This project was conducted to extend our knowledge of ecosystem-level carbon and water vapor fluxes. A major goal of the proposed study was to determine the extent to which C₄-dominated grasslands would sequester carbon and would act as a buffer against future increases in atmospheric CO₂ through a better understanding of the carbon and nitrogen interactions with soil microbes. Our approach centered on water use efficiency changes associated with elevated CO₂. We monitored above- and belowground biomass production, net carbon exchange and evapotranspiration at the chamber level with natural precipitation in the original nine plots and manipulated water availability in four experimental chambers. Grazing impact was not assessed in the current study, but, because of the importance of herbivory in natural systems, we determined the effect of elevated CO₂ on regrowth biomass accumulation, water relations, and physiology following top removal.

The experimental site for the tallgrass prairie research was located in pristine tallgrass prairie north of and adjacent to the Kansas State University campus. Vegetation on the site was a mixture of C₃ and C₄ species and was dominated by big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash]. Subdominants included Kentucky bluegrass (Poa pratensis L.), sideoats grama [Bouteloua curtipendula (Michx.) Torr.], and tall dropseed [Sporobolus asper var. asper (Michx.) Kunth]. Members of the sedge family made up 5-10% of the composition. Principal forbs included ironweed [Vernonia baldwinii subsp. interior(Small) Faust], western ragweed (Ambrosia psilostachya DC.), Louisiana sagewort (Artemesia ludoviciana Nutt.), and manyflower scurfpea [Psoralea tenuiflora var. floribunda (Nutt.) Rydb.]. Average peak biomass occurs in early August at 425 g m^-2 of which 35 g m^-2 is from forbs. Soils in the area are transitional from Ustolls to Udolls (Tully series: fine, mixed, mesic, montmorillonitic, Pachic Argiustolls). Slope on the area is 5%. Fire has been infrequent, occurring two to three times in 10 years. Past history has included primarily winter grazing by cow-calf pairs. The 30-year average annual precipitation is 84 cm, with 52 cm occurring during the growing season.