Variations of δ 13 C in the surface soil organic carbon pool

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 δ 13 C value on both local and regional scales. Savanna soil δ 13 C 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 δ 13 C value of −28.4±0.7‰ (1σ), while tropical grasslands (C 4 ‐dominated) have an average δ 13 C value of −15.5±0.8‰ (1σ) and temperate grasslands (C 3 ‐dominated) −26.0±l.l‰ (1σ). Despite extreme variability between savanna samples, there is a consistent relationship between δ 13 C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low δ 13 C values and high SOC contents, and tropical grasslands with high δ 13 C values and low SOC contents. The relationship suggests that an integrated regional δ 13 C value for SOC is a useful proxy for terrestrial carbon storage. River sediment δ 13 C values from the transect region reflect the δ 13 C values obtained for the regional soils, with a bias toward the C 3 end‐member. Size‐fractionated “average” soils from a variety of biomes suggest that little isotopic fractionation accompanies degradation but that in mixed C 3 /C 4 biomes, C 3 ‐derived carbon is preferentially incorporated into the coarse size fractions, while C 4 ‐derived carbon is preferentially added to the fine size fractions.