青藏高原高寒草甸净生态系统碳交换对散射辐射变化的响应

青藏高原是地球上接收太阳辐射能最多的地区之一,具有世界上最高的高寒草甸生态系统,对区域乃至全球碳循环起着重要作用.为了探究太阳辐射变化对高寒草甸生态系统碳动态的影响,本研究利用涡度相关技术和微气象观测系统对高寒草甸生态系统CO₂净交换(NEE)、太阳总辐射、散射辐射及其相关环境要素进行观测;根据晴空指数(CI,到达地面的太阳辐射与大气上界太阳辐射的比值)将天空状况划分为晴天(CI≥0.7)、多云(0.32·m^-2·s^-1)对应的光量子通量密度(PPFD)约为1400 μmol·m^-2·s^-1,出现在CI为0.6～0.7范围内的多云天空,高于CI≥0.7的最高值(-0.57 mg CO₂·m^-2·s^-1)(NEE负值为碳吸收,正值为排放,为方便起见在此均用绝对值描述);CI<0.6条件下,NEE随散射辐射的增加呈显著的对数增加;CI在0.6～0.7范围内,NEE达到最大值,CI≥0.7时,NEE随CI的上升呈降低趋势,说明生态系统的光合作用可能出现了光抑制现象,且散射辐射的增加有利于提高生态系统固碳能力;生态系统呼吸(R_e)随温度升高呈明显的指数上升趋势,高寒草甸NEE最高值对应的温度为15 ℃,当温度高于15 ℃时,NEE随温度的升高呈下降趋势.晴天状况下,温度升高增加了R_e,进而降低了NEE.当饱和水汽压差(VPD)<0.6 kPa时,NEE随VPD增加呈增加趋势;当VPD>0.6 kPa时,NEE随VPD的升高呈缓慢下降趋势,说明相对较高的VPD抑制了生态系统的光合作用.晴天的强辐射并不能促进青藏高原高寒草甸的碳吸收能力,而晴空指数在0.6～0.7范围的多云天气最有利于生态系统碳固定.

Responses of net ecosystem carbon exchange to diffuse radiation in an alpine meadow on the Qinghai-Tibetan Plateau,China.

Qinghai-Tibetan Plateau, one of the regions on the earth that receives the most solar radiation, is the world’s highest alpine meadow ecosystem, with significance to regional and global carbon cycles. To examine the effects of solar radiation on ecosystem carbon dynamics in an alpine meadow, the net ecosystem CO₂ exchange (NEE), solar radiation, diffuse radiation, and related environmental variables were measured using eddy-covariance technique and micro-meteorological system. Sky conditions were divided into three categories of clear days (CI≥0.7), cloudy days (0.32·m^-2·s^-1) appeared in photosynthetic photon flux density (PPFD) of approximately 1400 μmol·m^-2·s^-1, corresponding the CI of 0.6-0.7 in cloudy days, which was higher than the maximum NEE in clear days (-0.57 mg CO₂·m^-2·s^-1) (negative and positive NEE represented carbon uptake and emission, respectively, here absolute value was used to describe NEE). With the increases of diffuse radiation, NEE increased logarithmically when CI<0.6, reached the highest value when CI ranged from 0.6 to 0.7, and then decreased when CI≥0.7, indicating that photo-inhibition might occur under high solar radiation conditions and increasing diffuse radiation would improve carbon sequestration of alpine meadow. Ecosystem respiration (R_e) increased exponentially with the increases of air temperature (T_a). The maximum NEE was found at the air temperature of about 15 ℃ from the relationship of NEE and air temperature, but NEE tended to decrease with increasing air temperature when T_a>15 ℃. Under clear sky day conditions, R_e increased with increasing CI due to the increases of air temperature, with negative effects on NEE. NEE increased with the increases of VPD up to 0.6 kPa, then slowly decreased when VPD>0.6 kPa, illustrating that NEE was reduced due to the relatively high VPD. Our results suggested that strong solar radiation on clear days would not increase carbon uptake capacity of alpine meadow, while cloudy days with clearness index of 0.6-0.7 would help increase carbon sequestration on the Qinghai-Tibetan Plateau.