Shortgrass Steppe Ecosystem Dynamics and Trace Gas Exchange Under Elevated CO2

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9524068 Future atmospheric carbon dioxide concentrations will almost certainly be at least twice as high as current concentrations. It is crucial that research address how such a dramatic change will affect ecosystems, especially those needed for food production. The vast semiarid grasslands of the Great Plains has been grazed extensively for thousands of years, and is comprised of grass species that have "normal" (C3) and water-efficient (C4) photosynthetic pathways. It is not known how elevated atmospheric carbon dioxide concentration will affect the shortgrass steppe ecosystem of the western United States. Increasing carbon dioxide concentrations will likely impact the productivity of arid and semiarid ecosystems through increased carbon, nitrogen and water use efficiencies in individual plants and may affect the competitive balance between C3 and C4 plants because of their different responses. Changing patterns of growth, nutrient use, and species composition may alter soil nitrogen and carbon cycles and affect the long term viability of the grassland. This research couples the use of large, open-top chambers for field carbon dioxide enrichment with studies of plant physiology and soil nutrient balance through soil nitrogen cycling and trace gas flux measurements. There are three basic objectives in this research. First, it will determine the impact of doubling carbon dioxide on photosynthesis, productivity, water and nitrogen use efficiency, plant water relations, and allocation of carbon and nitrogen above- and below-ground in two dominant grass species, one a C3 grass and the other a C4 grass. Second, it will determine the impact of doubling carbon dioxide on soil water and nitrogen dynamics. The knowledge gained from these studies will be incorporated into simulation models that will allow for realistic extrapolation through tim e and space of soil moisture, nutrient cycling, plant productivity, and overall ecosystem response. This program will provide insight into the effect of elevated carbon dioxide on this ecosystem to help maintain long term viability of the shortgrass steppe.