finding 12.1 : key-message-12-1

Estimates for soil carbon stocks in the conterminous United States plus Alaska range from 142 to 154 petagrams of carbon (Pg C) to 1 m in depth. Estimates for Canada average about 262 Pg C, but sampling is less extensive. Soil carbon for Mexico is calculated as 18 Pg C (1 m in depth), but there is some uncertainty in this value (medium confidence).



This finding is from chapter 12 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: The value range of soil carbon to a depth of 1 m for the United States is based on several compilations: Alaska is estimated in Mishra and Riley (2012) as 77 Pg C, an increase from the value reported by Bliss and Maursetter (2010) of 48 Pg. The sampling for the Mishra and Riley (2012) estimate is quite extensive, and land types for areal weighting are well known and documented. Modern estimates for the conterminous United States (CONUS) span the range from the U.S. Geological Survey (USGS) estimate of Sundquist et al. (2009) at 77 Pg C and the Rapid Carbon Assessment (RaCA, initiated by the Soil Science Division of the U.S. Department of Agriculture’s National Resources Conservation Service in 2010) estimate (Soil Survey and Loecke 2016) at 65 Pg C (see Table 12.1). The RaCA estimate is based on 144,833 soil samples and extrapolation using detailed soil maps. The soil carbon value of 9 Pg C for Mexico is based on Paz Pellat et al. (2016), but that estimate is based on sampling to a depth of only 30 cm. Based on conversion factors in Jobbágy and Jackson (2000), a conservative extrapolation to 1 m yields a value of 18 Pg C. The estimates for Canada are from Tarnocai (1997, 2006). This assessment recognizes that 1 m is a very arbitrary depth to consider; Batjes (1996) reported a 60% increase in the global soil organic carbon (SOC) budget when the second meter of soil was included.

New information and remaining uncertainties: There is medium high confidence in the estimates from CONUS due to new extensive and intensive sampling, although estimates for specific land-use classes still vary with different estimates. Confidence is relatively high for estimates in the agricultural areas of Canada but lower for forested areas. In Canada, uncertainty for the large peatlands areas in the boreal and Arctic regions is high due to low-sampling intensity and low-resolution mapping of peatland types. Uncertainty for estimates from Mexico are likely high due to low sampling coverage, and available data are only to a depth of 30 cm.

Assessment of confidence based on evidence: Soil carbon was extensively sampled in three independent studies for CONUS, so the confidence for the range of values reported here is very high. Due to the complex nature of estimating soil carbon in boreal and peat regions, the uncertainty is greater surrounding values for Canada. There is low confidence in values reported for Mexico as sampling is not as extensive and the depth of sampling is not as great.

Provenance
This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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