Biotic and abiotic factors affecting the δ13C of soil respired CO2 in a Mediterranean oak woodland

The flux (Rs) and carbon isotopic composition (delta C-13(Rs)) of soil respired CO2 was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the delta C-13 composition of leaf, root and soil organic matter (delta C-13(SOM)) and metabolites. Simulations of R-s and delta C-13(Rs) were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of delta C-13(Rs) over the study period were within the range of root metabolite and average delta C-13(SOM) values, but enriched in C-13 relative to the bulk delta C-13 of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO2 fluxes and the underlying features of delta C-13(Rs). Observed diel variations of 0.5% in delta C-13(Rs) were predicted by the model in response to temperature-related shifts in production rates along a similar to 3 parts per thousand gradient observed in the profile of delta C-13(SOM). However, observed delta C-13(Rs) varied by over 2 parts per thousand, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of delta C-13(Rs).