Modeling interactions among carbon dioxide, nitrogen, and climate on energy exchange of wheat in a free air carbon dioxide experiment

Changes in mass and energy exchange by crops under rising atmospheric CO(2) 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 COL Enrichment (FACE) experiment in which wheat (Triticum aestivum L.) was grown under 548 vs. 363 pmol 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 mu mol 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 mu mol 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.