Nitrogen dynamics under elevated carbon dioxide

The effect of elevated carbon dioxide concentration ([CO₂]) on greenhouse gas emissions from semi-arid cropping systems is poorly understood. Two studies were reported during 2010, one from the Australian Grains FACE experiment, and the other from the CAAS China mini-FACE project.

Closed static chambers were used to measure the short-term fluxes of nitrous oxide (N2O), CO2 and methane (CH₄) at the Australian Grains Free-Air Carbon dioxide Enrichment (AGFACE) facility within a spring wheat (Triticum aestivum L. cv. Yitpi) crop in southern Australia. Gas measurements were conducted at various key growth stages of wheat, viz., stem elongation, booting, anthesis, dough development and ripening. The targeted atmospheric [CO₂]s were 380 (ambient) and 550 (elevated) mol mol–1 for both rainfed and supplementary irrigated treatments. Elevated [CO₂] increased the emissions of N₂O and CO₂ by 101% and 37-42%, respectively, during the vegetative stage of wheat, but there were no significant effects measured during the remainder of the season. At this vegetative stage, elevated [CO₂] increased CH₄ emissions from 2.5 to 23.3 g C m–2 h–1. There was a small but significant interaction between elevated [CO₂] and supplementary irrigation on CO₂ flux, but not for the fluxes of N₂O and CH₄ flux. The results suggest that, under future atmospheric [CO₂], agricultural greenhouse gas emissions at the vegetative stage will increase, whereas supplementary irrigation is unlikely to affect the emissions from semi-arid cropping systems.

The fluxes of N₂O, CO₂ and CH₄ from a semi-arid wheat cropping systems were measured using closed chambers under ambient (420 18 mol mol–1) and elevated (565 37 mol mol–1) at the Free-Air Carbon dioxide Enrichment experimental facility in northern China. Measurements were made over five weeks on a wheat crop (Triticum aestivum L. cv. Zhongmai 175). Elevated [CO₂] increased N₂O and CO₂ emission from soil by 60% and 15%, respectively, but had no significant effect on CH4 flux. There was no significant interaction between [CO₂] and N application rate on these gas fluxes, probably because soil N was not limiting. At least 22% increase in C storage is required to offset the observed increase in greenhouse gas emissions under elevated [CO₂]. The data from this experiment was published in Soil Biology and Biochemistry 43, 458-461. ANZ-04