The stable carbon isotope composition of soil organic carbon and pedogenic carbonates along a bioclimatic gradient in the Palouse region, Washington State, USA

A horizon soil organic carbon (SOC) δ13C values varied from −24.3‰ to −25.9‰ PDB. Values in the arid portion of the gradient (200 to approximately 500 mm MAP) generally decreased and linear regression of SOM 13C vs. MAP was significant (r2=0.71, p=0.02). Trends in plant-13C of two grass species (Agropyron spicatum and Festuca idahoensis) found throughout this portion of the gradient were similar to that of SOC. Mean pedogenic carbonate δ13C values varied from −4.1‰ to −10.8‰ PDB. Linear regression was significant for carbonate 13C vs. MAP (r2=0.79, p=0.007), estimated above-ground productivity (r2=0.88, p=0.002) and soil carbon content (r2=0.83, p=0.004). Carbonate δ13C values at the most arid site exhibited higher variability than other sites (presumably due to greater spatial variation in plant respiration vs. atmospheric diffusion). Our data suggest that carbon isotopic relationships among ecosystem components may prove useful in determining ecosystem level properties in modern systems, and potentially in ancient systems as well.