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Ecological processes dominate the 13 C land disequilibrium in a Rocky Mountain subalpine forest
Author(s) -
Bowling D. R.,
Ballantyne A. P.,
Miller J. B.,
Burns S. P.,
Conway T. J.,
Menzer O.,
Stephens B. B.,
Vaughn B. H.
Publication year - 2014
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1002/2013gb004686
Subject(s) - disequilibrium , biosphere , environmental science , atmospheric sciences , atmosphere (unit) , isotopes of carbon , terrestrial ecosystem , photosynthesis , ecosystem , ecology , geology , botany , geography , total organic carbon , biology , meteorology , medicine , ophthalmology
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.