Maize, in rotation with soybean, forms the largest continuous ecosystem in temperate North America, therefore changes to the biosphere-atmosphere exchange of water vapor and energy of these crops are likely to have an impact on the Midwestern US climate and hydrological cycle. As a C 4 crop, maize photosynthesis is already CO 2-saturated at current CO 2 concentrations ([CO 2]) and the primary response of maize to elevated [CO 2] is decreased stomatal conductance ( gs). If maize photosynthesis is not stimulated in elevated [CO 2], then reduced gs is not offset by greater canopy leaf area, which could potentially result in a greater ET reduction relative to that previously reported in soybean, a C 3 species. The objective of this study is to quantify the impact of elevated [CO 2] on canopy energy and water fluxes of maize ( Zea mays). Maize was grown under ambient and elevated [CO 2] (550 mol mol -1 during 2004 and 2006 and 585 mol mol -1 during 2010) 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 measurements of sensible ( H) and soil heat fluxes, and net radiation. Relative to control, elevated [CO 2] decreased maize ET (7-11%; P<0.01) along with lesser soil moisture depletion, while H increased (25-30 W m -2; P<0.01) along with higher canopy temperature (0.5-0.6°C). This reduction in maize ET in elevated [CO 2] is approximately half that previously reported for soybean. A partitioning analysis showed that transpiration contributed less to total ET for maize compared to soybean, indicating a smaller role of stomata in dictating the ET response to elevated [CO 2]. Nonetheless, both maize and soybean had significantly decreased ET and increased H, highlighting the critical role of elevated [CO 2] in altering future hydrology and climate of the region that is extensively cropped with these species.
