An unexpected actor: role of arid environments in terrestrial carbon uptake

Anthropogenic climate change has become a household phrase, spreading throughout our culture as more and more research describes the effect of fossil fuel emissions on both CO2 levels in the atmosphere and average global surface temperatures.  But our society’s CO2 emissions are part of a much larger carbon cycle that has existed for much longer than our species.  The cycle involves, in one direction, the natural emission of CO2 from volcanic eruptions and outgassing from the Earth’s mantle, balanced in the other direction by the uptake of CO2 by the ocean and land.

800px-Carbon-cycle-full

The ocean uptake is well understood, as carbon dioxide dissolves in surface waters to create calcium carbonates, carbonic acids, and the like.  The carbonates then slowly drop to the bottom of the ocean, leak into the Earth’s mantle, and are emitted again through eruptions and outgassing to complete the cycle.  Our role as fossil fuel users can be seen above in the blue ‘fossil carbon’ line, as we dig into the crust and combust fossil carbons that have entered the soil eons ago as organic matter.

Terrestial carbon uptake, on the other hand, is a bit of a  mystery.  We know that it exists, but the science community has usually inferred it to be whatever is left over after taking into account ocean and human-directed mechanisms to balance the emission and uptake.  However, a very cool, 10-year study has just appeared in Nature Climate Change that definitively shows the important role that arid environments play in carbon uptake.  This is crucial information, as arid environments have largely been ignored in this capacity due to their lack of plant matter, relative to forested environments, that are usually considered to be the main actors in taking up carbon through photosynthetic processes.  This information will be especially important as we begin to develop in more arid environments, as this will likely take away from the carbon uptake potential of these ecosystems and leave more of the CO2 sitting in the atmosphere.

Figure courtesty of Desert Research Institute: University of Nevada - Las Vegas

Figure courtesty of Desert Research Institute: University of Nevada – Las Vegas

This is the satellite view of of a plot of land used in the study.  Nine such samples exist as part of the Nevada Desert Free-Air CO2 Enrichment Facility (NDFF, for less of a mouthful) in the Mojave Desert.  Each site is 23 meters in diameter, and, over a ten-year period (1997-2007), three of each were exposed to three different CO2 fumigation conditions: 1) no fumigation; 2) ambient CO2 fumigation (current); and 3) CO2 fumigation matching the level expected to be in the atmosphere around 2050 (elevated).

After the ten years, biomass and soil were collected and their carbon content was measured.  Significantly greater organic carbon was found in the elevated condition, demonstrating that the environment sucked up more organic carbon if more was in the surrounding atmosphere.  This was mainly found in the soil and not in plants, which the researchers say contrast more fertile ecosystems, which show increased plant mass with increased atmospheric CO2.  The research found that this increased carbon content in the soil was likely due to increased photosynthetic processes, as plants under elevated CO2 conditions almost double their photosynthetic rates.

This is one of the first studies to demonstrate significant carbon uptake in an arid ecosystem.  These findings should stimulate more research on carbon uptake in unexpected terrestrial environments (i.e., non-forested) so we can begin to get a clearer picture of the exact mechanisms of terrestrial capture.  This knowledge will be imperative as our civilization continues to expand into new territory, possibly cutting off more and more avenues of carbon uptake that will leave more CO2 in the atmosphere to expedite global warming.

Reference

 

ResearchBlogging.org

Evans, R., Koyama, A., Sonderegger, D., Charlet, T., Newingham, B., Fenstermaker, L., Harlow, B., Jin, V., Ogle, K., Smith, S., & Nowak, R. (2014). Greater ecosystem carbon in the Mojave Desert after ten years exposure to elevated CO2 Nature Climate Change DOI: 10.1038/nclimate2184

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