This study examines the regional distribution of the stable isotopes of organic carbon in the surface soils (SOC) of a variety of biomes including forests, savannas, and grasslands. A transect through tropical/subtropical biomes in northern Australia demonstrates that forest and grassland soils exhibit comparatively small variations in &dgr;13C value on both local and regional scales. Savanna soil &dgr;13C values exhibit extreme variability at all spatial scales with samples separated by only a few meters differing by up to 6.6?, and a total range of values for savanna samples from -15.9 to -26.6?. Forest surface SOC has an average &dgr;13C value of -28.4¿0.7?(1&sgr;), while tropical grasslands (C4-dominated) have an average &dgr;13C value of -15.5¿0.8?(1&sgr;) and temperate grasslands (C3-dominated) -26.0¿1.1?(1&sgr;). Despite extreme variability between savanna samples, there is a consistent relationship between &dgr;13C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low &dgr;13C values and high SOC contents, and tropical grasslands with high &dgr;13C values and low SOC contents. The relationship suggests that an integrated regional &dgr;13C value for SOC is a useful proxy for terrestrial carbon storage. River sediment &dgr;13C values from the transect region reflect the &dgr;13C values obtained for the regional soils, with a bias toward the C3 end-member. Size-fractionated average soils from a variety of biomes suggest that little isotopic fractionation accompanies degradation but that in mixed C3/C4 biomes, C3-derived carbon is preferentially incorporated into the coarse size fractions, while C4-derived carbon is preferentially added to the fine size fractions.¿ 1997 American Geophysical Union