Environmental controls on the stable carbon isotopic composition of soil organic carbon: implications for modelling the distribution of C-3 and C-4 plants, Australia

We use multivariate statistics to examine the continental-scale pattern, of the stable carbon isotopic composition (delta C-13) of soil organic carbon (SOC) from a data set collected throughout the natural range of variation in climatic, edaphic and biotic controls in Australia. Climate and soil texture (percent of mineral particles <63 mu m) are found to be the dominant controls on delta C-13(SOC). Of the environmental variables analysed, the strongest correlations to delta C-13(SOC) do not simply Occur with respect to mean annual temperature or precipitation, but rather to ecosystem-scale measures of water availability such as mean annual vapour pressure deficit (VPD) and an index of the annual flux of available water available to plants (M. After the variance of delta C-13(SOC) attributed to W was removed, the proportion of particles <= 63 mu m diameter remained the only secondarily significant correlation (p < 0.05). Based on this observation, we also develop a model of the primary climatic control on delta C-13(SOC), which is rooted in the assumption of optimized water-use efficiency of C-3 and C-4 vegetation. and can be extrapolated to continental or global data with readily available environmental data. The model describes optimized water-use efficiency controls on delta C-13(SOC) in terms of a function of the variable W. We estimate model parameters of climatic control on delta C-13(SOC) using an analysis of surface samples (0-5 cm) of sandy soils (<10% mineral particles <= 63 mu m diameter) from which other edaphic and biotic controls are minimized. This simple model function is modified to account for variation of delta C-13(SOC) due to variation of respiration rates and variable incorporation of the terrestrial Suess effect with mean annual temperature (MAT). Model regression of delta C-13(SOC) to these continental-scale climate data accounts for 92% of the variance observed using a model function with simple variables (W and MAT) and physically meaningful constants. We also examine edaphic controls on delta C-13(SOC) using particle size separates from soil textural gradients within four climatic zones of Australia. These data indicate the protection of C-13-enriched old, stable SOC in association with fine mineral particles, consistent with variable incorporation of the terrestrial Suess effect.