Nitrogen dynamics under elevated carbon dioxide

Elevated atmospheric CO₂ affects growth and yield which then affect processes controlling the supply and losses of N to sustain these increases. This research was undertaken to measure the effects of elevated CO₂ on crop N demand, fertilizer N recovery, symbiotic N₂ fixation, residual N availability, and greenhouse gas emissions from cropping systems in southern Australia (Horsham) and northern China using free-air CO₂ enrichment (FACE) facilities and glasshouse chambers. Elevated CO₂ generally increased crop biomass (11 to 84%) and grain yield (10 to 70%) across a range of crops, except when the wheat crop was grown under a hot and dry period, or when legumes experienced P deficiency. Results in the literature indicate that grain N removal worldwide is likely to increase by an average of 17% in crops grown under elevated CO₂.

Wheat was no more effective at sourcing N from fertilizer, so that the CO₂-induced increase in plant N uptake (18 to 44%) was satisfied mostly by increased uptake of indigenous N (19 to 50%) at both sites. A glasshouse experiment showed that incorporating crop residues lowered the recovery from soil. Under FACE conditions in Changping, elevated CO₂ increased the proportion (from 59 to 79%) and the amount (from 166 to 275 kg N/ha) of shoot N derived in soybean. A glasshouse experiment then showed that the rate of N fixation in chickpea, field pea and barrel medic under elevated CO₂ depended on P supply, with improved N fixation to CO₂ occurring only with adequate P supply. Elevated [CO₂] increased emissions of N₂O (108%), CO₂ (29%) and CH₄ from soil at Horsham, with changes being greater early in the season. At the Changping site, elevated [CO₂] increased N₂O (60%) and CO₂ (15%) emission, but had no significant effect on CH₄ flux. Elevated CO₂, therefore, can be expected to lead to higher overall N₂O emission by 27% and 75% under low N and high N inputs, respectively.

Results of the present research suggest that under future elevated CO₂ atmospheres there will be an increase in crop demand for N. To meet this demand requires higher fertilizer N application rates and the greater use of legume intercropping using locally appropriate agricultural management practices. Increases in the terrestrial C sink may be less than expected as CO₂-induced increase in greenhouse gas emissions will be significant as atmospheric CO₂ rises. ANZ-04