Summary

 

Author

Alexander Knohl

Summary

Measuring the isotopic composition of ecosystem scale fluxes of CO₂, water vapor and other trace gases in terrestrial ecosystems provides additional information on ecosystem-atmosphere interaction. It can reveal insights into ecosystem scale isotope discrimination for CO₂ (Griffis et al. 2008, Sturm et al. 2012), provide an approximation of leaf water ¹⁸O enrichment at ecosystem scale (Lee et al. 2007a, Griffis et al. 2010) and quantify the isoforcing of ecosystems, i.e. the imprint of ecosystems to the isotopic composition of the atmosphere (Griffis et al. 2007, Lee et al. 2009). Furthermore, knowing the isotopic composition of ecosystem scale fluxes also provides independent constraints for evaluating ecosystem models (Baldocchi & Bowling 2003) and helps to partition the net ecosystem exchange of CO₂ into its components, assimilation and respiration (Bowling et al. 2001, Ogée et al. 2004, Knohl & Buchmann 2005).

Materials/Equipment

Measuring the isotopic composition of ecosystem scale fluxes is, however, technically challenging as it typically requires online and direct field measurements of the isotopes in the respective trace gas. Only the development of laser spectrometer for stable isotope analysis enables us to carry out such measurements. Before that people used relaxed eddy accumulation, a micrometeorological approach by conditionally sampling up and down draft of air parcels (Bowling et al. 1999), or the EC/flask approach using assumptions about constant relationships of the isotopic composition and the mixing ratio (Bowling et al. 2001, Ogée et al. 2004, Knohl & Buchmann 2005).

Measurement approaches

Using laser spectrometer two approaches are typically applied for ecosystem scale flux measurements: (a) the flux gradient approach and (b) the eddy covariance approach.

Flux gradient

The flux gradient approach is well suited for short vegetation and when continuous, but not very fast (a few seconds) measurements are available. Isotopic measurements are carried out simultaneously or alternately in two heights above the vegetation. The isotopic composition, e.g. ¹⁸O in H₂O, of the flux is then calculated (Lee et al. 2007b) as

where R_x is the ratio of the molar flux of the heavy isotopologue (a) to the molar flux of the light isotopologue (b). c₁ and c₂ denote the calibrated mixing ratio measurements of the respective isotopologues in height 1 and height 2. The molar flux ratio (R_x) is then converted to the delta notation ( _x) in reference to an isotopic standard (R_s, e.g. VPDB or VSMOW).

Eddy covariance

The eddy covariance approach is well suited over tall vegetation, e.g. forest, but requires high frequency measurements (5-10 Hz) of the isotopic composition of the respective gas. The eddy flux F_a (μmol m ² s ¹) is calculated from the covariance of the vertical wind speed w (m s ¹) and the mole fraction c_a of the isotopologue (μmol mol ¹) measured above the canopy (Sturm et al. 2012):

where V_m (m³ mol ¹) is the molar volume of air, the covariance (cov) of w and c_a is typically calculated over 30 minutes intervals. The delta value of the flux ( _x) can then be derived from the eddy flux ratio of the heavy isotopologue (F_a) to the light isotopologue (F_b) in reference to an isotopic standard (R_s).

Literature references

Baldocchi DD, Bowling DR (2003) Modelling the discrimination of (CO2)-C-13 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales. Plant, Cell and Environment, 26, 231-244.

Bowling DR, Baldocchi DD, Monson RK (1999) Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest. Global Biogeochemical Cycles, 13, 903-922.

Bowling DR, Tans PP, Monson RK (2001) Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2. Global Change Biology, 7, 127-145.

Griffis TJ, Sargent SD, Baker JM et al. (2008) Direct measurement of biosphere-atmosphere isotopic CO(2) exchange using the eddy covariance technique. Journal of Geophysical Research-Atmospheres, 113.

Griffis TJ, Sargent SD, Lee X et al. (2010) Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance. Boundary-Layer Meteorology, 137, 307-326.

Griffis TJ, Zhang J, Baker JM, Kljun N, Billmark K (2007) Determining carbon isotope signatures from micrometeorological measurements: Implications for studying biosphere-atmosphere exchange processes. Boundary-Layer Meteorology, 123, 295-316.

Knohl A, Buchmann N (2005) Partitioning the net CO2 flux of a deciduous forest into respiration and assimilation using stable carbon isotopes. Global Biogeochemical Cycles, 19, GB4008.

Lee X, Kim K, Smith R (2007a) Temporal variations of the 18O/16O signal of the whole-canopy transpiration in a temperate forest. Global Biogeochemical Cycles, 21.

Lee XH, Griffis TJ, Baker JM, Billmark KA, Kim K, Welp LR (2009) Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapor isotopes. Global Biogeochemical Cycles, 23.

Lee XH, Kim K, Smith R (2007b) Temporal variations of the O-18/O-16 signal of the whole-canopy transpiration in a temperate forest. Global Biogeochemical Cycles, 21, Gb3013.

Ogée J, Peylin P, Cuntz M et al. (2004) Partitioning net ecosystem carbon exchange into net assimilation and respiration with canopy-scale isotopic measurements: An error propagation analysis with (CO2)-C-13 and (COO)-O-18 data. Global Biogeochemical Cycles, 18.

Sturm P, Eugster W, Knohl A (2012) Eddy covariance measurements of CO2 isotopologues with a quantum cascade laser absorption spectrometer. Agricultural and Forest Meteorology, 152, 73-82.