How isotopes in water and carbon dioxide are used in the National Ecological Observatory Network sensor design

Background/Question/Methods

Stable isotope analyses provide robust information on the structure, function, and processes of ecological systems.  As such, they provide unique information as part of each National Ecological Observatory Network (NEON) Fundamental Instrument Unit (FIU).  NEON’s science grand challenge questions surround the ecology of: biodiversity, ecohydrology, biogeochemical cycles, infectious diseases, climate change, invasive species, land-use, and emerging issues–are all addressed at some level through the use of isotope measurements.  For example, δ¹⁸O and δD undergo predictable transformations in the hydrological cycle that allow tracing of water sources geographical origin, reconstruction of climate, and calculation of energy balance and evaporation.  δ¹³C can be used as an integrated measure of plant stress and plant water use efficiency as reflected in the ratio of leaf internal and external CO₂ concentration (ci/ca).  δ¹³C discrimination by a range of biotic processes also assist in our ability to partition larger fluxes into their sources. Because of the scale of this initiative, new challenges in how to apply these techniques have emerged.  Managing the standards and long-term (30-y) traceability key among them.  For example, the amount of primary δ¹⁸O standards (IAEA) made annually is only a small fraction of what is needed, and they are held as a community resource.

Results/Conclusions

Here, we describe the results of NEON’s strategy to ensure the representativeness of new innovative isotope measurements (laser spectroscopy) at scales of the continent (>1000s km) to microclimate (vertical profiles through the plant canopy).  These results include i) how large quantities of working standards can be made, how they are traceable to primary-community held-standards and how they can me maintained over NEON’s 30-y operational life, ii) the design and uncertainties in automating isotopic measurements across the continent, iii) innovative strategies for calibration, and iv) QA/QC methods to optimize the science returns.