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Understanding the Stable Isotope Composition of Biosphere-Atmosphere CO 2 Exchange

Abstract : Stable isotopes of atmospheric carbon dioxide (CO 2) contain a wealth of information regarding biosphere-atmosphere interactions. The carbon isotope ratio of CO 2 (δ 13 C) reflects the terrestrial carbon cycle including processes of photosynthesis, respiration, and decomposition. The oxygen isotope ratio (δ 18 O) reflects terrestrial carbon and water coupling due to CO 2-H 2 O oxygen exchange. Isotopic CO 2 measurements, in combination with ecosystem-isotopic exchange models, allow for the quantification of patterns and mechanisms regulating terrestrial carbon and water cycles, as well as for hypothesis development, data interpretation, and forecasting. Isotopic measurements and models have evolved significantly over the past two decades, resulting in organizations that promote model-measurement networks, e.g., the U.S. National Science Foundation's Biosphere-Atmosphere Stable Isotope Network , the European Stable Isotopes in Bio-sphere-Atmosphere Exchange Network, and the U.S. National Environmental Observatory Network. Unfortunately, technical limitations associated with atmospheric flask sampling and subsequent analyses via mass spectrometry have constrained high-frequency data useful for in-depth quantification of patterns and controls. As a consequence, state-of-the-art models of ecosystem stable isotope exchange have evolved beyond their counterpart measurement systems, leaving model assumptions vulnerable due to a lack of validation data sets. The recent development of laser-based systems for high-frequency, continuous sampling of CO 2 isotopologues (at least one atom with a different number of neutrons), i.e., 12 CO 2 , 13 CO 2 , and 12 C 16 O 18 O, allows, for the first time, rigorous model testing at compatible timescales. Numerous laser systems are in development and all have advantages and disadvantages ; however, all of them share the potential of having precision and accuracy on par with traditional flask/mass spectrometry techniques while providing continuous, high-frequency data (i.e., <1-15 minutes). Model-measurement comparisons can now be used to interpret observations and identify knowledge gaps via sensitivity analyses, thereby creating a closed loop of hypotheses-observation interpretation hypotheses. Biosphere-Atmosphere Isotopic Exchange We compared 2 years of near-continuous measurements of nocturnal ecosystem-respired CO 2 (δ 13 C R and δ 18 O R) from a semi-arid juniper woodland-located at the Los Alamos National Laboratory in northern New Mexico-with outputs from three models of ecosystem stable isotope exchange. Our objective was to determine if new insight into the strengths and weaknesses of models could be revealed using continuous, high-frequency observations. Measurements collected via tunable diode laser spectroscopy (Campbell Scien-tific's TGA100) were used to test the models: CanIsotope (canopy-isotope) for δ 13 C R (a simple steady state model), SIM (simple isotope model) for δ 18 O R , and ISOLSM (isotope land surface model) for both δ 13 C R and δ 18 O R. Though not an exhaustive comparison of all models, these three models represent end points in the spectrum from detailed to simple process representations. All three models received similar input data: meteorology , leaf area, photosynthetic capacity, and δ 18 O of precipitation, soil, and stem water. All three models provided some encouraging successes when simulations were compared with observations. The models reproduced the observations of ecosystem CO 2 , latent and sensible heat fluxes, and the ratio of CO 2 partial pressure within canopy foliage relative to the partial pressure of CO 2 in the surrounding atmosphere. Also, the timing and direction of δ 18 O R variation was well captured (Figure 1). Thus, a range of model complexity and assumptions captured photosynthesis and δ 18 O dynamics associated with the large daily and seasonal variation in humidity and precipitation that occurs at this woodland. Models captured δ 18 O R better than δ 13 C R. This demonstrates our limited understanding of carbon cycling compared with oxygen isotope exchanges between soil and leaf H 2 O and CO 2. The discrepancy partially occurred because these models lack carbon pools, and thus they simulated δ 13 C of photosynthate rather than δ 13 C R , assumed that carbon translocation to respiration was immediate, and assumed that no respiratory fractionation occurred (we note that some models exist that do represent these processes). In addition, the models assumed isohydric (constant) regulation of leaf water potential, which leads to strong feedbacks between water availability and stomatal conductance. However, juniper has anisohy-dric (nonconstant) leaf water potential regulation , as do many semi arid species, leading the region north of 60°N will be completed in the 30-arc-second resolution by inserting existing global data (GTOPO30 and HYDRO1k). Higher-resolution elevation data will be utilized in future updates where available (United States, Canada, Europe). To date, the focus in data processing has been to provide the following core layers: void-filled and hydrologically conditioned elevation and drainage directions at all resolutions; and flow accumulation, river network, and first-order basin outlines at 15-and 30-arc-second resolutions. Future developments will add new layers of information and enhanced attributes. The following updates are planned: slope; full-resolution flow accumulations; flow distances; stream network and subbasin delineations according to various river-ordering schemes, including Pfafstetter coding [Verdin and Verdin, 1999]; and mod-eled flow quantities. As the upscaling algorithm of HydroSHEDS is flexible, all data sets may be provided at additional resolutions in the future.
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Submitted on : Sunday, November 1, 2020 - 11:37:26 PM
Last modification on : Wednesday, March 10, 2021 - 3:08:50 AM

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Nate Mcdowell, Dennis Baldocchi, Margaret Barbour, Chris Bickford, Matthias Cuntz, et al.. Understanding the Stable Isotope Composition of Biosphere-Atmosphere CO 2 Exchange. Eos, Transactions American Geophysical Union, American Geophysical Union (AGU), 2008, 89 (10), pp.94. ⟨10.1029/2008EO100002⟩. ⟨hal-02985169⟩

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