Quantity and Spatial Variability of Soil Carbon in the Conterminous United States Yinyan Guo, Ronald Amundson,* Peng Gong, and Qian Yu ABSTRACT We estimated the soil organic carbon (SOC) and soil inorganic car- bon (SIC) inventory for the conterminous USA using the State Soil Geographic Database (STATSGO). The relative contribution of each soil order and Land Resource Region (LRR) to the national SOC and SIC inventory was determined. There are 302 to 1499 310 8 Mg of SOC and 226 to 937 3 10 8 Mg of SIC in the upper 2 m of soil in the conterminous USA. About 30 and 80% of the upper 2-m SOC is in the 0- to 0.20- and 0- to 1.0-m soil layers, respectively. For SIC, only about 8% of the upper 2-m SIC is in the upper 0.2 m, and about 50% is in the top 1.0-m layer. The relative spatial variability of SOC increases dramatically as soil depth increases while the largest rela- tive variability of SIC is in the surface layer. Because of its large area (27% of the soil area in the conterminous USA), Mollisols are the larg- est contributors both to the SOC stock (about 31 to 39%) and to the SIC stock (about 43 to 44%) in the conterminous USA. The results of this study provide a view of soil C partitioning by taxonomic group and land resource area, information that may be useful for assessing the impact of land use and climatic change on SOC and SIC pools. A N ACCURATE ASSESSMENT of U.S. soil C storage is needed as a baseline for evaluating the overall U.S. C budget and the impact of land cover/use change on the inventory. Numerous studies have estimated the U.S. SOC pools at local and regional scales (Franzmeier et al., 1985; Sims and Neilsen, 1986; Huntington et al., 1988; Davidson and Lefebvre, 1993; Davidson, 1995; Homann et al., 1998; Brejda et al., 2001; Galbraith et al., 2003). Kern (1994), using data from 3700 pedons, esti- mated that U.S. soils have between 62.1 and 84.5 Pg (Pg 5 10 15 g) of SOC in the upper 1.0 m. Bliss et al. (1995), using averages (midpoint values) from the STATSGO, estimated SOC storage (total soil profile) in 40 states. Lacelle et al. (2001) later generated a map of SOC in the upper 1.0 m of the soils of North America, with the U.S. portion calculated using the midpoint values from the STASTGO database. Soil inorganic C is also a large C pool. However, studies on SIC storage and content have only focused on local or regional assessments (Schlesinger, 1982; Grossman et al., 1995; Monger and Matrinez-Rios, 2000). Esti- mates of the SIC pools at a national or global scale have been more tentative than estimates of SOC pools (Lal et al., 1998b). Nonetheless, most of the SIC, which exists as carbonates, is believed to occur in soils of arid and semiarid regions (Grossman et al., 1995; Schlesinger, 1997; Lal et al., 1998b). Monger and Matrinez-Rios (2000) estimated the amount of soil carbonate in grazing lands of the USA by focusing on the woodlands, shrublands, and grasslands that occur within aridic, ustic, and xeric moisture regimes using random sampling for at least 25 sites per ecoregion. An estimation of SIC storage for the entire USA has not been made. The STATSGO database is not only amenable for exploring the national distribution of soil properties, but also for examining soil properties within LRR and among the taxa within Soil Taxonomy categories. At this time, no systematic studies of SOC and SIC parti- tioning by LRRs or soil orders at a national scale have been performed. The STATSGO database used in this study is a geo- graphic information system (GIS) based relational data- base compiled by the National Resources Conservation Service (NRCS), which was made by generalizing de- tailed soil survey data. The level of detail in STATSGO is based on its intended use for planning and manage- ment covering state, multi-state, and regional areas. Most importantly, it is the only soil database currently available for evaluating national soil resources (SCS, 1992; Reybold and Gale, 1989). The mapping scale for the STATSGO data is 1:250 000 (with the exception of Alaska) with a minimum mapping unit area of 6.25 km 2 , equivalent to a square cell of 2.5 by 2.5 km. The basic structure of STATSGO is the map unit and its compo- nents. Components are the finest horizontal entities (units) for data recording. A map unit may contain 1 to 21 com- ponents. In the conterminous USA (excluding water, urban land, bare rock, and other non-soil bodies), there are 10 441 STATSGO map units (74 590 polygons) and 111 247 components (regions within the map units). For each component, its area percentage (%) within the map unit, its soil classification (Soil Survey Staff, 1999), and its properties for each soil layer (‘‘O’’ horizon ex- cluded) are reported in a relational database format by experienced local soil scientists based on soil survey results. The purpose of this study is to calculate total SOC and SIC inventories, as well as the contents (e.g., con- centrations, kg m 22 ), within three depth intervals (0–0.2, 0–1.0, and 0–2.0 m) for the conterminous USA using the STATSGO database, and to examine the partitioning of SOC and SIC pools by natural land resource region and by soil orders in Soil Taxonomy. Our analysis of SOC expands previous STATSGO SOC analyses, and our analyses of SIC for the nation is, to our knowledge, an Division of Ecosystem Sciences and Center for Assessment and Moni- toring of Forest and Environmental Resources (CAMFER), 137 Mul- ford Hall, College of Natural Resources, Univ. of California, Berkeley, CA 94720-3114. Received 16 May 2005. *Corresponding author (earthy@ nature.berkeley.edu). Published in Soil Sci. Soc. Am. J. 70:590–600 (2006). Pedology, Soil & Water Management & Conservation doi:10.2136/sssaj2005.0162 ª Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: GIS, geographical information system; LRR, land re- source region; MRLA, major land resource areas; NRCS, Natural Re- sources Conservation Service; OM, organic matter; SIC, soil inorganic carbon; SOC, soil organic carbon; STATSGO, state soil geographi- cal database. Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. 590 Published online February 27, 2006