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

Fossil fuel combustion has increased atmospheric CO 2 by ≈ 115 µmol mol −1 since 1750 and decreased its carbon isotope composition ( δ 13 C) by 1.7–2‰ (the 13 C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ 13 C of CO 2 released by terrestrial ecosystems is expected to differ from the δ 13 C of CO 2 assimilated by land plants during photosynthesis. This isotopic difference between land‐atmosphere respiration ( δ R ) and photosynthetic assimilation ( δ A ) fluxes gives rise to the 13 C 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 ( δ R > δ A ). A 7 year record of biosphere‐atmosphere carbon exchange was used to evaluate the seasonality of δ A and δ R , and the 13 C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ 13 C of net land‐atmosphere exchange during day and night and combined with tower‐based flux observations to assess δ A and δ R . 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). Seasonality in D appeared to be driven by photosynthetic discrimination (Δ canopy ) responding to environmental factors. Possible explanations for negative D include (1) changes in Δ canopy over decades as CO 2 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. Carbon isotope contents of photosynthesis and respiration differ Isotope fractionation of land photosynthesis dominates over the 13 C Suess effect Biogeochemical C isotope models must include terrestrial ecological processes