The effect of all land management practices on carbon (C) balances are under scrutiny, including the management of short-rotation woody biomass crops. Studies of invasion of old fields by forests, and limited information from short-rotation plantations,lead to a hypothesis that such plantations will increase soil C by 10-25 Mg ha(-1) over a 10- to 15-year rotation. Soil C under other land uses, such as row crops or pasture, has usually been reduced from native, undisturbed levels. Highly productive woody crops will add substantial C to soil, both above-and below-ground. In addition, within 2-3 years after plantation establishment, mulching by leaf litter and lack of cultivation will slow decomposition and further help retain C. We collected data from five hybrid poplar plantations in Minnesota, from 6 to 15 years old, and found no differences in soil C compared to adjacent row crops or hayland. A simple analysis of C balance indicated an initial decline and then an increase in soil C, consistent with literature reports. The analysis also indicated that structural roots are very important in plantation soil-C balance. Before definitive statements can be made about the C balance of short-rotation plantations, selected plantations must be followed through one or more rotations using standard protocols to facilitate intercomparisons.