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Effects of Elevated CO2 on C3 Grass Microcosms: Implications for Global Carbon Cycling
Lutze, J.L. and Gifford, R.M.  1998.  Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth.  Global Change Biology 4: 851-861.

What was done
The authors grew microcosms of the C3 grass Danthonia richardsonii for four years in glasshouses with atmospheric CO2 concentrations of 360 or 720 ppm and three levels of soil nitrogen in order to determine the effects of elevated CO2 and soil nutrition on microcosm carbon gain and water use.  Destructive harvests of plant material occurred at six-month intervals throughout the project.

What was learned
Carbon accumulation in microcosms was strongly correlated with soil nitrogen; thus, nitrogen was limiting productivity, even in the highest fertilization treatment.  Nevertheless, when averaged across all harvests and nitrogen regimes, elevated CO2 significantly increased total microcosm carbon gains by 15-34%, in spite of a 9% reduction in leaf area index.  In addition, elevated CO2 increased senesced leaf material per unit ground area at all harvests and nitrogen treatments by an average of 31%.  Consequently, these phenomena led to final soil carbon contents in CO2-enriched microcosms that were 4, 9, and 17% greater than what was observed in ambient microcosms at low, medium, and high soil nitrogen treatments, respectively.  Elevated CO2 also reduced microcosm water use by 25% across all nitrogen treatments, thereby allowing greater soil volumetric water contents to exist in CO2-enriched microcosms.

What it means
As the amount of CO2 in the air increases, swards of Danthonia richardsonii will likely respond by increasing total community carbon gains, even if soil nitrogen is limiting to growth.  In addition, reductions in water use and corresponding increases in soil water content at elevated CO2 concentrations may have "important implications for microcosm, and potentially ecosystem, function perhaps as important as those of CO2 directly on photosynthesis."  In analyzing the smallest microcosm carbon gain (15% at low nitrogen), the authors concluded that if all terrestrial ecosystems responded to elevated CO2 in a similar way, this phenomenon would account for all of the "missing carbon" depicted in most global carbon cycle models.

Reviewed 15 April 1999