| This thesis is in two parts, with each part presenting dynamic modeling of crop productivity. Part I. The CROPGRO crop growth model has been adapted to analyze experimental data for soybean crops grown under controlled growth chamber environments at elevated {dollar}rm COsb2{dollar} levels (1,200 {dollar}mu{dollar}mol {dollar}rm molsp{lcub}-1{rcub}).{dollar} Data on assimilate partitioning, plant development, photosynthesis, respiration, and canopy light absorption are used for model development and testing. Modified CROPGRO is used to model high pressure sodium and metal halide lamp experiments. Model simulations generally predict yield and biomass within {dollar}pm{dollar}10%, and crop life-cycle radiation-use efficiency within {dollar}{lcub}-{rcub}10{dollar} to 20% of the data. The model successfully reproduces available gas-exchange data. While it is found that accounting for incident diffuse light is necessary in order to properly characterize growth chamber environments, modified CROPGRO is not very sensitive to changes in the fraction of incident diffuse light over the range of 0.60 to 0.80, a typical range for growth chamber environments. Part II. A full assessment of the impacts of land clearance and crop production on atmospheric {dollar}rm COsb2{dollar} requires a systems approach. By considering long-term soil carbon changes and fossil fuel energy inputs, it is shown that increased crop productivity will alleviate {dollar}rm COsb2{dollar} release to the atmosphere primarily by preventing additional land cultivation. In model simulations of long-term scenarios, which consider population growth, high yield crop systems preclude the release of 50 to 200 Pg C to the atmosphere, depending on whether abandoned cropland is returned to nature, or converted to energy crops as a fossil fuel substitute, respectively. In these scenarios, the global land clearance avoided is 300 Mha. By avoiding land cultivation, high-yield crop systems thus preserve natural ecosystems. The water requirements for crop production in these scenarios depend on the land-yield combinations of irrigated and rainfed cropland, and crop water-use and irrigation efficiencies. An ongoing commitment to improve crop productivity may thus result in long-term environmental benefits, which should be included in environmental impact analyses of increased crop yields. |
