Free-air CO₂ enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat

In order to determine the likely effects of the increasing atmospheric CO₂ concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 μmol/mol CO₂ (or 200 μmol/mol above current ambient levels of about 360 μmol/mol) using a free-air CO₂ enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, R(n); soil heat flux; air and soil temperatures; Canopy temperature, T(s); and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime T(s) about 0.6°and 1.1°C at high and low N, respectively. Daily total R(n) was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.