Variations of θ13C in the surface soil organic carbon pool

Abstract

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 θ13C value on both local and regional scales. Savanna soil θ13C 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 θ13C value of -28.4±0.7‰ (1σ), while tropical grasslands (C4-dominated) have an average θ13C value of -15.5±0.8‰ (1σ) and temperate grasslands (C3-dominated) -26.0±1.l‰ (1σ). Despite extreme variability between savanna samples, there is a consistent relationship between θ13C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low θ13C values and high SOC contents, and tropical grasslands with high θ13C values and low SOC contents. The relationship suggests that an integrated regional θ13C value for SOC is a useful proxy for terrestrial carbon storage. River sediment θ13C values from the transect region reflect the θ13C 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.