散射辐射对西藏高原高寒草甸净生态系统CO_2交换的影响

Influence of diffuse radiation on the net CO_2 exchange of alpine meadow ecosystem on Tibet Plateau

2003-2006年在当雄草原站用涡度相关法对西藏高原广泛分布的高寒草甸生态系统的碳通量和常规气象数据进行了连续观测。基于这些数据,根据净生态系统CO2交换量（NEE）对晴朗指数（k）和土壤温度的响应特征,分析了净生态系统CO2交换量与散射辐射之间的关系。依据地面接受的散射辐射量把天气划分为云隙天、晴天和多云天。结果表明,散射辐射不能提高西藏高原高寒草甸生态系统的碳吸收水平。该生态系统的碳收支过程主要受光合有效辐射控制,碳排放过程主要受土壤温度控制;且NEE随k的变化趋势受散射辐射的影响较小,生态系统碳收支更多地受太阳辐射对土壤强烈加温的影响。三种散射辐射天气条件下,NEE随k的变化趋势基本一致,先增加后减小;NEE达最大值时的土壤温度皆为15℃左右,k值皆为0.7-0.8。

The net CO2 exchange between the atmosphere and alpine meadow ecosystem （NEE） and standard meteorological parameters were measured by Eddy covariance technique in Damxung County, Tibet from 2004 to 2006. Based on the data, net CO2 exchange response characteristic of alpine meadow ecosystem on Tibet Plateau to diffuse radiation was analyzed. The ground conditions can be separated into three types： cloud-gap sky condition, clear-sky condition and overcast-sky condition by the clearness index as a criterion. Although in cloud-gap and overcast situations the diffuse radiation can be enhanced, the ground surface under a cloud-gap sky receives the same amount of direct beam radiation as under a clear sky, while under overcast sky, the direct beam radiation is reduced. We compared the enhanced NEE from three sky conditions for 5° interval of solar elevation angles and 5℃ interval of soil temperature. The results show that the scatterplots between the net ecosystem CO2 exchange and clear index present a different pattern in three ground conditions. It is shown that, no matter whether the site is under a clear sky, a cloud-gap sky or an overcast sky, the carbon uptake does not tend to be enhanced. That is to say, the trend of change on the net ecosystem CO2 exchange with the clear index was not affected hy diffuse radiation, but affected by coupling effect of the solar radiation intensity and the soil temperature. During the daytime, photosynthetically active radiation dominated the carbon absorption, and soil temperature dominated the carbon emission. Generally speaking, the maximal NEE occurs under three skies with the soil temperature at 15℃, and the clearness index between 0.7 and 0.8.