Carbon and water isotope ratios constrain the processes driving exchange of water and carbon between ecosystems and the atmosphere. Prior studies in a wide range of ecosystems have demonstrated the utility of isotope ratio measurements, but these measurements have often been of short duration for non-overlapping time periods, and have been made without standardized measurement and calibration protocols. The National Ecological Observatory Network (NEON) holds the promise to address several of these shortcomings by measuring isotope ratios in carbon dioxide and water vapor across the United States in a wide variety of ecosystems, with common instrumentation and processing across sites. However, the data provided by NEON are currently not calibrated to international standard scales nor corrected for known concentration-dependent spectral biases. Here, we present an R package we have developed that applies a uniform calibration framework to NEON’s atmospheric isotope data, and use it to examine cross-network patterns in δ13C and δ18O and their variability using data calibrated by this code base.
Results/Conclusions
Currently, δ13C measurements are available for 46 sites and water isotope measurements are available for 20 sites. Three reference materials are measured daily at each NEON site for each molecule (e.g., carbon dioxide or water vapor). We use two of these standards to define a correction equation, and the third standard to validate and estimate the error in our calibration. We focus our analysis on the carbon isotope data here. After calibration of NEON carbon isotope values, we estimate the median error to be 0.2‰. We expect this median site error to drop through time, as some of the early data exhibited higher biases. Across forested and agricultural sites, annual gradients of 50 ppm CO2 and 1-2‰ in δ13C from the bottom to the top of the eddy covariance tower are common. Summer gradients in both CO2 and δ13C are higher magnitude and more variable than winter gradients, particularly for deciduous sites. Shrubland, grassland, and tundra sites exhibit minimal vertical gradients. Finally, we estimated the δ13C value of ecosystem respiration using Keeling plots from nighttime CO2 and δ13C values. Median δ13C values of ecosystem respiration ranged from -30.7‰ (Niwot Ridge) to -15.6‰ (Lajas Experimental Station). Based on these results, we suggest that NEON atmospheric isotope data record meaningful environmental variation in space and time and we provide an R package for community use to produce calibrated isotope ratios by applying consistent algorithms across the network.