TY - JOUR
T1 - Does vapor pressure deficit drive the seasonality of δ13C of the net land-atmosphere CO2 exchange across the United States?
AU - Raczka, B.
AU - Biraud, S. C.
AU - Ehleringer, J. R.
AU - Lai, C. T.
AU - Miller, J. B.
AU - Pataki, D. E.
AU - Saleska, S. R.
AU - Torn, M. S.
AU - Vaughn, B. H.
AU - Wehr, R.
AU - Bowling, D. R.
N1 - Funding Information:
Thank you to Ashley Ballantyne for helpful discussion regarding the mixing model methods, interpretation of results, and helpful comments on a manuscript draft. Thanks to the AmeriFlux site investigators for sharing the flux tower data, including Sonia Wharton who provided flux tower data and discussion for the Wind River site. Thanks to Ed Dlugokencky and Paul Novelli for providing the flask observations from the Niwot Ridge tundra site. The authors thank Daniel Mendoza of the University of Utah who provided Hestia fossil fuel emissions data for Salt Lake City. Thank you to Ryan Bares who helped with the collection and organization of the Salt Lake City isotope data. The support and resources from the Center for High Performance Computing at the University of Utah are gratefully acknowledged. This research was supported by the US Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science (TES) Program under award number DE-SC0010625, and the US National Science Foundation Macrosystems Biology Program under award EF-1137336. Additional support was provided by the Atmospheric Radiation Measurement Program (ARM) under award number DE-AC02-05CH11231 and through the NOAA Climate Program Office’s Atmospheric Chemistry, Carbon Cycle, and Climate Program, grant NA14OAR4310178. Funding for AmeriFlux data resources and operation of the sites was provided by the U.S. Department of Energy’s Office of Science. Harvard Forest is an AmeriFlux core site also supported by NSF as part of the Harvard Forest LTER. The laser spectrometer measurements at the Harvard Forest were supported by the DOE TES program under award DE-SC0006741. The location of the data sets used for the analysis is given in the methods of this manuscript (sections 2.1–2.4).
Publisher Copyright:
©2017. American Geophysical Union. All Rights Reserved.
PY - 2017/8
Y1 - 2017/8
N2 - The seasonal pattern of the carbon isotope content (δ13C) of atmospheric CO2 depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ13C of the net land-atmosphere CO2 flux (δsource) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δsource at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal δsource for each site after removing background influence and, when possible, removing δ13C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March–September) variation in δsource at the forest sites (0.5‰ variation). We did not find a consistent seasonal relationship between VPD and δsource across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δsource. In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δsource (5‰) dominated by seasonal transitions in C3/C4 grass productivity and in fossil fuel emissions, respectively. Our findings were sensitive to the location used to account for atmospheric background variation within the mixing model method that determined δsource. Special consideration should be given to background location depending on whether the intent is to understand site level dynamics or regional scale impacts of land-atmosphere exchange. The seasonal amplitude in δ13C of land-atmosphere CO2 exchange (δsource) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δsource were at grassland and urban sites, driven by changes in C3/C4 grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δsource when background atmospheric observations are remote and/or prone to anthropogenic influence.
AB - The seasonal pattern of the carbon isotope content (δ13C) of atmospheric CO2 depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ13C of the net land-atmosphere CO2 flux (δsource) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δsource at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal δsource for each site after removing background influence and, when possible, removing δ13C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March–September) variation in δsource at the forest sites (0.5‰ variation). We did not find a consistent seasonal relationship between VPD and δsource across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δsource. In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δsource (5‰) dominated by seasonal transitions in C3/C4 grass productivity and in fossil fuel emissions, respectively. Our findings were sensitive to the location used to account for atmospheric background variation within the mixing model method that determined δsource. Special consideration should be given to background location depending on whether the intent is to understand site level dynamics or regional scale impacts of land-atmosphere exchange. The seasonal amplitude in δ13C of land-atmosphere CO2 exchange (δsource) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δsource were at grassland and urban sites, driven by changes in C3/C4 grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δsource when background atmospheric observations are remote and/or prone to anthropogenic influence.
KW - C discrimination
KW - land-atmosphere exchange
KW - stable carbon isotopes
KW - vapor pressure deficit
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U2 - 10.1002/2017JG003795
DO - 10.1002/2017JG003795
M3 - Article
AN - SCOPUS:85029158289
VL - 122
SP - 1969
EP - 1987
JO - Journal of Geophysical Research: Biogeosciences
JF - Journal of Geophysical Research: Biogeosciences
SN - 2169-8953
IS - 8
ER -