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

Abstract

Abstract. In order to determine the likely effects of the increasing atmospheric GO 2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550/xmol/mol CO2 (or 200/xmol/mol above current ambient levels of about 360/xmol/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, Ts; 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 ø and 1.1øC 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. IntroductionThe CO 2 concentration of the atmosphere is increasing, and climate modelers have predicted a consequent global warming as well as changes in precipitation patterns. The report of the IPCC [Intergovernmental Panel on Climate Change, 1996] projects CO2 increasing from present day concentrations of about 360/xmol/mol to about 500/xmol/mol by the end of the next century if emissions are maintained at 1994 levels. They further project that the increase in CO2 plus that of other radiatively active "greenhouse" gases (methane, nitrous oxide, chlorofluorocarbons (CFCs), ozone) will cause an increase in global mean temperature of 0.9 ø to 3.5øC depending on future emission rates. Some regions might receive increases in precipitation, while others might receive less. However, these projected changes in climate are very uncertain.Increasing CO2 concentration has been shown to cause partial closure of plant leaf stomata, which reduces the conductance of water vapor from inside the leaf stomatal cavities to the outside air [Morison, 1987] [1997] exposed grassland to elevated CO2 using open-top chambers and attempted to measure ET with smaller gas-exchange chambers. They found reductions of ET of 12-39% due to elevated CO2 on sandstone-derived soil, but on serpentinederived soil, ET actually increased from -1% to + 14%. They believe that the latter increase is because the serpentine canopy was sparse, so there was more E (evaporation from soil) than T, and E would not be affected by CO2. Thus the prior experimental work using chambers has been somewhat variable but explainable based on leaf area growth, canopy, and stomatal effects. Except for the results of Dugas et al. [1997], however, generally the effects of CO2 on ET have been small. 1179

Projected Dryland Cropping System Shifts in the Pacific Northwest in Response to Climate Change

“…With very limited water availability in the CF zone, winter wheat utilized the projected additional winter precipitation (two winter seasons for one crop season) to support increased yields; thus transpiration increased slightly from the baseline value of 146 mm to 150 and 156 mm in 2070s under RCP 4.5 and 8.5, respectively. Decreased evapotranspiration driven by elevated CO 2 (and unmodified temperature) has been documented in FACE experiments (e.g., Kimball et al, 1999). To understand crop yield responses to future conditions, it is also of interest to consider the spatial distribution of crop transpiration-use efficiency (TUE = g above-ground biomass produced per kg of water transpired), shown for winter wheat in Figure 3.…”

Global effects of doubled atmospheric CO2 content on evapotranspiration, soil moisture and runoff under potential natural vegetation

“…Since such structural vegetation responses almost necessarily accompany the physiological effect of increased [CO 2 ], the decrease in stomatal conductance induced by the latter is usually not fully translated into lower canopy-and ecosystem-level transpiration. It is still under debate how large the net effect of physiological and structural vegetation responses to atmospheric CO 2 enrichment might be upon water balances at scales from local (Kimball et al, 1999;Hui et al, 2001) to global (Betts et al, 1997;Kergoat et al, 2002). Open questions are, for instance, whether the global net effect on plant transpiration would be positive or negative; how this effect might vary between regions and between seasons; and to what degree the transpirational effects would be translated into changes in other components of the terrestrial water balance, such as runoff.…”

Effect of a high-end CO2-emission scenario on hydrology

“…Increases in CO 2 -concentrations have been shown to affect the stomatal conductance of the leaf surface for plants (Kimball et al 1993), meaning that higher levels of CO 2 allow the plant to reduce the stomata opening, thus lowering water vapor loss resulting in reduced evapotranspiration (Samarakoon & Gifford 1995, Kimball et al 1999, Conley et al 2001, Krujit et al 2008). This response is somewhat uncertain as other influences are still debated.…”