and fluxes, and major controlling processes, including land use change and wildfire, in the conterminous United States (CONUS) and provide a brief outlook of ongoing science requirements for wetland carbon research. Understanding the land area covered by wetlands, the carbon stock in wetland soil, and the potential for emissions due to disturbances, helps build a more complete picture of CONUS wetlands and an understanding of best management practices.
2.2. WETLAND DISTRIBUTION, TYPES, AND CARBON STOCK IN THE UNITED STATES
Two inventory programs in the United States regularly map and track the nation’s wetlands. The U.S. Geological Survey’s NLCD (Jin et al., 2018) produces land cover maps of the entire country approximately every five years. Wetlands are mapped based on their aboveground vegetation type and thus classified as woody or herbaceous wetlands. The most recent release of NLCD (2016), with a 30 m × 30 m cell size, suggests 351,000 km2 of woody wetlands and 119,000 km2 of herbaceous wetlands, for a total of 471,000 km2, exist in the CONUS. The NWI program of USFWS produces detailed wetland maps with a focus on hydrological conditions (Dahl, 2011). In the most recent inventory in 2009, the NWI found 446,000 km2 of CONUS wetland area using stratified random sampling (Dahl, 2011). While these wetland spatial products are not independent, different estimates by the NLCD and NWI reflect differences in definitions, methodologies, and data sources. It is estimated that the state of Alaska contains an additional 110,000 km2 of wetlands (Homer et al., 2015). Alaskan and Hawaiian wetlands are not included in this chapter due to limited data availability compared to CONUS lands.
Wetland soils have also been characterized using data derived from U.S. Environmental Protection Agency (US EPA) National Wetland Condition Assessment (NWCA) and SSURGO data that corresponds to the location of a wetland in the NLCD 2011 database. SSURGO provides a series of maps that link to data tables, however the SOC data relies on bulk density values that can be taken from literature or expert opinion rather than laboratory measurements (Holmquist et al, 2018). The NWCA database is available as a collection of sample points determined to be representative using statistical analysis (Nahlik and Fennessy, 2016). Fig. 2.1 shows the distribution of percent organic carbon by weight and soil organic carbon density in the inland wetlands of the CONUS in the NWCA 2011 database, showing a peak at a lower carbon content that tapers off as the carbon content increases.
Figure 2.1 Distribution of percent organic carbon by weight (a) and soil organic carbon density (b) in NWCA 2011 inland wetland soils.
(Source; Based on National Wetland Condition Assessment 2011, U.S. Environmental Protection Agency.)
The following tables (Tables 2.1 and 2.2) show the soil organic carbon stock in CONUS inland soils from SSURGO divided by regions and vegetation cover. Vegetation cover is derived from NLCD 2011.
2.3. EFFECTS OF LAND USE CHANGE IN RECENT DECADES ON WETLAND CARBON
Four hundred years ago, prior to the extensive agricultural settlement in the U.S., there were approximately 894,000 km2 of wetlands in CONUS (Dahl, 1990). Approximately 53% of the total wetland area changed conditions as a result of draining for agriculture and other uses, with major conversions between the 1780s and 1980s (Mitsch & Gosselink, 2015; Bridgham et al., 2007). Wetland conversions slowed due to the adoption of the “no net loss” policy by the U.S. Federal Government in 1989, which required that damaged wetlands be replaced or “mitigated” with functionally similar wetlands (Dahl, 2011). Restoring, enhancing, or creating a new wetland is permissible to mitigate loss of a natural wetland or impact to aquatic resources (U.S. Environmental Protection Agency, 2019), although the long‐term sustainability and efficacy of these mitigation wetlands may differ from natural wetlands (Wolf et al., 2011).
Disturbed wetlands can release GHGs including carbon dioxide, methane, and nitrous oxide (Moomaw et al., 2018). It is important to recognize the range of ways a wetland may respond to disturbance when understanding the effect of land use change on wetlands and wetland carbon. Disturbance of anaerobic conditions, such as draining, can increase decomposition of organic soils and continue to promote the release of GHGs even if the wetland’s condition is restored (Neubauer & Verhoeven, 2019). Wetland emissions of carbon dioxide and methane vary based on ecosystem conditions, including the depth of the water table and disturbances in the areas around the wetland (Kolka et al., 2018). Generally, wetland carbon is affected by the balance between sequestration through plant growth and burial and release through microbial activity.
Table 2.1 This table uses CONUS inland wetland SOC stock data from SSURGO and tidal region SOC stock data from Holmquist et al. (2018)
| Region | Area km2 | Percent of CONUS Area | Mean SOC Stock kg/m2 | Total SOC Stock Tg |
|---|---|---|---|---|
| Coastal Plains | 191,000 | 44 | 17.6 | 3,360 |
| Eastern Mountains Upper Midwest | 132,000 | 30 | 40.0 | 5,267 |
| Interior Plains | 51,000 | 12 | 18.7 | 947 |
| Tidal | 40,000 | 9 | 27.0 | 1,080 |
| West | 24,000 | 5 | 11.5 | 272 |
| Inland Total | 398,000 | 91 | 24.7 | 9,846 |
| Total | 438,000 | 100 | 24.9 | 10,926 |
Data was selected from cells matching NLCD 2011 inland wetland classes or C‐CAP tidal wetland classes, with a tidal region boundary provided by Holmquist et al. (2018). SOC stock values are for the top 1 m of soil.
Table 2.2 Inland CONUS wetland carbon by vegetation cover for the top 1 m of soil, using data from SSURGO extracted based on wetland classes identified in NLCD 2011
| Class | Class Area km2 | Mean SOC Stock kg/m2 | Total SOC Stock Tg |
|---|---|---|---|
| Herbaceous Wetlands (Inland) | 79,000 | 28.30 | 2,236 |
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