Controls of soil organic carbon (SOC) cycling in temperate forests of the western United States remain poorly understood. To address mechanisms of SOC cycling and stabilization, we examined the distribution of SOC in various density fractions of soils formed under ponderosa pine in Arizona. Soils were sampled from four parent materials (rhyolite, granite, basalt, and limestone) to capture a range of soil chemical and physical properties. The density fractions included a free light fraction (FLF) separated at 1.8 g cm-3, an occluded free light fraction (OCC) separated at 1.8 g cm-3 after sonication at 1500 J g soil-1, and the remaining dense or mineral fraction (MIN). Our results indicated that the relative proportion of SOC in each aggregate fraction varied by parent material, suggesting that soil physical and chemical properties, such as pH, clay content, and the soil mineral assemblage control the physical distribution of SOC. Furthermore, regression analysis suggested an aggregate stability control of SOC distribution as evidenced by significant relationships of SOC to free Fe-oxides and clay content among the various parent materials. Future work will quantify the mean residence time of SOC in each fraction within each parent material to further define the mechanisms controlling SOC cycling in these ecosystems.