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Apparent seasonal cycle in isotopic discrimination of carbon in the atmosphere and biosphere due to vapor pressure deficit
Author(s) -
Ballantyne A. P.,
Miller J. B.,
Tans P. P.
Publication year - 2010
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 - 10.1029/2009gb003623
Subject(s) - biosphere , atmosphere (unit) , atmospheric sciences , troposphere , environmental science , seasonality , vapour pressure deficit , biosphere model , water vapor , climatology , carbon cycle , isotopes of carbon , chemistry , geology , total organic carbon , transpiration , photosynthesis , meteorology , environmental chemistry , ecology , geography , biochemistry , ecosystem , biology , organic chemistry
We explore seasonal variability in isotopic fractionation by analyzing observational data from the atmosphere and the biosphere, as well as simulated data from a global model. Using simulated values of atmospheric CO 2 and its carbon isotopic composition, we evaluated different methods for specifying background concentrations when calculating the isotopic signature of source CO 2 ( δ s ) to the atmosphere. Based on this evaluation, we determined that free troposphere measurements should be used when available as a background reference when calculating δ s from boundary layer observations. We then estimate the seasonal distribution of δ s from monthly climatologies derived from several atmospheric sampling sites. This approach yields significant seasonal variations in δ s with more enriched values during the summer months that exceed the uncertainty of δ s estimated for any given month. Intra‐annual measurements of δ 13 C in the cellulose of Pinus taeda growing in the southeastern U.S. also reveal seasonal isotopic variations that are consistent in phase but not necessarily amplitude with atmospherically derived estimates of δ s . Coherent seasonal patterns in δ s inferred from the atmosphere and observed in the biosphere were not consistent with the seasonal isotopic discrimination simulated by a commonly used biosphere model. However, δ s seasonality consistent with observations from the atmosphere and biosphere was retrieved with a revised biosphere model when stomatal conductance, and thus isotopic discrimination, was allowed to vary in response to vapor pressure deficit rather than relative humidity. Therefore, in regions where vapor pressure deficit and relative humidity are positively covariant over the growth season, such as the sub‐tropics, different stomatal conductance models may yield very different estimates of CO 2 and H 2 O exchange between the biosphere and atmosphere.