Our research focuses on quantifying stable isotope ratios (13C/12C and 18O/16O) associated with photosynthetic and respiratory exchanges across the biosphere-atmosphere boundary at AmeriFlux sites – including Harvard Forest, Howland Forest, Rannells Flint Hills Prairie, Niwot Ridge Forest, and Wind River Canopy Crane Site, spanning the dominant ecosystem types of the United States. Photosynthesis and respiration in terrestrial ecosystems have opposite effects on diurnal and seasonal patterns on atmospheric CO2 concentration and isotope ratios. This isotopic variation contains information about the functioning of different terrestrial ecosystems and is filtered out in a flask network that is largely focused at marine sampling sites. The goal of our project is to better capture isotopic effects of ecosystem-atmosphere interaction at diurnal, seasonal and interannual time scales by long-term monitoring 13C of CO2 exchange with the atmosphere at weekly intervals. Specifically, our research addresses the following fundamental questions to meet the need to have a stronger characterization of the isotopic composition of CO2 in terrestrial biosphere-atmosphere exchange identified by the U.S. Carbon Cycle Science Plan (1999): Providing long-term observations of Δ13C and δ18) of CO2 in canopy boundary layer from a diverse ecosystem types in the United States Dissecting δ13C of respiratory CO2 fluxes into above- and belowground components Partitioning net ecosystem exchange (NEE) into assimilation and respiration components Modeling canopy CO2 fluxes and isotopic discrimination The current project will also contribute to The Biosphere - Atmosphere Stable Isotope Network (BASIN), aiming at improving our understanding of carbon and water cycle processes at the ecosystem, regional, and global scales. The data sharing and integration will allow for cross-site comparisons and provide an opportunity to assess any continental or global effects.