Ecological processes dominate the 13C land disequilibrium in a Rocky Mountain subalpine forest

Fossil fuel combustion has increased atmospheric CO2 by approximate to 115 mu mol mol(-1) since 1750 and decreased its carbon isotope composition (delta C-13) by 1.7-2 (the C-13 Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the delta C-13 of CO2 released by terrestrial ecosystems is expected to differ from the C-13 of CO2 assimilated by land plants during photosynthesis. This isotopic difference between land-atmosphere respiration (delta(R)) and photosynthetic assimilation (delta(A)) fluxes gives rise to the C-13 land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus, D is generally positive (delta(R)>delta(A)). A 7 year record of biosphere-atmosphere carbon exchange was used to evaluate the seasonality of delta(A) and delta(R), and the C-13 land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the delta C-13 of net land-atmosphere exchange during day and night and combined with tower-based flux observations to assess delta(A) and delta(R). The disequilibrium varied seasonally and when flux-weighted was opposite in sign than expected from the Suess effect (D= -0.75 +/- 0.21 parts per thousand or -0.880.10 parts per thousand depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Delta(canopy)) responding to environmental factors. Possible explanations for negative D include (1) changes in Delta(canopy) over decades as CO2 and temperature have risen, and/or (2) post-photosynthetic fractionation processes leading to sequestration of isotopically enriched carbon in long-lived pools like wood and soil.