Temporal variation in δ13C of ecosystem respiration in the Pacific Northwest: links to moisture stress

We measured seasonal and interannual variations in δ13C values within the carbon reservoirs (leaves and soil) and CO2 fluxes (soil and ecosystem respired CO2) of an old growth coniferous forest in the Pacific Northwest USA with relation to local meteorological conditions. There were significant intra-annual and interannual differences in the carbon isotope ratios of CO2 respired at both the ecosystem (δ13CR) and the soil levels (δ13CR-soil), but only limited variations in the carbon isotope ratios of carbon stocks. The δ13CR values varied by as much as 4.4‰ over a growing season, while δ13CR-soil values changed as much as 6.2‰. The δ13C of soil organic carbon (δ13CSOC) and needle organic carbon (δ13CP) exhibited little or no significant changes over the course of this study. Carbon isotope discrimination within leaves (Δp) showed systematic decreases with increased canopy height, but remained fairly constant throughout the year (Δp=17.9‰–19.2‰ at the top of the canopy, Δp=19.6‰–20.9‰ at mid-canopy, Δp=23.3‰–25.1‰ at the canopy base). The temporal variation in the δ13C of soil and ecosystem respired CO2 was correlated (r=0.93, P<0.001) with soil moisture levels, with dry summer months having the most 13C-enriched values. The dynamic seasonal changes in δ13C of respired CO2 are hypothesized to be the result of fast cycling of recently fixed carbon back to the atmosphere. One scaling consequence of the seasonal and interannual variations in δ13CR is that inversion-based carbon-cycle models dependent on observed atmospheric CO2 concentration and isotope values may be improved by incorporating dynamic δ13CR values to interpret regional carbon sink strength.