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

For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important one. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. Here we review the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well to plant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.