Abiotic soil properties are crucial in determining biologically important variables such as soil habitat and nutrient availability but also inherently stabilize carbon through clay surface area interactions, soil aggregation and biochemical recalcitrance. Soil C pools protected by these mechanisms may be inherently limited (or saturatable) while other pools such as unprotected C may be more influenced by biotic or climatic factors. Previous attempts to quantify soil C sequestration capacity has focused on silt and clay protection and largely ignored the effects of soil structural protection through aggregation and biochemical protection. Our objectives were to assess the mechanisms of C stabilization in measurable C pools by quantifying the relative explanatory power of two contrasting hypotheses: one with no saturation limit (i.e., linear model) and one with an explicit soil C saturation limit (i.e., C saturation model) and to determine values of fraction C saturation capacity when appropriate. We isolated soil fractions corresponding to the C pools; namely, free POM (non-protected), microaggregate-associated C (physically-protected), silt and clay associated C (chemically protected) and non-hydrolyzable C (biochemically-protected) pools from eight long-term agroecosystem experiments across the US and Canada. We found support for C saturation in the chemically-protected pool for most sites and in the biochemically-protected pools for some sites, while the microaggregate and non-protected C pools exhibited non-saturating dynamics. Soil C saturation behavior was observed in soils from a variety of taxonomies, textures and climates suggesting that the C saturation of chemically- and biochemically- protected soil pools may be generalized and influenced soil C accumulation even at sites that appear to be far from a theoretical C saturation limit.