Technical Abstract: Biochar is a high surface-area, variable-charge organic material that may improve nutrient retention and soil C sequestration but its general beneficial properties have yet to be quantified in many soil types. Biochar has the potential to increase soil water-holding capacity, cation exchange capacity (CEC) and surface sorption capacity which decreases the leaching losses of nitrate and ammonium, and reduce the emission of N2O. Despite these properties, the magnitude of soil benefits will depend on the size, quantity, and individual characteristics of both the biochar and the amended soil. In order to determine the relative contribution of biochar to: 1) nutrient retention and 2) soil C stabilization versus losses through soil respiration, we established a relatively long-term (3 years) laboratory experiment. We added two sizes (>250 and <250 'm) of C3-derived biochar to two C4-soils (sandy, silty-clay loam) with and without fertilizer addition, and measured over time C and N losses trough respiration and leaching, respectively. At occasional destructive harvests, the contribution of char to soil organic matter fractions, separated by size and density, is quantified, and the potential for biochar to contribute to long term soil C stabilization assessed. Stable C isotope mixing model is applied to partition SOC versus biochar C in the measured C pools. Data from this experiment will be reported and results discussed in the context of the potential for biochar to promote soil organic carbon sequestration.