Effect of Elevated Carbon Dioxide on Nitrogen Dynamics and Greenhouse Gas Emissions in Grain Crop and Legume Pasture Systems: FACE Experiments and a Meta-Analysis

By 2070, atmospheric carbon dioxide concentration [CO2] is expected to be double that observed in 1950. In this higher [CO2] world, the sustainability of global crop production may be in jeopardy unless current nitrogen (N) management strategies are changed because of potential interactions between elevated atmospheric [CO2] and soil N dynamics. However, these interactions are poorly understood especially in semi-arid cropping systems. In this paper, experimental results are presented on the effects of elevated [CO2] on crop N demand, fertilizer N recovery, symbiotic N2 fixation and greenhouse gas emissions from cropping systems in southern Australia and northern China using free-air CO2 enrichment (FACE) facilities. Also discussed are the findings of a meta-analytic review of current literature which estimated quantitatively the effects of elevated [CO2] on soil N dynamics in grain crop and legume pasture systems. Results of experiments reported here and the meta-analysis suggest that under future elevated CO2 atmospheres (i) there will be an increase in crop demand for N, (ii) higher fertilizer N application rates and greater use of legume intercropping using locally appropriate agricultural management practices to meet the additional crop N demand, and (iii) increases in the terrestrial C sink may be less than expected since there will be a significant increase in greenhouse gas emissions (CO2-equivalent) associated with an increase in atmospheric [CO2].