Corn and wheat post-harvest biomass is usually tilled into the ground to improve organic matter in the soil and return nutrients to the soil, or fed to cows and other ruminants by grazing or by cutting.
In both cases you're burying a product of photosynthesis that would have been converted back to carbon dioxide through combustion or respiration. You're taking carbon out of the cycle. It doesn't matter whether the carbon was fixed millions of years ago or a couple of months ago. Carbon is carbon.