Biophysical impact from increasing carbon dioxide on U.S. agroecosystems

Increasing concentrations of atmospheric carbon dioxide will have impacts on many different aspects of the climate system including the surface energy budget. Several years of climatic and biological data have been collected for two major U.S. crops, corn and soybean, at the Soybean Free Air Concentration Enrichment (SoyFACE) site in Champaign, Illinois. Using these data we calibrated the Agro-IBIS (Integrated Biosphere Simulator, agricultural version) model to simulate the crop response to a CO2 enriched environment of 550 ppm. Agro-IBIS accounts for the different photosynthetic pathways in soybean (C3) and corn (C4). Previously the model overpredicted the CO2 fertilization effect by overestimating the leaf area index (LAI) of soybean. Realistic simulated LAI values are necessary for accurate simulation of transpiration, one component of the latent heat flux. We found that improving the phenology routine and adjusting the specific leaf area parameter results in a simulated LAI value that compares with the observations within the enriched environment. Due to limitations of the model calculations, we also decreased the stomatal conductance an additional 40% to simulate realistic transpiration values. After validation at the SoyFACE site, we ran Agro-IBIS over the U.S. east of the Rocky Mountains with current and elevated CO2 concentrations. Here we show the impact that the response of soybean to elevated CO2 is expected to have on the net radiation, soil heat flux, and latent and sensible heat fluxes across this domain. These predicted changes to the energy budget are important and need to be considered in all climate models.