采用涡度相关法对2005年生长季内蒙古锡林河流域羊草(Leymus chinensis)草原净生态系统交换(Net ecosystem exchange,NEE)进行了观测。观测结果表明：作为生长季降雨量仅有126 mm的干旱年，锡林河流域羊草草原生态系统受到强烈的干旱胁迫，其净生态系统碳交换的日动态表现为具有两个吸收高峰，净吸收峰值出现在8∶00和18∶00左右。最大的CO2吸收率为-0.38 mg CO2·m－2·s－1， 出现在6月底，与丰水年相比生态系统最大CO2吸收率下降了1倍。就整个生长季而言，不管是白天还是晚上2005年都表现为净CO2排放，整个生长季CO2净排放量为372.56 g CO2·m－2，是一个明显的CO2源。土壤含水量和土壤温度控制着生态系统CO2通量的大小，尤其是在白天，CO2通量和土壤含水量的变化呈现出显著的负相关关系，和土壤温度表现为正相关关系。
Background and Aims Many reports on global change have predicted major change in the temporal and spatial pattern of precipitation, which may have significant effects on temperate grasslands in arid and semi-arid regions. The responses of grasslands to environmental changes, especially amount and timing of precipitation, can be very different. Some studies indicate that drought may result in degradation of ecosystem function in NEE, even changing the ecosystem from a carbon sink to a carbon source.
Methods In order to quantify net ecosystem carbon exchange in Leymus chinensis steppe and its response to precipitation, we used the eddy covariance technique to measure carbon dioxide flux during the 2005 growing season in Xilin River Basin of Inner Mongolia Plateau in 2005. Only 126 mm precipitation fell during this growing season, far less than average; therefore, the steppe was in an extreme drought condition.
Key Results The daily pattern of CO2 uptake in this drought year was consistent bimodal, with peaks at 8∶00 and 18∶00. In normal years, the bimodal pattern occurred only when soil water stress occurred. Maximum half-hourly average CO2 uptake was -0.38 mg·m-2·s-1 in 2005, which was half that in typical growing seasons. Moreover, the ecosystem was a CO2 source most of the growing season, releasing about 0.05 mg CO2·m-2·s-1 at nighttime.
Conclusions The seasonal pattern of CO2 uptake closely followed that of aboveground biomass and was strongly affected by soil temperature and soil water content. The ecosystem emitted 372.56 g CO2·m-2during the growing season in 2005. The partial explanation is that much plant litter accumulated on the ground surface due to enclosure of the grassland since 1979, and this litter decomposed and resulted in a net release of CO2 to atmosphere.