Spatial Patterns of Soil Organic Carbon in the Contiguous United States

Abstract Spatial patterns and total amounts of soil organic C (SOC) are important data for studies of soil productivity, soil hydraulic properties, and the cycling of C‐based greenhouse gases. This study evaluated several approaches for characterizing SOC to determine their relative merits. The first approach entailed grouping data from a global pedon SOC database by type of ecosystem, resulting in a total of 78.0 Pg of C (Pg = 10 15 g) to 1‐m depth for the contiguous USA. In a second approach, a pedon database was aggregated using soil taxonomy, resulting in a total for the contiguous USA of 80.7 ± 18.6 Pg of C when the great group SOC was spatially distributed with Major Land Resource Areas (MLRAs) using the 1982 National Resource Inventory (NRI) and the Soil Interpretation Record databases. The third approach used pedon and spatial data from a global soil map grouped by soil unit that resulted in 84.5 Pg of C for the contiguous USA. Although the ecosystem and soil taxonomic approaches resulted in similar totals, the taxonomic approaches are recommended because they gave more realistic results in areas of Histosols, shallow soils, and soils with high rock fragment content. The ecosystem approach did not give reliable spatial patterns and is only useful for very broad‐scale work where precisely georeferenced data are not needed. Grouping data by great group provided more information than grouping by order or suborder. The approach based on soil taxonomy is very useful because it is based on the NRI statistical framework and it allows stratification by other NRI items, such as land use and vegetation.