Soil organic matter (SOM) anchors global terrestrial productivity and food and fiber supply. To maintain and restore SOM, we need better understanding of soil organic carbon (SOC) saturation capacity and the retention of above- and belowground inputs in SOM. Our analysis suggests root inputs are ~5 times more likely to be stabilized as SOM than an equivalent mass of aboveground litter. Microbes, particularly fungi and bacteria, and soil faunal food webs strongly influence SOM decomposition at shallower depths, whereas mineral associations drive stabilization below ~30 cm. Global uncertainties in the amounts and locations of SOM include the extent of wetland, peatland, and permafrost systems and factors such as shallow bedrock constraining soil depths. Considering these uncertainties, global SOC stocks to 2- and 3-m depth are 2270 and 2770 Pg, respectively, but could be as much as 700 Pg smaller. Sedimentary deposits below 3 m likely contain >500 Pg of additional SOC.