Nitrogen dynamics under elevated carbon dioxide

How will high carbon dioxide concentrations affect the cycling of nitrogen in crop production systems.


26 Feb 2012

2011 Annual Interpretive Summary

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