Does vapor pressure deficit drive the seasonality of δ 13C of the net land-atmosphere CO2 exchange across the United States?
Biraud, S. C.
Ehleringer, J. R.
Miller, J. B.
Pataki, D. E.
Saleska, S. R.
Torn, M. S.
Vaughn, B. H.
Bowling, D. R.
AffiliationUniv Arizona, Dept Ecol & Evolutionary Biol
MetadataShow full item record
PublisherAMER GEOPHYSICAL UNION
CitationDoes vapor pressure deficit drive the seasonality of δ 13C of the net land-atmosphere CO2 exchange across the United States? 2017, 122 (8):1969 Journal of Geophysical Research: Biogeosciences
Rights©2017. American Geophysical Union. All Rights Reserved.
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AbstractThe seasonal pattern of the carbon isotope content (delta C-13) of atmospheric CO2 depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the delta C-13 of the net land-atmosphere CO2 flux (delta(source)) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of (source) at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal delta(source) for each site after removing background influence and, when possible, removing delta C-13 variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March-September) variation in delta(source) at the forest sites (0.5 parts per thousand variation). We did not find a consistent seasonal relationship between VPD and delta(source) across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in (source). In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in (source) (5) dominated by seasonal transitions in C-3/C-4 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 (source). 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 delta C-13 of land-atmosphere CO2 exchange (delta(source)) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of delta(source) were at grassland and urban sites, driven by changes in C-3/C-4 grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate delta(source) when background atmospheric observations are remote and/or prone to anthropogenic influence.
Note6 month embargo; published online: 10 August 2017
VersionFinal published version
SponsorsCenter for High Performance Computing at the University of Utah; US Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science (TES) Program [DE-SC0010625]; US National Science Foundation Macrosystems Biology Program [EF-1137336]; Atmospheric Radiation Measurement Program (ARM) [DE-AC02-05CH11231]; NOAA Climate Program Office's Atmospheric Chemistry, Carbon Cycle, and Climate Program [NA14OAR4310178]; U.S. Department of Energy's Office of Science; NSF as part of the Harvard Forest LTER; DOE TES program [DE-SC0006741]