Atmosphere Level

Canopy Level

Leaf Level

Soil Level

Mass spec.

References

Links

As atmospheric carbon concentrations continue to increase at hyper-natural rates as a result of fossil feul emissions. It is important to understand how carbon is used by ecosystems for many reasons including: pinpointing missing carbon sinks, charting changes in stable isotope concentrations as a result of the increase in C₄ crop plants, and to understand how plants remove carbon from the atmosphere. One of the crucial levels of analysis is understanding carbon use on the canopy level. Because stable isotopes ¹³CO₂ concentrations from the atmosphere and the canopy are quite different about -8‰ and -27‰ respectively.

As it is possible to differentiate the two CO₂ sources, stable isotope analysis allows researchers to see how atmosphere and canopy interact and change. To date while the methods for the study of canopy level CO₂ emissions are generally understood and accepted; it is still hard to give accurate estimates under different temperature conditions, biomes, or seasons. More information is needed on how ¹³C changes over different environments and different time scales.

In most measurment sites net fluxes of CO₂ exchanged between ecosystems is measured. However this flux data can be altered by respiration within a canopy. Thus a forest that has a net annual increase in CO₂ fixation may indicate an increase in net photosynthesis or may be a result of some environmental change in temperature of humidity which indicates a decrease in respiration (Yakir 2000). Following is an account of the techniques and outcomes of canopy level analysis including Keeling plots and an example of stable isotopes being used to understand the transfer of carbon in C₃ vrs C₃ plants.

As the ecosystem's carbon signature is a result of atmospheric mixing, plant ¹³C emissions., and soil/microorganism emissions. each with their own ¹³C signature an approach that integrates these three areas has been used. The main work in this area has been to model the carbon isotope discrimination of whole canopies by looking at D¹³C¹⁶O¹⁶O to determine the Decosystem when integrated with soil and atmosphere level inputs. This is modeled below with the major outputs of various signatures diagrammed (Lloyd and Farquhar 1994):

The equations on the right of the diagram show the integration of all the inputs into the canopy level interactions. Where d¹³Ctrop is the carbon dioxide from the troposphere and d¹³Cresp is from the vegetation and soil micro-organisms. As these measurements (for the method on each scale see soil, individual plants, or the atmosphere) can be integrated over time and different land types with their own histories of carbon use, the results from data can give a view as to what happens when these various turnover rates come together to form the biosphere turnover rates for carbon.

Keeling Plots

To accomplish this integration across different scales and large spatial areas Keeling Plots are used. The Keeling technique manipulates the data found in the day by day changes in concentration and isotopic ratio of the atmospheric carbon dioxide within a canopy. At night the carbon dioxide concentration within the forest boundary layer increases as a result of plant nighttime respiration and soil microorganism respiration. Both of these carbon dioxide sources are depleted in ¹³C, so the forest boundary layer is likewise depleted in ¹³C.

By plotting the mixing of d¹³C on the y-axis and 1/CO₂ on the x-axis linear relationship is established, with the intercept of this plot being the isotope ratio of the respired CO₂. Thus, by sampling air at different levels in the air column i.e. just above the soil, at canopy height, and by aircraft d¹³C, measurements from different producers of d¹³C can be determined and clearly visualized across large time and spatial scales. Large differences in spatial scales can be compared in similar terms. Furthermore by sampling from different places in the canopy the d¹³C results reflect the d¹³C signatures from different spatial scales above, inside, and below the canopy. Presenting a hopefully comprehensive signature from numerous differently aged carbon pools that have different turnover rates and d¹³C values. Below is an example of Keeling plots being used to plot ecosystem level changes among different spatial and time scales showing how much CO₂ left the ecosystem and what its signature was:

(Flanagan and Ehleringer 1998)

These results can be used in numerous ways to understand specific changes occurring to a certain ecosystem over time and varying conditions such as: evergreen vrs. deciduous forest, stand structure, site history, sesonality, effects of weather and many more applications.

For example the impact of massive land-use changes such as the conversion of C₃ forests to crop plants or pasture grasses which are typically C₄ plants. As the d¹³C signatures of C₃ and C₄ are very different during photosynthesis, comparisons of carbon turnover and fixation can be determined. This has been done in numerous placed like the measurements from crop rotation of alfalfa and corn on a certain agricultural site. An alfalfa plant typically has a D_leaf of 20‰ whereas an alfalfa stand growing on soil that recently housed a corn stand the D_resp is 13.8‰. Likewise an individual corn plant has a D_leaf of 4-5‰ but when corn is planted on recently rotated alfalfa it has a D_e of 13.2‰. This means that the soil microorganism's releasing the stored d¹³C signature of the organic material that was there before the rotation occurred. It means that as much as 45% of the D_resp can be made up of the carbon signature of the microorganisms that are barracking down soil organic matter. Thus, hypothetically one wold be able to find instances of human habitation on land dating back as far as 4000yrs by analyzing the D_resp for a particular area (Buchmann 1998)