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

Isotopic signatures of soil components are commonly used to infer past ecologic and climatic shifts in the soil record. The theory behind the fractionation of isotopes that occurs during ecosystem processes is well understood; however, few isotopic studies have explored ecosystem relationships in modem soils. We discuss relationships of stable carbon isotopic signatures in plant tissue, soil organic carbon (SOC), laboratory-respired CO2, and modem carbonates at 10 sites (seven containing pedogenic carbonates) along a C-3-dominated climatic gradient (mean annual precipitation (MAP) ranging from 200 to 1000 mm) in the Palouse region of eastern Washington state. A horizon soil organic carbon (SOC) delta(13)C values varied from -24.3parts per thousand to -25.9parts per thousand PDB. Values in the and portion of the gradient (200 to approximately 500 nun MAP) generally decreased and linear regression of SOM C-13 VS. MAP was significant (r(2) = 0.71,p = 0.02). Trends in plant-C-13 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 delta(13)C values varied from -4.1parts per thousand to - 10.8parts per thousand PDB. Linear regression was significant for carbonate C-13 vs. MAP (r(2) = 0.79, p = 0.007), estimated above-ground productivity (r(2) = 0.88, p = 0.002) and soil carbon content (r(2) = 0.83, p = 0.004). Carbonate delta(13)C values at the most and 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 modem systems, and potentially in ancient systems as well. (C) 2004 Elsevier B.V. All rights reserved.