Nitrogen Dynamics in Grain Crop and Legume Pasture Systems Under Elevated Carbon Dioxide: A Meta-Analysis.

Understanding nitrogen (N) removal and replenishment is crucial to crop sustainability under rising atmospheric carbon dioxide concentration ([CO₂]). The effects of elevated [CO₂] on N dynamics in grain crop and legume pasture systems were evaluated using meta-analytic techniques (366 observations from 127 studies). The information analysed for non-legume crops included grain N removal, residue C:N ratio, nitrous oxide (N₂O) emission and fertilizer N recovery. In addition to these parameters, nodule number and mass, nitrogenase activity, the percentage and amount of N fixed from the atmosphere were also assessed in legumes. Elevated [CO₂] increased grain N removal (11–36%) of C₃ non-legumes, legumes and C₄ crops. The C:N ratio of residues from C₃ non-legumes and legumes increased under elevated [CO₂] by 16% and 8%, respectively, but the increase for C₄ crops (9%) was not statistically significant. Elevated [CO₂] increased the plant uptake of fertilizer N by 17%, and N₂O emission by 27%. Under elevated [CO₂], there was a 38% increase in the amount of N fixed from the atmosphere by legumes, which was accompanied by greater whole plant nodule number (33%), nodule mass (39%), nitrogenase activity (37%) and %N derived from the atmosphere (10%; non-significant). These results suggest that N demand and removal in grain cropping systems will increase under future CO₂-enriched environments, and that current N management practice needs to be revised.