The Prairie Soil - Forest Soil Boundary.

What does it mean — soil types vary across the landscape? In what way do they vary and in response to what factors? At a glance, the 1973 image in Buol, Hole, and McCracken (Figure 11.3) helps answer these questions. The image illustrates a prairie soil with a thick organic-rich A horizon merging into an adjacent forest soil with leaf litter (O horizon) overlying an eluvial-illuvial (A-E-Bt) sequence. The image is qualitative and observational, but has served as the precursors to quantitative and mechanistic research showing that soil morphological differences are linked to the way organic carbon enters soil profiles. Carbon in the prairie soil is injection below the land surface by fibrous roots that have high turnover rates. In contrast, carbon in forest soil is mixed with the mineral horizons at the land surface after the carbon drops from trees as leaves, twigs, and limbs. If extended into a drier climate, the image can show the desert-prairie boundary. Here, not only is there a change in organic carbon, there is also a change in inorganic carbon (CaCO₃), which gets shallower and more plentiful as annual rainfall becomes less, as shown in yet another classic image (see Jenny and Leonard. 1934. Soil Sci 38:363-382).