Variations of delta C-13 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 delta(13)C value on both local and regional scales. Savanna soil delta(13)C values exhibit extreme variability at all spatial scales with samples separated by only a few meters differing by up to 6.6 parts per thousand, and a total range of values for savanna samples from -15.9 to -26.6 parts per thousand. Forest surface SOC has an average delta(13)C value of -28.4+/-0.7 parts per thousand (1 sigma), while tropical grasslands (C-4-dominated) have an average delta(13)C value of -15.5+/-0.8 parts per thousand (lo) and temperate grasslands (C-4-dominated) -26.0+/-1.1 parts per thousand(1 sigma). Despite extreme variability between savanna samples, there is a consistent relationship between delta(13)C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low delta(13)C values and high SOC contents, and tropical grasslands with high delta(13)C values and low SOC contents. The relationship suggests that an integrated regional delta(13)C value for SOC is a useful proxy for terrestrial carbon storage. River sediment delta(13)C values from the transect region reflect the delta(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.