A new instrument for stable isotope measurements of ¹³C and ¹⁸O in CO₂ – instrument performance and ecological application of the Delta Ray IRIS analyzer

Abstract. We used the recently developed commercially available Delta Ray isotope ratio infrared spectrometer (IRIS) to continuously measure the CO₂ concentration c and its isotopic composition δ¹³C and δ¹⁸O in a managed beech forest in central Germany. Our objectives are (a) to characterize the Delta Ray IRIS and evaluate its internal calibration procedure and (b) to quantify the seasonal variability of c, δ¹³C, δ¹⁸O and the isotopic composition of nighttime net ecosystem CO₂ exchange (respiration) R_eco¹³C and R_eco¹⁸O derived from Keeling plot intercepts. The analyzer's minimal Allan deviation (as a measure of precision) was below 0.01 ppm for the CO₂ concentration and below 0.03 ‰ for both δ values. The potential accuracy (defined as the 1σ deviation from the respective linear regression that was used for calibration) was approximately 0.45 ppm for c, 0.24 ‰ for ¹³C and 0.3 ‰ for ¹⁸O. For repeated measurements of a target gas in the field, the long-term standard deviation from the mean was 0.3 ppm for c and below 0.3 ‰ for both δ values. We used measurements of nine different inlet heights to evaluate the isotopic compositions of nighttime net ecosystem CO₂ exchange R_eco¹³C and R_eco¹⁸O in a 3-month measurement campaign in a beech forest in autumn 2015. During this period, an early snow and frost event occurred, coinciding with a change in the observed characteristics of both R_eco¹³C and R_eco¹⁸O. Before the first snow, R_eco¹³C correlated significantly (p  <  10⁻⁴) with time-lagged net radiation R_n, a driver of photosynthesis and photosynthetic discrimination against ¹³C. This correlation became insignificant (p  >  0.1) for the period after the first snow, indicating a decoupling of δ¹³C of respiration from recent assimilates. For ¹⁸O, we measured a decrease of 30 ‰ within 10 days in R_eco¹⁸O after the snow event, potentially reflecting the influence of ¹⁸O depleted snow on soil moisture. This decrease was 10 times larger than the corresponding decrease in δ¹⁸O in ambient CO₂ (below 3 ‰) and took 3 times longer to recover (3 weeks vs. 1 week). In summary, we conclude that (1) the new Delta Ray IRIS with its internal calibration procedure provides an opportunity to precisely and accurately measure c, δ¹³C and δ¹⁸O at field sites and (2) even short snow or frost events might have strong effects on the isotopic composition (in particular ¹⁸O) of CO₂ exchange on an ecosystem scale.