Determining carbon isotope signatures from micrometeorological measurements: Implications for studying biosphere–atmosphere exchange processes |
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Authors: | T J Griffis J Zhang J M Baker N Kljun K Billmark |
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Institution: | (1) Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, USA;(2) United States Department of Agriculture – Agriculture, Research Service, St. Paul, MN 55108, USA;(3) Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland |
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Abstract: | In recent years considerable effort has been focused on combining micrometeorological and stable isotope techniques to partition
net fluxes and to study biosphere–atmosphere exchange processes. While much progress has been achieved over the last decade,
some new issues are beginning to emerge as technological advances, such as laser spectroscopy, permit isotopic fluxes to be
measured more easily and continuously in the field. Traditional investigations have quantified the isotopic composition of
biosphere-atmosphere exchange by using the Keeling two-member mixing model (the classic Keeling plot). An alternative method,
based on a new capacity to measure isotopic mixing ratios, is to determine the isotope composition of biosphere–atmosphere
exchange from the ratio of flux measurements. The objective of this study was to critically evaluate these methods for quantifying
the isotopic composition of ecosystem respiration (δR) over a period of three growing seasons (2003–2005) within a heterogeneous landscape consisting of C3 and C4 species. For C4 canopies, the mixing model approach produced δR values that were 4–6‰ lower (isotopically lighter) than the flux-gradient method. The analyses presented here strongly suggest
that differences between flux and concentration footprint functions are the main factor influencing the inequality between
the mixing model and flux-gradient approaches. A mixing model approach, which is based on the concentration footprint, can
have a source area influence more than 20-fold greater than the flux footprint. These results highlight the fact that isotopic
flux partitioning is susceptible to problems arising from combining signals (concentration and fluxes) that represent very
different spatial scales (footprint). This problem is likely to be most pronounced within heterogeneous terrain. However,
even under ideal conditions, the mismatch between concentration and flux footprints could have a detrimental impact on isotopic
flux partitioning where very small differences in isotopic signals must be resolved. |
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Keywords: | Carbon cycling Ecosystem respiration Flux-gradient Heterogeneous terrain Keeling plot Lagrangian flux and concentration footprints Similarity theory Stable isotopes |
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