The total carbon (C_tot) concentration of soils is simply the sum of organic carbon (C_org) and inorganic carbon (C_inorg). Organic carbon is that portion of the total carbon in soils that is chemically incorporated into soil organic matter. Inorganic carbon, as quantified in this study is that portion of the total carbon in soils that occurs as a major component of the carbonate minerals calcite, dolomite, and aragonite. In this survey, C_org was not analyzed directly, but rather was calculated as the difference between C_tot and C_inorg. The two portions that make up C_tot can have very different qualities and very different modes of occurrence. Thus, the C_tot maps portray somewhat disparate information that reflects the combined factors that control the distribution of these two contrasting carbon forms.

Total carbon in this study was only determined for samples from the soil A and C horizons. The soil A horizon has a median concentration of 1.78 weight percent (wt. %) C_tot and the soil C horizon has a median concentration of 0.62 wt. % C_tot (see the summary statistics [open in new window]). The higher median concentration in the soil A horizon is caused primarily by the accumulation of organic matter in this near–surface sample material as compared to the deeper soil C horizon, which accumulates relatively little organic matter. The geochemical maps for C_tot illustrate the combined distribution of C_org and C_inorg. For example, the C_tot map for the soil A horizon reflects high C_org in forested areas in eastern New York, New Hampshire, Vermont, and Maine. High C_tot concentrations also occur in soils developed on carbonate–rich rocks in the Texas carbonate terrane and in the Everglades (USDA, 2006) of southern Florida. Thus, the factors controlling the distribution of C_tot are quite dissimilar. Those factors are discussed in more detail in the interpretations of C_org and C_inorg, and the reader is referred to those web pages for an explanation of the geochemical patterns related to soil carbon.