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降雨的年际变化可导致碳通量显著变化。因此我们利用涡度相关(EC)技术观测西藏高原北部的一个高寒草甸连续2年(2005–2006)的CO2净生态系统交换(NEE),以分析不同降水年份下CO_2通量的差异和控制因素。2005和2006年的降水量分别为489.9 mm和241.1 mm,相比于476.0 mm的多年平均值,前者属于平水年,后者属于干旱年。2006年的NEE的年际累积表现为净排放量(87.70 g C m~(-2) yr~(-1)),而2005年则表现为非常微弱的吸收(-2.35 g C m~(-2) yr~(-1))。因此该高寒草甸在平水年是碳中性的,但在干旱年是碳源,这说明如果未来气候变暖继续恶化加剧土壤干旱的条件下,高寒草甸有可能成为一个CO_2释放源。在干旱年份,总初级生产力(GEE)、叶面积指数(LAI)以及生态系统碳吸收持续的时间都明显降低,由此引发干旱年份生长季旺盛时期每日NEE最大吸收速率、最大光合速率(Pmax)以及表观量子效率(α)只是平水年的30%–50%。在其他因子的调控方面,半小时尺度的GEE和NEE与光合有效辐射(PAR)密切相关,但这种响应会受空气温度(T_a),土壤水分含量(SWC)和水汽压亏缺(VPD)的影响。NEE的吸收速率会随着T_a和VPD的升高以及SWC的下降而减少。当PAR超过合适的范围值时,由于较高的辐射加剧了土壤干旱的情况,会减少白天NEE的吸收速率。NEE吸收速率的最适T_a和VPD值分别为12.7℃和0.42 KPa,而且NEE的吸收速率也会随着SWC的增加而增大。LAI的季节变异能够解释GEE和NEE变异的77%。半小时尺度上的生态系统呼吸(R_e)的变异主要依赖于土壤温度(T_s),但SWC会在一定程度上调控R_e对T_s的响应。
Inter-annual rainfall changes can lead to significant changes in carbon flux. Therefore, we used the eddy covariance (EC) technique to observe the CO2 net ecosystem exchange (NEE) of an alpine meadow in the northern part of the Tibetan Plateau for two consecutive years (2005-2006) to analyze the differences and control of CO2 fluxes under different precipitation years factor. The precipitation in 2005 and 2006 were 489.9 mm and 241.1 mm respectively. Compared with the multi-year average of 476.0 mm, the former belongs to the flat water year and the latter belongs to the drought year. The annual cumulative NEE in 2006 showed a net emission (87.70 g C m -2 yr -1), while in 2005 it showed a very weak absorption (-2.35 g C m ~ (- 2) yr ~ (-1)). Therefore, the alpine meadow is carbon neutral in flat water years but a carbon source in drought years, indicating that alpine meadow may become a CO 2 release source in the future if climate warming continues to deteriorate and aggravate soil drought. During the drought years, the total primary productivity (GEE), leaf area index (LAI) and duration of carbon uptake by ecosystems decreased significantly, which led to the daily maximum NEE absorption rate and maximum photosynthetic rate (Pmax ) And apparent quantum efficiency (α) are only 30% -50% of the normal water year. In the regulation of other factors, GEE and NEE on the half-hour scale are closely related to photosynthetically active radiation (PAR), but this response will be affected by air temperature (T_a), soil moisture content (SWC) and water vapor pressure deficit (VPD) Impact. The absorption rate of NEE decreases with the increase of T_a and VPD and the decrease of SWC. When PAR exceeds a suitable range, the rate of NEE absorption during the day decreases due to the higher radiation exacerbating soil drought. The optimum T_a and VPD values of NEE absorption rate were 12.7 ℃ and 0.42 KPa respectively, and the absorption rate of NEE also increased with the increase of SWC. The seasonal variation of LAI can account for 77% of the GEE and NEE variations. The variation of ecosystem respiration (R_e) at half-hour scale mainly depends on the soil temperature (T_s), but SWC will control the response of R_e to T_s to a certain extent.