Abstract
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) Reco13C and Reco18O 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 Reco13C and Reco18O 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 Reco13C and Reco18O. Before the first snow, Reco13C correlated significantly (p < 10−4) with time-lagged net radiation Rn, 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 Reco18O 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.
Highlights
The stable isotopic compositions of CO2 and water vapor have been intensely used to study ecosystem gas exchange (Yakir and Sternberg, 2000)
The objectives of our study are (a) to characterize the Delta Ray isotope ratio infrared spectrometer (IRIS) and its performance under field conditions as well as (b) to quantify the seasonal variability of δ13C, δ18O and the isotopic composition of CO2 exchange for both δ13C and δ18O derived from Keeling plot intercepts
The measured Allan deviation σA followed this slope up to averaging times for approximately 300 s for δ-value measurements and approximately 200 s for concentration measurements. At these timescales the analyzer showed its maximum precision of 0.02 ‰ VPDB for δ13C, 0.03 ‰ VPDB-CO2 for δ18O and 0.007 ppm for CO2 concentration
Summary
The stable isotopic compositions of CO2 and water vapor have been intensely used to study ecosystem gas exchange (Yakir and Sternberg, 2000). The temporal variability of the isotopic composition of respiration for example has been studied on timescales ranging from sub-diurnal (Barbour et al, 2011) to seasonal (Ekblad and Högberg, 2001; Bowling et al, 2002; Knohl et al, 2005). The 18O composition of ecosystem CO2 exchange Re1c8oO, on the other hand, is interesting for studying the coupled CO2 and water cycle (see e.g., Yakir and Wang, 1996)
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