One of the dozen benefits from planting a cover crop like annual ryegrass is to sequester, or uptake, available nitrogen (N) in the soil. This is accomplished mostly by reducing the amount of N that leaches out of the field over winter and spring.
Annual ryegrass is among the most popular cover crops for a variety of reasons, including erosion-proofing your crop acres. Before that, it germinates easily and grows well in cool weather, whether planted in the fall after corn harvest or interseeded with corn in the spring. If planted in the fall, it maximizes root growth and N uptake before cold weather limits growth. If interseeded, it establishes among knee-high corn then goes dormant in the shade of a corn canopy, then goes to town after fall harvest.
Perhaps the biggest asset of annual ryegrass is the depth of its roots. In no-tilled fields after a few years to work its wonders, ryegrass roots can be found to depths of 4 and 5 feet, far below other cover crops. But even in new-to-cover-crop acres, ryegrass roots can easily sink to 3 feet over the winter, breaking up compaction on the way to accessing nutrients deeper in the profile.
But further savings can be realized when considering that annual ryegrass (and other cover crops) sequester available N in their leaves and roots. Then, once terminated in the spring (with glysophate), the cover crop residue composts in the field, releasing N just when the corn needs it most, in late spring and early summer. With a cover crop like this, you can reduce your input of N fertilizer by up to half, depending on other factors.