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

Because carbon is stored in the terrestrial biosphere for decades and longer, the δ¹³C of CO₂ released by terrestrial ecosystems is expected to differ from the δ¹³C of CO₂ assimilated by land plants during photosynthesis. This isotopic difference between land-atmosphere respiration (δR) and photosynthetic assimilation (δA) fluxes gives rise to the ¹³C 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 (δR > δA). A 7 year record of biosphere-atmosphere carbon exchange was used to evaluate the seasonality of δA and δR, and the ¹³C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ¹³C of net land-atmosphere exchange during day and night and combined with tower-based flux observations to assess δA and δR. The disequilibrium varied seasonally and when flux-weighted was opposite in sign than expected from the Suess effect (D = −0.75 ± 0.21‰ or −0.88 ± 0.10‰ depending on method).