Nitrogen dynamics in grain crop and legume pasture systems under elevated atmospheric carbon dioxide concentration: A meta-analysis.

Understanding nitrogen (N) removal and replenishment is crucial to crop sustainability under rising atmospheric carbon dioxide concentration ([ CO₂]). While a significant portion of N is removed in grains, the soil N taken from agroecosystems can be replenished by fertilizer application and N₂ fixation by legumes. 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, fertilizer N recovery and nitrous oxide (N₂O) emission. 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 of C₃ non-legumes (11%), legumes (36%) and C₄ crops (14%). 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. 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). Elevated [ CO₂] increased the plant uptake of fertilizer N by 17%, and N₂O emission by 27%. These results suggest that N demand and removal in grain cropping systems will increase under future CO₂-enriched environments, and that current N management practices (fertilizer application and legume incorporation) will need to be revised. [ABSTRACT FROM AUTHOR]