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

D. R. Bowling, A. P. Ballantyne, J. B. Miller, S. P. Burns, T. J. Conway, O. Menzer, B. B. Stephens, B. H. Vaughn

Research output: Contribution to journalArticlepeer-review

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 ecosystems is expected to differ from the δ13C of CO 2 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 (Δ canopy) 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. Key Points Carbon isotope contents of photosynthesis and respiration differ Isotope fractionation of land photosynthesis dominates over the 13C Suess effect Biogeochemical C isotope models must include terrestrial ecological processes

Original languageEnglish
Pages (from-to)352-370
Number of pages19
JournalGlobal Biogeochemical Cycles
Volume28
Issue number4
DOIs
StatePublished - Apr 2014

Keywords

  • biogeochemical cycles
  • biosphere/atmosphere interactions
  • carbon cycling
  • carbon isotope discrimination
  • conifer
  • forest ecology

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