In-situ carbon isotopes of soil CO2 in a subalpine forest over three summers

Carbon isotope ratios of CO2 are useful to examine aspects of biosphere-atmosphere gas exchange, ecosystem physiological processes, decomposition, and other processes. Most investigations focused on biosphere-atmosphere exchange have focused on isotope ratios within forest air, and not within the soil. We investigated aspects of belowground carbon cycling using continuous measurements of natural abundance stable isotopes in a high-elevation subalpine forest in Colorado, U.S.A (the Niwot Ridge AmeriFlux forest). Vertical profiles of CO2 and δ13C of CO2 were measured within and above the soil (from -35 cm depth to + 120 cm above the soil surface) over the summers of 2007-2009. Measurements were made of a full profile every 2-3 hours using a tunable diode laser absorption spectrometer. CO2 within the soil profile varied with depth and with time, over a range from near-ambient to above 5000 ppm. δ13C of soil CO2 also varied strongly with depth and time and ranged from -8 to -22 permil. Both parameters varied on a diel basis, and changed rapidly in response to rain events. The responses to temperature and moisture were not identical for CO2 and δ13C, and varied with depth. Isotope variation was largely decoupled from CO2 variation when CO2 was above 2500 ppm (at the deepest sampling locations). The isotope ratio of the soil surface flux was inferred from mixing relationships and assumptions of diffusional isotope fractionation. δ13C of the surface flux was generally in the range -27 to -28.5 permil, which is somewhat more negative than bulk values for litter, roots, and soil organic matter. There was evidence for diel variability in δ13C of the surface flux, but this was not always observed.