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MODELING

Modeling Interactions among Carbon Dioxide, Nitrogen, and Climate on Energy Exchange of Wheat in a Free Air Carbon Dioxide Experiment

^a Dep. of Renewable Resources, Univ. of Alberta, Edmonton, AB, Canada T6G 2H1
^b USDA-ARS, U.S. Water Conserv. Lab., 4331 E. Broadway, Phoenix, AZ 85040
^c Laboratory of Tree Ring Res., Univ. of Arizona, Tucson, AZ
^d Dep. of Soil, Water, and Environ. Sci., Univ. of Arizona, Tucson, AZ

Corresponding author (robert.grant{at}ualberta.ca)

Received for publication June 13, 2000. Changes in mass and energy exchange by crops under rising atmospheric CO₂ concentration (C_a) may be affected by N and weather; C_a interacts with weather on mass and energy exchange through limitations on latent heat flux imposed by stomatal conductance, which is affected by C_a, and aerodynamic conductance, which is affected by weather. We examined the bases for these interactions with the ecosystem model ecosys. Simulation results were tested with energy flux data from a Free Air CO₂ Enrichment (FACE) experiment in which wheat (Triticum aestivum L.) was grown under 548 vs. 363 µmol mol^-1 C_a and fertilized with 7 vs. 35 g N m^-2. Both model and experimental results indicated that raising C_a from 363 to 548 µmol mol^-1 reduced midday latent heat fluxes by ca. 50 W m^-2 for wheat fertilized with 35 g N m^-2, and by ca. 100 W m^-2 for wheat fertilized with only 7 g N m^-2 when N deficits developed later in the growing season. These reductions were smaller under low wind speeds (<5 km h^-1) and stable boundary conditions when aerodynamic conductance became the dominant constraint to transpiration. At a seasonal time scale, raising C_a from 363 to 548 µmol mol^-1 reduced simulated (measured) evapotranspiration of wheat by 9% (7%) when fertilized with 35 g N m^-2, and by 16% (19%) with 7 g N m^-2. Changes with C_a in mass and energy exchange used in climate change studies should therefore reflect the site-specific availability of N, as well as climate attributes such as wind speed.

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