientific investigations necessary to improve research in this important field. Background information in contemporary and historical land use change and land management decisions, such as changes to hydrology and wildfire, and effects on wetlands are discussed, along with the need to further investigate and create more precise maps of wetland location, type, and carbon stocks.

2.1. INTRODUCTION

      Wetlands are of special interest for carbon management and emissions mitigation due to the high potential for carbon sequestration in their hydric soils. Organic carbon storage in hydric soils, coupled with aboveground carbon stocks, poise wetlands (both woody and herbaceous) as among the most carbon‐rich ecosystems on Earth. Although they represent a small fraction of global land area, these wetlands contain a large fraction of global soil carbon stocks. While differences in carbon storage among wetland types vary, coastal and inland wetlands overall remain consistently higher in carbon stocks than upland ecosystems (Nahlik & Fennessy, 2016).

Wetland soil carbon stocks can be lost rapidly through direct and indirect disturbances. Emissions of carbon dioxide are the dominant pathway of stock loss, but erosion and lateral flux may be under appreciated (Kolka et al., 2018; Lauerwald et al., 2020). Accurate data on wetland condition and classification is necessary to describe vulnerability and potential carbon dioxide emissions through changes in wetland carbon dynamics and extent. Wetland soil pools are highly vulnerable to disturbance. The Intergovernmental Panel on Climate Change (IPCC) guidelines (Kennedy, 2014) document a wide range of management and land use actions that play roles in emissions of carbon dioxide from wetlands, including drainage, excavation, fires, grazing, and forestry. Peat from wetlands is often harvested for fuel and/or horticulture, and this process exposes vulnerable soils to degradation and enhanced carbon dioxide emissions. Additional management practices and research topics of importance for tidal wetlands include impoundments for waterfowl habitat, ditching and other actions related to mosquito control, and inadvertent tidal restrictions and impoundments due to transportation infrastructure. Further, other greenhouse gases (GHG), including methane and nitrous oxide, are emitted from saturated soils and influenced by hydrologic and chemical alterations including impoundment and nitrogen loading. Methane is of particular importance in understanding wetland carbon flux.

      Overall, emissions of methane gas from wetlands contribute to approximately one‐third of all global methane emissions (natural and anthropogenic) (Zhang et al., 2017). Natural methane emissions, including wetland emission, are the most important source of uncertainty in the methane budget (Saunois et al., 2020). Methane emissions may increase due to a wide range of effects including permafrost thaw in high‐latitude regions, increased precipitation, and temperature (which increases microbial activity in the soil), and increased atmospheric carbon dioxide (which affects water utilization in the ecosystem); these effects may be exacerbated with climate change, leading to higher rates of methane emissions (Zhang et al., 2017). Global annual methane emissions from wetlands are estimated by a range of climate models to increase from the present value of 172 Tg CH₄/yr and are expected to increase to between 222 Tg CH₄/yr and 338 Tg CH₄/yr by 2100 (Zhang et al., 2017). This highlights the importance of finding management solutions to minimize wetland methane emissions. Methane emissions from wetlands may be lower in some models due to reclassifications of wetland vs. inland water emissions (Saunois et al., 2020).

      Landcover, hydrology, soil type, degree of tidal influence, and level of disturbance can be used to describe wetland condition when tracking carbon dynamics. These factors can be used along with soil carbon stock estimates to account for carbon storage potential in wetland ecosystems. Land cover classifications for wetlands (e.g., National Land Cover Database [NLCD] or Coastal Change Analysis Program [C‐CAP]) typically include vegetation descriptions, such as woody vs. herbaceous plant cover (Jin et al., 2016; National Oceanic and Atmospheric Administration [NOAA], 2018). The U.S. Fish and Wildlife Service (USFWS) National Wetlands Inventory (NWI) maps wetlands with the addition of hydrologic and soil information (Cowardin et al., 1978), such as binary salinity classes of estuarine and palustrine soil classifications (e.g., mineral, organic, histic; Kolka et al., 2018). Such classifications are the most relevant to ecosystem carbon stock assessments, given the potential for high soil carbon densities (up to 0.2 g carbon per cubic centimeter of soil, e.g., Glaser et al., 2012) and deep soil profiles (to 10 m depth, e.g., Drexler et al., 2003) supported by hydrologic and soil information. Tidal hydrology is difficult to map but critical to characterizing carbon stabilization processes and vulnerability. Despite the importance of soil cores, wetlands (with hydric soil characteristics) are currently undersampled in the U.S. Department of Agriculture (USDA) Soil Survey Geographic database (SSURGO). Maps of wetlands classified as hydric (active and former wetlands) and hydric flooded/ponded (active wetlands) show lower acreages than that of NWI or NLCD (Sundquist et al., 2011). Soil carbon distribution of wetland components within the SSURGO database needs quality assessment to assure spatial and profile representation with accurate bulk density measurements that account for disturbances, other land change, effects of climate change, and the high spatial and temporal variability. It is necessary to show the impacts of land change on SOC stocks in order to understand the extent to which wetland disturbances may alter the rate of soil organic carbon (SOC) storage and flux.

      The objective of this chapter is to provide an overview of recent estimates