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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
Woody Wetlands (Inland)	319,000	23.89	7,610

Draining wetland soils for agriculture, forestry, or development leads to emissions of a large quantity of carbon that had been sequestered in the soils for centuries (Kolka et al., 2018; Moomaw et al., 2018). Wood harvesting and wildland fires are additional examples of disturbances that release soil carbon from wetlands (Moomaw et al., 2018). Wetland soils can contain more than 40% organic carbon by weight, but agricultural soils typically have a soil organic carbon range from 0.5–2%, and disturbances are shown to significantly reduce wetland soil carbon even when a full land conversion has not taken place (Nahlik & Fennessy, 2016). This drastic difference between agricultural and wetland SOC density gives an indication of the magnitude of soil carbon that could be lost over time if wetland soils are converted to agricultural or other uses. Disturbances to wetlands and conversions in favor of other land uses cannot be easily undone because restored and created wetlands in their early years have significantly lower rates of biogeochemical functioning than reference wetlands (Moreno‐Mateos et al., 2012).

An analysis of CONUS wetland class change shows the approximate amount of sequestered soil carbon vulnerable to changes in land use. The loss of wetlands to agriculture and development is explored in this chapter through an analysis of Sleeter et al. (2018) on contemporary land use change. From 1974 to 2007, nearly 17,000 km² of CONUS land transferred into or out of an inland wetland classification, with a net loss of nearly 5,000 km² of inland CONUS wetland area.

Some of the changes documented by Sleeter et al. (2018) are due to changes in moisture availability. For example, droughts in the late 1980s and early 1990s followed by a rapid change from drought to wet conditions in 1993 (Huang et al., 2011) resulted in the wettest period seen in study area in North Dakota in 130 years (Winter & Rosenberry, 1998). Much of the change in wetland land cover may be due to natural wetland expansion and contraction due to changes in soil moisture. It is common for some wetlands, including prairie pothole wetlands, to expand during wet years and contract in dry years. These wetlands are linked by groundwater hydrology or aboveground flow, and may even be combined during wet periods. One such period was from 1993 to 1998, when record high ground and surface water levels were recorded due to high precipitation. These wet‐dry cycles (also called oscillatory fluctuations) are common and have been documented over thousands of years in prairie pothole wetlands through proxy data such as tree rings. These cycles can cause changes in species presence and abundance and occur over long periods of time, estimated to range from 4–35 years (Valk, 2005). Moisture trends may be caused in part by increased snowmelt and warmer air temperatures in the Prairie Pothole region (McKenna et al., 2017).

Wetland gains and losses in the last several decades (1974–2007) in the United States were not distributed uniformly. A small number of ecoregions represented a large amount of the total change, with rates of change over 100 km² per year in some areas (Sleeter et al., 2018). Some of these ecoregions are at a higher risk of pressure from agriculture and development, while others benefit from land changes and land use changes conducive to wetland development such as wetland restoration. Some of the larger changes documented are in the Southern Coastal Plain ecoregion, which lost the most wetland area during the years studied: 4,700 km² net loss. Of this, 4,600 km² were lost to development. The largest gains were in the Western Corn Belt Plains, which gained the most wetland area of any ecoregion. There was a gain of 880 km² from agricultural lands that became wetlands, however other wetland areas were lost to agricultural expansion or development, leaving a net gain of 750 km² of wetland area.

Table 2.3 Burned areas in wetlands are compared to burned areas in all CONUS landcover classes in select years

Year	Percent of CONUS Area Burned	Percent of Wetland Area Burned	Total Burned Area km2	Wetland Burned Area km2	Wetland Burned Area as Percent of Total Burned Area
1984	0.08	0.06	5,600	300	4.42
1990	0.23	0.44	15,700	1,900	12.24
2000	0.72	0.78	52,700	3,400	6.45
2011	0.85	1.0	62,000	4,400	7.03
2015	0.37	0.27	27,100	1,200	4.33

Burned areas were determined through MTBS.