Submitted to: The Earth's Changing Land Global Change and Terrestrial Ecosystems and Land
Publication Type: Abstract Only
Publication Acceptance Date: May 26, 1998
Publication Date: N/A
Spring wheat (Triticum aestivum L. cv Yecora rojo) was exposed to CO2 concentrations enriched to 200 umol/mol above ambient (ca. 370 umol/mol) using free-air CO2 enrichment (FACE) apparatus in an open field. Half of each main CO2 plot received ample nitrogen (350 kg N/ha), while the other half received 70 kg N/ha, intended to provide a low-N stress. Net radiation (Rn) was measured with net radiometers, soil heat flux (G) with soil heat flux plates, soil temperature with thermocouples, canopy temperature with carefully calibrated infrared thermometers, air wet and dry bulb temperatures with psychrometers, leaf wetness with conductance plates, and wind speed with cup anemometers. Sensible heat flux (H) was determined from the temperature difference between crop surface and air divided by an aerodynamic resistance calculated from wind speed. Evapotranspiration (ET) was determined as latent heat flux (LET) from the residual of net radiation minus soil heat flux minus sensible heat flux, where L is the latent heat of vaporization. LET = Rn - G - H. The elevated CO2 concentration in the FACE plots increased foliage temperatures by 0.6 and 1.1 degrees C at high and low N, respectively, during the daytime for much of the growing season. Soil heat flux (G) was generally very small, sensible heat flux (H) was moderate, and net radiation (Rn) was the largest component of the energy balance, so latent heat flux (LET) tended to follow net radiation. However, associated with the increases in foliage temperature, there were small but consistent increases in sensible heat flux. Comparing daily totals, the FACE treatment reduced ET about 6 and 16 percent at the high and low nitrogen levels, respectively.