1-Abstract. Carbon budgets in soils can be computed by a simple equation in which inputs are accounted by the humifi cation of residues added to the soil, and outputs are estimated by the respiration of soil organic carbon (C S). It is generally accepted that physical protection in soil aggregates and chemical recalcitrance limit the rate of C S decomposition. We propose that soybean-based crop sequences will have lower steady state C S stocks than those based on corn, because of lower C inputs

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... and enhanced soil respiration of C S due the so-called priming effect. To address this hypothesis, we studied C S turnover from 13 C natural abundance changes in no-till corn and soybean crops. Soybean and corn crops were sown with a complete randomized design with three replicates on an old pasture with an intermediate soil 13 C signal of-16.34. Soils were sampled at the beginning and after two years of continuous no till cropping. Aboveground and belowground C inputs were measured each year for corn and soybean crops. Soil and plant samples were analyzed for C, nitrogen and 13 C/ 12 C ratio. Corn C inputs where signifi cantly higher than soybean (10.7 and 7.0 Mg ha-1), but no signifi cant changes in C S where observed among treatments (68 Mg ha-1 in the fi rst 0.3 m). However, the Cs turnover and humifi cation rates under corn (0.015 and 0.18 y-1) were higher than under soybean (0.006 and 0.11 y-1