Carbon partitioning and rhizodeposition by corn (Zea mays L.) and bromegrass (Bromus inermis Leyss.) were evaluated after 1 and 2 yr of cropping. 14C-labelled CO2 was used as a tracer of assimilated C. Most of the 14C assimilated by corn was translocated to the aboveground plant (nearly 90%), whereas the amounts located above and below ground in bromegrasss were approximately 60 and 40%, respectively. The total amount of rhizodeposition by bromegrass was twice that of corn after both 1 and 2 yr of cropping. A model was developed to predict the amount of carbon input by the below-ground systems of both crops on a field scale. The model indicates that the annual carbon rhizodeposition by bromegrass would be nearly five times higher than the below-ground carbon input under corn. Key words: Carbon partitioning, rhizodeposition, soil structure, root exudates