Rayleigh distillation and the depth profile of 13C/12C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC(delta C-13(SOC)) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC (C) and delta C-13(SOC). Within 1 in of the soil surface, delta C-13(SOC) in the coarse-textured soil increases by 1.3 to 1.6 parts per thousand, while delta C-13(SOC) from the fine-textured soil increase by as much as 3.8 to 5.5 parts per thousand. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of delta C-13(SOC) in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of C-13 in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of delta C-13(SOC) in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios. Copyright (c) 2005 Elsevier Ltd