Abstract

Fossil fuel combustion has increased atmospheric CO₂ by ~ 115 µmol mol-1 since 1750 and decreased its carbon isotope composition (δ¹³C) by 1.7-2‰ (the ¹³C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ¹³C of CO₂ released by terrestrial ecosystem... Show moreFossil fuel combustion has increased atmospheric CO₂ by ~ 115 µmol mol-1 since 1750 and decreased its carbon isotope composition (δ¹³C) by 1.7-2‰ (the ¹³C Suess effect). 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). Seasonality in D appeared to be driven by photosynthetic discrimination (Δcanopy) responding to environmental factors. Possible explanations for negative D include (1) changes in Δcanopy 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. Show less