Spatial covariation of soil organic carbon, clay content, and drainage class at a regional scale

Several factors affecting stocks of soil organic-C have been identified, including climate, soil texture, and drainage, but how these factors and their influence vary spatially is not well documented. The State Soil Geographic Data Base (STATSGO) was used to estimate soil organic-C stocks of Montana and Kansas and to map spatial variation of soil properties. Regressions across map units of area-weighted estimates of soil organic-C, clay content, and drainage class show that clay content is positively correlated with organic-C in Kansas, bur that drainage class is a better indicator of soil with high and low organic-C stocks in Montana. About 85% of Kansas is covered by Mollisols. These grasslands of the North American Great Plains are where the paradigm relating clay content to stabilization of soil organic-C was developed. In contrast, clay content does not covary with soil organic-C across Montana. Only 30% of Montana is covered by Mollisols; the remainder includes rangeland, covered primarily by Aridisols and Entisols, and forests, covered by Inceptisols, Spodosols, and Histosols. Although other unidentified factors contribute to spatial variation in soil organic-C stocks in Montana, drainage class distinguishes the C-rich and the C-poor soils. When taken with similar results correlating soil C stocks with drainage class in a separate study of Maine, soil wetness emerges as an important controller of soil organic-C in northern states of the USA. Another objective was to compare STATSGO estimates (1:250,000 scale) of area covered by soil orders with estimates from the FAO/UNESCO Soils Map of the World (1:5,000,000). Agreement was excellent in Kansas and reasonably good in Montana. When used with regionally specific estimates for soil-C, the FAO map holds promise for regional and global extrapolation of soil C stocks.