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Discrimination against C 18 O 16 O during photosynthesis and the oxygen isotope ratio of respired CO 2 in boreal forest ecosystems
Author(s) -
Flanagan Lawrence B.,
Brooks J. Renee,
Varney Gregory T.,
Ehleringer James R.
Publication year - 1997
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 - 10.1029/96gb03941
Subject(s) - photosynthesis , taiga , boreal ecosystem , isotopes of oxygen , environmental chemistry , black spruce , oxygen , isotopes of carbon , oxygen 18 , stable isotope ratio , soil water , chemistry , ecosystem , terrestrial ecosystem , soil respiration , environmental science , ecology , soil science , total organic carbon , biology , quantum mechanics , biochemistry , physics , organic chemistry , nuclear chemistry
Our objective was to analyze factors that influence changes in the oxygen isotope ratio (δ 18 O) of atmospheric CO 2 within boreal forest ecosystems. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem‐Atmosphere Study (BOREAS). In terrestrial ecosystems the δ 18 O value of atmospheric CO 2 is strongly influenced by isotope effects that occur during photosynthesis and respiration. Of primary importance is an equilibrium isotope effect that occurs between oxygen in CO 2 and oxygen in soil water and plant chloroplast water. During the equilibrium reaction the oxygen isotope ratio of CO 2 becomes enriched in 18 O relative to that of water. We measured seasonal changes in the oxygen isotope ratio of (1) water input to the ecosystems (precipitation), (2) water taken up by the major plant species from the soil (plant stem water), and (3) water in plant leaves. We used this information in calculations of isotope discrimination during photosynthesis and soil respiration. Discrimination against C 18 O 16 O during photosynthetic gas exchange (Δ A ) (influenced by equilibration with chloroplast water) averaged approximately 21‰ at midday and was similar for all forest types. In contrast, CO 2 released during plant and soil respiration had an average δ 18 O value of −14.4‰ but was less depleted in 18 O than would be expected for respired CO 2 in isotopic equilibrium with soil water. This effect was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface and the observation that water in the upper moss layers can have an oxygen isotope ratio substantially different from water in deeper soil layers.