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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.