Initial Summary of Soil Carbon Stocks From the Rapid Assessment of Carbon Project.

The Rapid Assessment of Carbon (RaCA) project was undertaken to inventory soil carbon stocks to a depth of 1m across the conterminous United States. A multi-level stratified sampling scheme was used to spread the evaluation and workload across the country. The USDA-NRCS separates the country into major land resource area (MLRA) regional office areas (MOs).  For logistical reasons, the RaCA sampling was stratified by MO first. Within each MO, RaCA sites were selected based on soil groups and broad land cover/use categories. Soil groups were created from individual soil series properties and linked to SSURGO map units by dominant component.  Land cover/use was initially assigned based on National Resource Inventory and National Land Cover Dataset information. Both soil group and land cover/use were verified before sampling a site.  At each site, 5 pedons were sampled at the exact site location and 30m in each cardinal direction.  Pedons were sampled from 0-5cm and by horizon to 100cm.  Samples of known volume were taken for the upper 50cm so that bulk density could be calculated. Bulk/grab samples were taken from 50 – 100 cm and bulk density will be modeled with pedotransfer functions for those samples.  A visible-near infrared spectrometer was used to predict soil organic and inorganic carbon content on all samples.  A subset of 3% of pedons will have laboratory analysis done for quality control and error estimation.  This study was designed to give an average carbon stock for individual MOs and across the U.S.  The results can also be used to examine average differences in land cover/use carbon stocks across a wide variety of soil conditions.