Effect of solar radiation on net ecosystem CO2 exchange of alpine meadow on the Tibetan Plateau

On the Tibetan Plateau, the alpine meadow is the most widespread vegetation type. The alpine meadow has a low biological productivity and low vegetation coverage in the growing season. The daytime NEE between the atmosphere and the alpine meadow ecosystem was influenced by solar radiation. To analyze the characteristics of change in NEE and to calculate the parameters related to photosynthesis and respiration in different solar radiation environments, the NEE measurements were taken in Damxung from July to August in 2003, 2004, 2005 and 2006 using the eddy covariance technique. Solar radiation was grouped into three levels according to the net radiation, which was more than 155 W m(-2) d(-1) on clear days, 144 +/- 5W m(-2) d(-1) on partly cloudy days and less than 134 W m(-2) d(-1) on cloudy days. The diurnal relationships between NEE and PAR varied with differences in solar radiation, which was a rectangular hyperbola form on clear days, two different concave curves on partly cloudy days and an irregular triangle form on cloudy days. The mean CO2 absorption rate showed a decreasing trend with increasing solar radiation. The daytime absorption maximum occurred around 10:00 on clear days with an average of slightly less -0.2 mg m(-2) d(-1), around 11:00 on partly cloudy days with an average of about -0.2 mg m(-2) d(-1), and around 12:00 on cloudy days with an average of about -0.25 mg m(-2) d(-1). As solar radiation increased, the A(max) and the Q(10) decreased. However, the R-10 increased and the maximum of the alpha occurred on partly cloudy days. The optimum net solar radiation was about 134-155W m(-2) d(-1), which induced a PAR of about 1800-2000 mu mol m(-2) s(-1) and soil temperature at a depth of 5 cm of about 14 degrees C. Therefore, on the Tibetan Plateau, the alpine meadow ecosystem will have a higher carbon absorption potential while solar radiation decreases in the future.