Future carbon dioxide concentration decreases canopy evapotranspiration and soil water depletion by field grown maize

As atmospheric concentrations of carbon dioxide (CO2) are increasing at a rate faster than the ‘worst case scenarios' presented by the Intergovernmental Panel on Climate Change (IPCC) it is increasingly important to understand the implications of rising CO2 on major ecosystems. Together with soybean, maize is the dominant species in the largest continuous ecosystem type in temperate North America. Thus, any influence of atmospheric changes on maize is likely to have a significant impact on the regional hydrological cycle. As a C4 crop, photosynthesis in maize is already CO2-saturated at current day CO2 concentrations and the primary response of maize to elevated CO2 is decreased stomatal conductance. In the absence of stimulated photosynthesis and productivity at elevated CO2, reduced stomatal conductance is not offset by greater canopy leaf area and so reductions in evapotranspiration (ET) and changes in hydrology have the potential to be greater than for C3 species. Thus, the objective of this study was to quantify the impact of elevated CO2 on ecosystem energy fluxes and water use of maize (Zea mays). Maize was grown under ambient CO2 (376 ìmol mol-1) and elevated CO2 (550 ìmol mol-1) during 2004 and 2006 using Free-Air Concentration Enrichment (FACE) technology at the SoyFACE facility in Urbana, Illinois. Maize ET was determined using a residual energy balance approach based on micrometeorological measurements of sensible heat flux, soil heat flux and net radiation. When maize was grown in elevated CO2, ET decreased (p<0.01) resulting in 10 % reduction in canopy water use and decreased soil moisture depletion; while sensible heat flux increased (p<0.01) by 20 % along with increased canopy temperature up to 0.3 °C. The present findings are consistent with previous experimental results of soybean grown at same elevated CO2 level. Coupled with similar responses of soybean to elevated CO2, the decrease in ET and subsequent increases in sensible heat flux highlight the critical role of elevated CO2 in altering future hydrology and climate of the region that is extensively cropped with these species.