Effects Of Elevated CO₂ On Greenhouse Gases Emissions From Rice-Wheat Rotation Cropland

FACE (Free-air CO₂ enrichment) facilities provide an optimum experimental platform for studying the effects of elevated CO₂ on ecosystem processes, since disturbance effects imposed by controlled-environment facilities, such as open top chambers, are avoided. In this study, we employed FACE facilities for investigating the effects of N and straw applications on CH₄ and N₂O emissions from rice-wheat rotation cropland and CO₂ exchange between the cropland and atmosphere under elevating the present atmospheric CO₂ by 200 μmol mol^-1. Wheat straw incorporation into soil stimulated CH₄ emission during the early rice growth season. The stimulation of CH₄ emission by wheat straw incorporation was decreased by the elevated CO₂ following 150kgN ha^-1 N application in rice growth season. CH₄ emission was enhanced by the elevated CO₂. Under the ambient CO₂, applying urea-based N fertilizers at a rate of 250 kg N ha^-1 decreased the seasonal CH₄ emission in comparison with the 150 kg N ha^-1 treatment. However, under the elevated CO₂, the high application rate of N fertilizer further increased the seasonal CH₄ emission in comparison with the low level N amendment treatment. In wheat growth season, the elevated CO₂ would stimulate CH₄ absorption by dry aerobic soil from atmosphere. The amount of CH₄ absorption by soil in wheat growth season was less than 3% that of CH₄ emission from cropland in rice growth season. In the early rice growth season, wheat straw application decreased N₂O emission from the cropland under the ambient CO₂ while increased N₂O emission under the elevated CO₂. N₂O emitted more from the cropland in 2001-2002 rotation than in 2002-2003 rotation. The elevated CO₂ had no significant impact on N₂O emission in the first rotation while significantly decreased N2O emission in the second rotation. Dark respiration rate of aboveground rice plant expressed on the basis of dry weight biomass was not significantly affected by the elevated CO2 throughout the entire growth season. As the elevated CO2 significantly stimulated rice growth, Dark respiration rate of aboveground rice plant expressed on the basis of growth ground area was increased under the elevated CO2. Finally, the results indicate that the elevated CO2 stimulated carbon sequestration from atmosphere to rice ecosystem.