Final Report for Award DE-FG02-03ER63640

The carbon isotope ratios of carbon dioxide fluxes from terrestrial ecosystems are key measurements needed to constrain interpretations of carbon sinks in North American carbon cycle analyses. The completed research was a multi-faceted effort addressing photosynthetic and respiratory isotope exchanges across the biosphere-atmosphere boundary at five AmeriFlux sites (Harvard Forest, Howland Forest, Rannalls Ranch, Niwot Ridge Forest, and Wind River Crane Site), spanning the dominant ecosystem types of the United States. The sampling and analysis protocols developed in this project have become the fundamental analytical approach for all sites measuring ecosystem isotope studies across the United States and Canada. It is the first network of long-term observations to characterize the isotopic composition of the biosphere-atmosphere CO2 flux. We focused on understanding the magnitude of changes in the carbon isotope ratio of respiration and of photosynthetic discrimination on seasonal and interannual bases. Focusing at AmeriFlux sites provided a direct link to NEE measurements associated with studies of the North American carbon cycle and an opportunity to provide mechanistic insights relating observed isotope changes and the controls over carbon sequestration and loss on seasonal and interannual bases. An additional component of our research linked directly with eddy covariance monitoring to partition NEE into assimilation and respiratory components. The completed project promoted cross-site analyses and resulting publications applicable at AmeriFlux and other long-term carbon cycle research sites. Lastly, the online monitoring of carbon dioxide in the Salt Lake Valley and the intermittent monitoring of absolute carbon dioxide concentrations at different AmeriFlux sites contributed public awareness and data sets that can be used in public education and as a basis for public policies related to carbon dioxid