Keeling Plots

The isotopic composition of soil or ecosystem respiration can be determined with the Keeling plot method. This method has long shown that the carbon isotope ratio of ecosystem respiration is highly variable in space. Part of this variation is due to the differences in photosynthetic pathways between C3 and C4 plants. However, even in ecosystems containing only C3 plants, there is considerable variation.

As technological improvements have allowed more frequent sampling of the carbon isotope composition of ecosystem respiration, researchers have also found variation over time. In fact, within C3 ecosystems the temporal variability can be greater than the spatial variability. What causes all of this variation? There are several possible causes. Changes in temperature, humidity, soil moisture etc. affect photosynthetic discrimation against heavy carbon isotopes through the affect on ci/ca. This carbon is respired back to the atmosphere on some time scale. Increasingly, it appears that much of the carbon that is respired from ecosystems was fixed relatively recently - some of it on a time scale of days. Another possible cause of spatial and temporal variability is changes in the proportion of plant and soil respiration, which may have different isotope ratios. Data is somewhat lacking on changes in the proportion of different respiration sources, which can be difficult to measure. Researchers are attempting to apply the differences in isotope ratios of ecosystem carbon sources to separate respiration into its component parts. This works particularly well when there has been a change in the proportion of C3 and C4 plants in the ecosystem. Researchers have also used the differences in the isotope ratio of photosynthesis and respiration to separate Net Ecosystem Exchange, the net gain or loss of carbon from the ecosystem, into the one way fluxes of photosynthetic uptake and respiratory loss.

What is a Keeling Plot?

We can determine the isotope ratio of respired CO2 from the soil or from whole ecosystems with Keeling plots.  To construct a Keeling plot you must capture CO2 samples during a period when the CO2 concentration is changing over time. At the night when photosynthesis has ceased, the CO2 concentration above the soil or the plant canopy increases as the ecosystem respires. During this time you can plot the isotope ratio of sampled CO2 on the y-axis, and the inverse of the CO2 concentration, 1/CO2, on the x-axis. This should create a straight line because of the mixing of respired and atmospheric CO2.  If the CO2 concentration is near the atmospheric value of 365 ppm, then the sample mostly contains atmospheric CO2, and d13C is near -8‰.  When the CO2 concentration rises above the atmospheric concentration, it contains a larger proportion of respired CO2, which has a more negative carbon isotope ratio.

The equation for a straight line is:

 y = intercept + slope*x    

The intercept is the value of y when x if zero. The intercept of a Keeling plot is the isotope ratio of respiration in the absence of dilution by atmospheric CO2.  If CO2 is sampled at the soil surface, the Keeling intercept is the isotope ratio of soil respiration; if it's sampled in the plant canopy the intercept is the isotope ratio of ecosystem respiration.  It is also possible to sample CO2 higher in the troposphere, where the carbon isotope ratio represents an entire region.  A sample Keeling plot is shown here.

We can also generate Keeling plots for d18O in CO2.  We have described the photosynthetic discrimination of CO2 with respect to oxygen isotopes and the effect of the exchange of oxygen with leaf water. Leaf water is highly enriched in 18O due to equilibrium fractionation in evaporation. Water in other plant parts such as stems and roots is not enriched, rather it has the same isotopic composition of the source of water in the soil.  The CO2 that is respired from these plant parts will equilibrate with this water, and also have the same isotope composition as the water source, which is depleted in 18O. Soil respired CO2 also equilibrates with soil water; it is generally affected by the water at about 5-15 cm depth; above 5 cm CO2 leaves the soil too rapidly to exchange with water, and below 15 cm isotopic exchange is negated by diffusional effects as the CO2 molecules move upwards.  At 5 - 15 cm depth, soil water is generally depleted in 18O due to the isotopic composition of precipitation discussed below.


Bowling DR., McDowell NG, Bond BJ, Law BE, Ehleringer JR. 2002. 13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit. Oecologia 131: 113-124.

Ekblad A, Högberg P. 2001. Natural abundance of 13C in CO2 respired from forest soils reveals speed of link between tree photosynthesis and root respiration. Oecologia 127: 305-308.

Flanagan LB, Ehleringer JR. 1998. Ecosystem-atmosphere CO2 exchange: interpreting signals of change using stable isotope ratios. TREE 13 (1): 10-14.

Högberg P, Nordgren A, Buchmann N, Taylor AFS, Ekblad A, Högberg M, Nyberg G, Ottosson-Löfvenius M, Read DJ. 2001. Large-scale forest girdling shows that current photosynthesis drives soil respiration. Nature 411: 789-792.

Pataki DE, Ehleringer JR, Flanagan LB, Yakir D, Bowling DR, Still C, Buchmann N, Kaplan JO, Berry JA. 2003. The application and interpretation of Keeling plots in terrestrial carbon cycle research. Global Biogeochemical Cycles, 17(1).

Yakir D, Sternberg LL. 2000. The use of stable isotopes to study ecosystem gas exchange. Oecologia: 123, 297-311.


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