- Raczka, B
- Biraud, SC
- Ehleringer, JR
- Lai, CT
- Miller, JB
- Pataki, DE
- Saleska, SR
- Torn, MS
- Vaughn, BH
- Wehr, R
- Bowling, DR
- et al.
The seasonal pattern of the carbon isotope content (δ C) of atmospheric CO depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ C of the net land-atmosphere CO flux (δ ) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δ 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 for each site after removing background influence and, when possible, removing δ C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March–September) variation in δ at the forest sites (0.5‰ variation). We did not find a consistent seasonal relationship between VPD and δ across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δ . In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δ (5‰) dominated by seasonal transitions in C /C 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 . 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 C of land-atmosphere CO exchange (δ ) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δ were at grassland and urban sites, driven by changes in C /C grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δ when background atmospheric observations are remote and/or prone to anthropogenic influence. 13 13 13 13 2 2 source source source source source source source 3 4 source 2 source source 3 4 source