Abstract: Grassland is a dominant vegetation type on the Yunnan-Guizhou Plateau, and comparatively little is known about the CO₂ flux and its diurnal and seasonal variation. Based on in-situ measurement using eddy covariance methodology from October 2017 to August 2018, the diurnal cycle and seasonal variations of CO₂ flux for a grassland ecosystem of Yunnan-Guizhou Plateau were analyzed, and key factors determining CO₂ flux were determined. The CO₂ flux had obvious diurnal variation, with net uptake during the day and net emission at night, and the diurnal cycle being more significant in growing season than in the non-growing season. Overall, the grassland ecosystem was a sink of atmospheric CO₂, which also varied seasonally with high uptake rate in spring and summer and low rate in autumn and winter. Specifically, the peak CO₂ uptake rate occurred in June and lowest was observed in January. The diurnal and seasonal variation in the CO₂ flux were associated with solar radiation, air temperature, soil temperature and wind speed fluctuations. Global warming would enhance the carbon sequestration capacity of the grassland ecosystem, given the CO₂ flux was sensitive to temperature. Our results suggested that the grassland ecosystem acted as carbon sink and absorbed 425.14 g CO₂·m^-2 from the atmosphere during the study period.

Key words: Yunnan-Guizhou Plateau, grassland ecosystem, CO₂ flux, environmental factors