![]() ![]() Extrapolating to this full target population requires the assumption that the unsampled area follows the same trends as the sampled area. The survey design, however, targeted a total of 38.4 million hectares, 13.4 million hectares of which (or 35%) could not be directly sampled primarily due to logistical difficulties 16. These sites, known as the inference population, represent 25 million hectares of wetlands in the conterminous United States and store a total of 7.54☐.59 PgC ( Table 2). Sites were selected from broadly defined NWCA Wetland Types ( Table 1) using a stratified-random, probabilistic sampling design 15, 16 (the Generalized Random Tessellation Stratified survey 17). To quantify carbon stocks (PgC), soil organic carbon concentration and bulk density data were collected by horizon from 120 cm-deep soil pits at 967 wetland sites across the conterminous United States ( Fig. Although we are unable to determine causality, our data also show that carbon stocks are significantly lower at wetland sites with most anthropogenic disturbance compared with sites with intermediate or least disturbance. Our data show that freshwater inland wetlands hold nearly 10-fold more carbon than the tidal saltwater sites that were assessed, in part due to the extensive area of inland wetlands compared with coastal sites-indicating their importance in regional carbon storage. Much of this carbon is stored within soil layers deeper than 30 cm and in freshwater inland wetlands-particularly those in the Midwest where wetlands with deep organic soils commonly occur in the northern tier states. We find that wetlands in the conterminous United States store a total of 11.52 PgC. These data provide empirical, unbiased, population-level estimates of soil carbon stocks with known confidence limits for targeted populations of wetlands at the national scale, and are not compiled based on the assumptions of a review of multiple sources, as earlier estimates have been (for example, ref. Here we provide a quantitative, robust estimate of wetland carbon storage in the conterminous United States as a function of soil depth, landscape position (inland versus tidal saline (that is, coastal)), and region, and an indication of how these stocks may be impacted by anthropogenic stressors using data from the US Environmental Protection Agency's (USEPA) 2011 National Wetland Condition Assessment (NWCA) 14. Accurate carbon accounting in wetlands is vital to reduce the risk of climate change contributions by identifying and protecting wetlands or wetland-dominated landscapes that hold disproportionately large carbon stocks, and to allow the inclusion of wetlands in carbon-offset programs, such as the United Nation’s programme Reducing Emissions from Deforestation and Forest Degradation (UN-REDD+) 13. Many studies have focused on quantifying the carbon held in terrestrial ecosystems (so-called green carbon) and, more recently, on the carbon held in tidal saline ecosystems, often referred to as blue carbon 10, 11, 12 however, our knowledge of carbon stored in inland freshwater wetlands, which we refer to here as teal carbon, is often overlooked or limited to site-specific studies. This is particularly true for freshwater inland wetlands that make up most of the wetland area comprising, for example, 95% of all wetlands in the conterminous United States 8, 9. In the United States, more than half of the historical wetland area has been lost due to anthropogenic activities 6 resulting in a net transfer of carbon from the soil to the atmosphere 7. As a result, wetlands can accumulate large carbon stores, making them an important sink for atmospheric carbon dioxide and holding up to or, in some cases, even more than 40% soil carbon 4, which is substantially greater than the 0.5–2% carbon commonly found in agricultural soils 5. ![]() The anoxic conditions characteristic of wetland soils slow decomposition and lead to the accumulation of organic matter. Wetlands contain a disproportionate amount of the earth’s total soil carbon holding between 20 and 30% of the estimated 1,500 Pg of global soil carbon 2 despite occupying 5–8% of its land surface 3. Soil carbon is vital in regulating climate, water supplies and biodiversity-all essential contributions to the provision of ecosystem services 1. ![]()
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