Technical Abstract: Cropping systems in the Northern Great Plains have shifted from fallow-based to legume-based systems. The introduction of grain legumes has impacted soil organisms, including nitrogen fixers, pathogens, mycorrhizae and fauna, and the processes that they perform. Legume seed exudates, rhizosphere exudates, and crop residues impact the soil biotic community. The benefits of legumes in rotation are in part due to the changes occurring in soil biology. The most widely studied legume-microbial association is dinitrogen fixation, which adds plant available nitrogen into the soil system. Many other associations also benefit from the legume in a cropping system. These include, but are not limited to, mycorrhizal associations that improve plant nutrient and water status, changes in the pathogen load and disease development and overall changes in the soil community. The microbial and faunal communities found on legume seeds and roots are different from those found on non-legumes. Residue and carbon quality characteristics of the legume also alter the soil microbial community. Legumes contribute to nitrous oxide emissions during nitrification and denitrification of fixed atmospheric N. Because less fertilizer N is used in legume-based cropping systems, overall greenhouse gas emissions are usually less than those in monoculture cereals. Inclusion of legumes in a cropping system can greatly alter the biotic component of an ecosystem and leads to mostly beneficial impacts.