A new instrument for stable isotope measurements of sup13/supC and sup18/supO in COsub2/sub – instrument performance and ecological application of the Delta Ray IRIS analyzer

摘要

We used the recently developed commercially available Delta Ray isotope ratio infrared spectrometer (IRIS) to continuously measure the COsub2/sub concentration ic/i and its isotopic composition iδ/isup13/supC and iδ/isup18/supO 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 ic/i, iδ/isup13/supC, iδ/isup18/supO and the isotopic composition of nighttime net ecosystem COsub2/sub exchange (respiration) iR/isubeco/subsup13/supC and iR/isubeco/subsup18/supO derived from Keeling plot intercepts. The analyzer's minimal Allan deviation (as a measure of precision) was below 0.01?ppm for the COsub2/sub concentration and below 0.03?‰ for both iδ/i values. The potential accuracy (defined as the 1iσ/i deviation from the respective linear regression that was used for calibration) was approximately 0.45?ppm for ic/i, 0.24?‰ for sup13/supC and 0.3?‰ for sup18/supO. For repeated measurements of a target gas in the field, the long-term standard deviation from the mean was 0.3?ppm for ic/i and below 0.3?‰ for both iδ/i values. We used measurements of nine different inlet heights to evaluate the isotopic compositions of nighttime net ecosystem COsub2/sub exchange iR/isubeco/subsup13/supC and iR/isubeco/subsup18/supO 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 iR/isubeco/subsup13/supC and iR/isubeco/subsup18/supO. Before the first snow, iR/isubeco/subsup13/supC correlated significantly (ip/i??&??10sup?4/sup) with time-lagged net radiation iR/isubn/sub, a driver of photosynthesis and photosynthetic discrimination against sup13/supC. This correlation became insignificant (ip/i??&??0.1) for the period after the first snow, indicating a decoupling of iδ/isup13/supC of respiration from recent assimilates. For sup18/supO, we measured a decrease of 30?‰ within 10?days in iR/isubeco/subsup18/supO after the snow event, potentially reflecting the influence of sup18/supO depleted snow on soil moisture. This decrease was 10?times larger than the corresponding decrease in iδ/isup18/supO in ambient COsub2/sub (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 ic/i, iδ/isup13/supC and iδ/isup18/supO at field sites and (2)?even short snow or frost events might have strong effects on the isotopic composition (in particular sup18/supO) of COsub2/sub exchange on an ecosystem scale.