Understanding the dynamics of long-term soil organic C (SOC) stabilization is crucial to accurate prediction of the size and duration of soil C sequestration. Currently, estimates of C sequestration potential are made without any explicit limit to soil C storage. However, the C saturation concept hypothesizes an upper limit to the amount of C capable of being stabilized; independent of the rate of C addition. SOC sequestration, occurs through (micro)-aggregation, intimate association with silt and clay particles, and biochemical stabilization through the formation of recalcitrant SOC compounds. By combining size, density and chemical fractionation methods within one fractionation scheme, we isolated four soil C pools that contribute to stabilization of soil C; 1) a microaggregate-protected C pool, 2) a silt- and clay-protected C pool, 3), a biochemically protected C pool and 4) an unprotected C pool. To examine the soil C saturation concept, we incubated soils from six agricultural sites that are close to (i.e. A-horizon) or far from (i.e. C-horizon) saturation with low and high addition rates of ¹³C-labeled wheat straw. After 2.5 years, the microaggregate and silt- and clay-protected fractions in five sites stabilized more added ¹³C in the low C soil (C-horizon) compared to the high C soil (A-horizon), indicating that the C-horizon was further from saturation the compared to the A-horizon. In the same five sites, the biochemically-protected fraction stabilized more C in the low compared to the high C soil, suggesting that the low C soil had a greater capacity to store added C in a highly stable form. This data corroborates C saturation behavior of microaggregate, silt-and clay-associated and biochemically protected C pools. Although these C stabilization dynamics are small in scale, the behavior of each soil C pool as it approaches saturation potentially influences local, regional, and landscape level soil C sequestration rates.