Stable and radiocarbon isotopes were used to investigate the role of soil clay content in the storage and dynamics of soil carbon in tropical forest soils. Organic matter in clay-rich Oxisols and Ultisols contains at least two distinct components: (1) material with light δ13C signatures and turnover times of decades or less; and (2) clay-associated, 13C-enriched, carbon with turnover times of decades at the surface to millennia at depths >20 cm. Soil texture, in this case clay content, exerts a major control on the amount of slowly cycling carbon and therefore influences the storage and dynamics of carbon in tropical forest soils. Soils in primary tropical forest have been proposed as a potentially large sink for anthropogenic carbon. Comparison of carbon stocks in Oxisols sampled near Manaus, Brazil, shows no measurable change in organic carbon stocks over the past 20 years. Simple models estimating the response of soil carbon pools to a sustained 0.5% yr−1 increase in productivity result in C storage rates of 0.09 to 0.13 MgC ha−1 yr−1 in soil organic matter, with additional potential storage of 0.18 to 0.27 MgC ha−1 yr−1 in surface litter and roots. Most storage occurs in organic matter pools with turnover times less than a decade. Export of carbon in dissolved form from upland terra firme Oxisols likely accounts for <0.2 MgC ha−1 yr−1, but more work is required to assess the export potential for periodically inundated Spodosols.