The Case of the Missing Carbon

 

August Jensen, Anna Dudow and Brook Cummings

 

CU Boulder, Fall 2006

 

Each year, humanity dumps roughly 8 billion metric tons of carbon into the atmosphere. But less than half of that total, 3.2 billion tons, remains in the atmosphere to warm the planet. Where is the missing carbon? Growing plants and vegetation remove carbon from the atmosphere by incorporating it into biomass such as larger plants and trees. Because of the natural ability of these organisms to channel CO2 into growth of massive tree trunks, branches and foliage, they may be consuming more CO2 as more becomes available. We hypothesized that as a result of anthropogenic excess CO2 concentrations, the organisms would increase their rate of absorption. If this hypothesis is true, then we should be able to increase CO2 consumption in plants by elevating the CO2 concentrations in a closed chamber.

         To test this hypothesis we placed juniper needles in a closed gas chamber fitted with a CO2 probe for measuring the concentration in parts per million (ppm). Carbon dioxide levels were elevated by exhalation into the chamber. We performed two trials with plant type and mass held constant and calculated the rate of change of CO2 versus CO2 concentration. The rate of CO2 consumption was calculated during segments of decay every 500 ppm, beginning at 2500 and ending at 1000 ppm. A higher concentration/surface area ratio might allow for greater CO2 availability in the photosynthetic pathway with enhanced efficiency as a result; therefore we predicted that the rate of CO2 absorption would decrease as the concentration level decreased within the chamber.

          Regression statistics obtained from individual trial analysis indicated that in each trial the CO2 concentration had a marginally significant effect on the rate of absorption (trial 1: R2 = 0.971, F = 0.109; trial 2: R2 = 0.992, F = 0.057). Our results agree with our predictions. Observed graphical trends, high R2 values and marginal levels of significance verify that plants increase their metabolic activity with greater CO2 availability. Results imply that carbon fixing organisms are sequestering the excess atmospheric CO2. However, a potential problem with the design of our experiment was that our trends may not directly apply on a global scale because our levels of CO2 were considerably greater than the global average of roughly 400 ppm. Nonetheless our trend still implies increased absorption, and at the rate of fossil fuel consumption and deforestation, global CO2 averages may very well reach the levels used in our experiment.

Similar studies published in scientific journals show related outcomes. Reich et al. (2001) recently found that more diverse plant ecosystems were better able to absorb carbon dioxide and nitrogen in response to the rising levels of atmospheric gases. It is this ecosystem-scale response that is providing a great service to life on earth. In a comparable study, Jackson et al. (2002) show a similar “free ride”, in which carbon absorption by natural ecosystems alleviates the rise in atmospheric CO2. However, soil nitrogen availability may limit the capacity of ecosystems to absorb the expected increases in atmospheric CO2.

         Although our hypothesis is marginally supported, a few modifications could potentially offer new insights into the metabolic activity of plants when exposed to unusual levels of CO2. Based on our results and successive understanding of CO2 absorption, we propose a modified hypothesis: small deviations of CO2 concentrations from the lower levels found in nature will alter the absorption rates of carbon fixing organisms. New methods would involve measurements taken at 400 ppm, the global average, and 550 ppm, the expected level over the next century. These lower levels would be included with the higher levels previously tested in our experiment. This would allow for a more pragmatic approach to understanding the effect that humans have on the carbon cycle, whether an organism’s ability to sequester CO2 from the atmosphere may delay the onset of global warming, and at what point these organisms reach a threshold in their ecosystem services.

 

Reich P.,  Knops J.,  Tilman D., Craine J., Ellsworth D., Tjoelker M., Lee T., and Bengston W.  2001. Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Nature 410:809-812

Gill R., Anderson L., Polley H., Johnson H., Jackson R. 2002.  Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2. Ecology 87:41-52