Carbon dioxide exchange between an alpine steppe ecosystem and the atmosphere on the Nam Co area of the Tibetan Plateau

The net ecosystem carbon dioxide (CO2) exchange (NEE) between the atmosphere and an alpine steppe ecosystem was measured by the Eddy covariance (EC) method on the Nam Co area of the Tibetan Plateau during the growing season in 2008 and 2009. The diurnal amplitude of NEE varied substantially during the growing seasons. The maximum CO2 uptake rates were 3.74 and 3.44 μmol CO2 m−2 s−1 in August 2008 and 2009, respectively. The peak daily CO2 uptake was observed on 16 July (DOY 198, 1.19 g C m−2 day−1) 2008, which occurred one month earlier than that in 2009 (DOY 228, 1.12 g C m−2day−1). This indicated that the alpine steppe had lower carbon sequestration potential comparing with other grasslands. The daily NEE responded to AGB in a linear manner (NEE = (−0.0073 ± 0.0027) × AGB) + (0.0533 ± 0.2049), R2 = 0.52). Fifty-two percent of the NEE variance could be explained by the variance of AGB. The amount and pattern of precipitation was significantly different between the two growing seasons. In 2008, the steppe received 493 mm from May to September. In contrast, the value was 327 mm in 2009 and more than 50% of this precipitation was received in August. The alpine steppe ecosystem became a weak carbon source in June 2009 (0.48 g C m−2) due to relatively low soil volumetric water content (VWC) and the relatively high temperature. The cumulative NEE for the growing season from May to September was 63.17 and 35.51 g C m−2 in 2008 and 2009, respectively. Variations in soil moisture had important effect on carbon exchange in this alpine steppe ecosystem. Both the photosynthesis and respiration were active under high soil moisture content, and suppressed during periods of water shortage.