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 (N2 O) 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 N2 O 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.