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Does vapor pressure deficit drive the seasonality of δ13C of the net land-atmosphere CO2 exchange across the United States?
- Source :
- Journal of Geophysical Research. Biogeosciences; Aug2017, Vol. 122 Issue 8, p1969-1987, 19p
- Publication Year :
- 2017
-
Abstract
- The seasonal pattern of the carbon isotope content (δ<superscript>13</superscript>C) of atmospheric CO<subscript>2</subscript> depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ<superscript>13</superscript>C of the net land-atmosphere CO<subscript>2</subscript> flux ( δ<subscript>source</subscript>) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δ<subscript>source</subscript> at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal δ<subscript>source</subscript> for each site after removing background influence and, when possible, removing δ<superscript>13</superscript>C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March-September) variation in δ<subscript>source</subscript> at the forest sites (0.5‰ variation). We did not find a consistent seasonal relationship between VPD and δ<subscript>source</subscript> across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δ<subscript>source</subscript>. In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δ<subscript>source</subscript> (5‰) dominated by seasonal transitions in C<subscript>3</subscript>/C<subscript>4</subscript> 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 δ<subscript>source</subscript> . 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 δ<superscript>13</superscript>C of land-atmosphere CO<subscript>2</subscript> exchange ( δ<subscript>source</subscript> ) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δ<subscript>source</subscript> were at grassland and urban sites, driven by changes in C<subscript>3</subscript>/C<subscript>4</subscript> grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δ<subscript>source</subscript> when background atmospheric observations are remote and/or prone to anthropogenic influence. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 21698953
- Volume :
- 122
- Issue :
- 8
- Database :
- Complementary Index
- Journal :
- Journal of Geophysical Research. Biogeosciences
- Publication Type :
- Academic Journal
- Accession number :
- 125084722
- Full Text :
- https://doi.org/10.1002/2017JG003795