N2O, CO2 and CH4 Fluxes Under Elevated CO2 From Wheat Cropping System in Semi-Arid Environment in Southern Australia and Northern China.

The effect of elevated carbon dioxide concentration ([CO₂]) on greenhouse gas emissions from semi-arid cropping systems is poorly understood. The fluxes of nitrous oxide (N₂O), CO₂ and methane (CH₄) from wheat (Triticum aestivum L.) field were measured using closed chambers under ambient and elevated (550 mmol mol^–1) [CO₂] at the FACE experimental facilities at Horsham (in southern Australia) and Changping (in northern China). At the Horsham site, elevated [CO₂] increased soil emissions of N₂O and CO₂ by 101% and 37–42%, respectively, during the vegetative growth 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 mg C m^–2 h^–1. While supplementary irrigation tended to reduce N₂O emissions when averaged across [CO₂] treatments, it increased CO₂ flux only at ambient [CO₂], but had no impact on CH₄ flux. At the Changping site, N₂O increased by 60% and CO₂ by 15% under elevated [CO₂] compared to ambient [CO₂], but there was no significant effect on CH₄ fluxes. Significantly more N₂O (37–1812%) and CO₂ (69%) was emitted under high fertilizer-N (188 kg N ha^–1) than low N (84 kg N ha^–1) input to irrigated plots in China. There was no significant interaction between [CO₂] and N application rate on the emission of the three gases. The positive feedback mechanism between elevated [CO₂] and the fluxes of the three gases at both sites implies the contribution of agricultural soils to global warming in future may be greater than current predictions. While supplementary irrigation tends to reduce the stimulatory effect of elevated [CO₂] on greenhouse gas production from Australian dryland agriculture, excessive N application by Chinese farmers will likely offset part of the extra C sequestered in the soil/plant system.

Keywords

Free-air carbon dioxide enrichment (FACE), nitrous oxide, carbon dioxide, methane, supplementary irrigation, nitrogen fertilizer, wheat soil