Bolinder, M. A., Andren, O., Katterer, T. and Parent, L.-E. 2008. Soil organic carbon sequestration potential for Canadian agricultural ecoregions calculated using the Introductory Carbon Balance Model. Call. J. Soil Sci. 88: 451-460. The potential for storage of atmospheric CO2-C as soil organic C (SOC) in agroecosystems depends largely oil soil biological activity and the quantity and quality of annual C inputs to Soil. In this Study we used the Introductory Carbon Balance Model (ICBM) approach driven by daily standard weather station data, specific soil properties and crop characteristics at the scale of Canadian agricultural ecoregions. The objectives were to calculate a climate-dependent soil biological activity parameter representative for annual agricultural crop production systems (r(e_crop)) and to estimate the effect of fallow (r(e_fallow)). These parameters are based on the daily product of soil temperature and stored water that influence biological activity in the arable layer, and are used to adjust the decomposition rates of the ICBM SOC pools. We also tested r(e_crop) and r(e_fallow) on SOC stock change data for different site and treatment combinations from long-term field experiments located in some of the ecoregions. An r(e_crop) value of 0.95 for western ecoregions was on average 0.23 units lower than that of the eastern ecoregions, indicating a lower decomposition rate of SOC. Although the estimated annual C inputs to soil for small-grain cereals were on average approximate to 7.5% higher in the eastern ecoregions (305 vs. 285 g C m(-2) yr(-1)), the overall results suggest that the western ecoregions would have a greater potential to maintain high SOC levels in the long term. However, these parameters varied between ecoregions and, consequently, the SOC sequestration potential was not always higher for the western ecoregions. The effect of fallow was on average approximate to 0.04, i.e., SOC decomposed slightly faster under fallow. Predictions for 24 out of 33 site and treatment combinations across Canada were significantly improved (P=0.003), compared with a previous application with the ICBM that did not differentiate between crops and fallow. The methodology used here enabled us to examine regional differences in the potential for SOC sequestration as a balance between annual C inputs to soil and soil biological activity.