腾格里荒漠红砂-珍珠群落CO2收支变化及其不同观测方法间的比较

由于荒漠生态系统植被覆盖度低、生产力低下,其在全球碳循环中的作用被长期忽视。为探讨荒漠生态系统碳收支各组分的变化规律,以腾格里荒漠红砂(Reaumuria soongorica Maxim.)-珍珠(Salsola passerina Beg.)群落为研究对象,采用静态箱式法研究了该群落的净生态系统CO2交换量(NEE)、生态系统呼吸、土壤呼吸的日变化规律,同时将该方法所获得的NEE结果与涡动相关法观测的结果进行了比较。结果表明:(1)红砂-珍珠群落NEE的日变化表现为,在6:00—9:00左右出现一个CO2吸收的高峰值,随后在12:00—15:00左右出现一个CO2释放高峰值。红砂种群、珍珠种群和整个群落NEE的平均值分别为0.018、0.020和0.028 mg CO2m-2s-1;(2)红砂种群、珍珠种群、土壤及整个群落生态系统呼吸速率的日变化规律一致,均表现为明显的单峰变化趋势,在12:00—15:00左右出现一个CO2释放的高峰值。红砂种群、珍珠种群、土壤和整个群落的生态系统呼吸的平均值分别为:0.121、0.062、0.029和0.040 mg CO2m-2s-1。以盖度为加权因子计算得到红砂种群、珍珠种群和土壤呼吸占生态系统呼吸的比例分别为:9%、21%和70%,由此可见,生态系统呼吸主要来源于土壤呼吸。(3)将箱式法和涡动相关法观测的NEE进行比较,结果表明两种方法观测的NEE变化规律基本一致,相关系数达到0.7。采用箱式法观测的NEE高于涡动相关法观测的结果,平均值分别0.028 mg CO2m-2s-1(箱式法)和0.015 mg CO2m-2s-1(涡动相关法),涡动相关法的观测结果与箱式法观测结果的比值为0.54。综上可得,荒漠生态系统土壤呼吸的变化速率决定了生态系统呼吸的变化规律,采用箱式法可能高估了荒漠生态系统CO2的释放量。

Variation of the carbon budget and comparison between static chamber method and eddy covariance method in the Reaumuria soongorica -Salsola passerina community in the Tennger Desert

Long-term field measurements of the exchange of CO₂ between terrestrial ecosystems and the atmosphere can contribute to our knowledge on the role of a particular ecosystem in the global carbon cycle. To date, little information is available on the carbon fluxes in a typical steppitied desert, especially in areas with annual precipitation less than 200 mm. The community dominated by Reaumuria soongorica Maxim. and Salsola passerina Bge. is one of the most common vegetation types in desert areas in China. In this program, the R.soongorica-S.passerina community in the Tennger Desert was selected as our study object. The diurnal variation of the net ecosystem CO₂ exchange (NEE), the gross primary productivity (GPP), the ecosystem respiration (Reco) and the soil respiration (Rsoil) were measured using the closed static chamber method. The results obtanined by the chamber method were compared with eddy covariance measurements and the main results were as follows: (1) NEE of the R.soongorica-S.passerina community exhibited a negative peak (negative values indicate CO₂ uptake by the desert ecosystem) in the morning between 6:00-9:00, and then increased and showed positive values (positive values indicate CO₂ release from the desert ecosystem to atmosphere) with the largest efflux peak occurring between 12:00-15:00. The diurnal variation of NEE in the R.soongorica-S.passerina community was similar to the S. passerina population, which means the variation of the R. soongorica-S. passerina community was determined by the S. passerina population. The mean NEEs of the R.soongorica population, S.passerina population and R.soongorica-S.passerina community were 0.018, 0.020 and 0.028 mgCO₂ m^-2 s^-1, respectively. (2) The diurnal variation of the respiration of the R. soongorica population, S. passerina population, soil and the whole community were similar, and exhibiting single-peak curves at daytime with the peak appearing at between 12:00-15:00, and the mean values of respiration of R. soongorica population, S. passerina population, soil and the whole community were 0.121, 0.062, 0.029, and 0.040 mgCO₂ m^-2 s^-1, respectively. We calibrated the carbon efflux using the coverage-weighted-average method and found that the R. soongorica population, S. passerina population and the soil system comprise 9%, 21% and 70% of the total R.soongorica-S.passerina community ecosystem respiration, respectively. Therefore the ecosystem respiration derived mainly from the soil system. (3) The variation of NEE monitored using the closed static chamber method was generally consistent with that by using eddy covariance method. The NEE measured by the closed static chamber was linearly correlated with the eddy covariance, and the correlative value was 0.7. The NEE measured by the closed static chamber was higher than the eddy covariance, with the mean values were 0.028 mgCO₂ m^-2 s^-1 (the former), 0.015 mg CO₂ m^-2 s^-1 (the latter), respectively. The result measured by the eddy covariance vs closed static chamber was 0.54. In conclusion, the carbon efflux of desert ecosystem maybe overestimated by using the closed static chamber method.