9652613 Wall The uncertainty regarding whether plants and ecosystems will respond to elevated atmospheric CO2 when nutrients are lacking under field conditions is one of the most important questions facing global change research. To help resolve this quandary, two CO2 by soil nitrogen experiments are being conducted on the wheat ecosystem in an open field using the free-air CO2 enrichment (FACE) approach. This proposal covers two important components of the carbon cycle -- photosynthesis (source) and belowground biomass distribution (sink) -- and thus will insure that all important physiological processes that are explicitly influenced by atmospheric CO2 enrichment are incorporated within an accurate comprehensive plant growth model responsive to elevated CO2 and changing climate variables called ECOSYS. Besides validating ECOSYS with regard to the interactions of CO2 and soil nitrogen supply, other important objectives are to determine the interactive effects of elevated CO2 and limiting soil nitrogen supply on (a) acclimation (down-regulation) of photosynthesis and (b) root growth and plant-soil nutrient status. Measurements of individual leaf and whole-canopy photosynthesis, respiration, and transpiration rates are being made throughout the season to quantify the diurnal and temporal source of carbon into the wheat ecosystem. Individual leaf photosynthesis will be integrated to whole-plant and community level and compared with measurements of whole canopy CO2 and H2O vapor fluxes. Similarly, soil cores are being taken at several crop growth stages, frozen, and stored. Nitrogen and carbon content of the root tissue will also be ascertained to determine if any atmospheric CO2 concentration influences the C/N ratio.