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

We measured seasonal and interannual variations in delta(13)C 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 (delta(13)C(R)) and the soil levels (delta(13)C(R-soil)), but only limited variations in the carbon isotope ratios of carbon stocks. The delta(13)C(R) values varied by as much as 4.4parts per thousand over a growing season, while delta(13)C(R-soil) values changed as much as 6.2parts per thousand. The delta(13)C of soil organic carbon (delta(13)C(SOC)) and needle organic carbon (delta(13)C(P)) exhibited little or no significant changes over the course of this study. Carbon isotope discrimination within leaves (Delta(p)) showed systematic decreases with increased canopy height, but remained fairly constant throughout the year (Delta(p)=17.9parts per thousand-19.2parts per thousand at the top of the canopy, Delta(p)=19.6parts per thousand-20.9parts per thousand at mid-canopy, Delta(p)=23.3parts per thousand-25.1parts per thousand at the canopy base). The temporal variation in the delta(13)C 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 C-13-enriched values. The dynamic seasonal changes in delta C-13 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 delta C-13(R) is that inversion-based carbon-cycle models dependent on observed atmospheric CO2 concentration and isotope values may be improved by incorporating dynamic delta C-13(R) values to interpret regional carbon sink strength.