Nope the soil is created by the grassland where only a small part of the biomass is aboveground. The fibrous roots of the grasses and other plants store the carbon, and more is added as dead plants and animals decay on the surface as well as underground. Weather does play a big part, however, as the lightning comes during seasons when conditions are apt to be dry. Up here that is early spring and mid fall. I'd bet trees would grow just fine in your blackbelt soils. The reason forest had not encroached into them long ago is almost certainly because of high fire frequency. North Dakota had less forested land than any other state, but the trees we plant flourish in the rich soils and often grow faster than in their original forest soils that are low in organic carbon. Native trees like aspen, jackpine, and pinyon juniper now dominate much former grassland. There are weather-related factors that also cause the loss of native grasslands, but I won't mention them here, as they are minor now that most native grasslands have long ago been converted to annual cropland.

Here is a link to the Komarek studies.
They spurred my 50-year interest in fire ecology.

http://talltimbers.org/wp-content/uploads/2014/03/Komarek1965_op.pdf