The effects of chamber pressurization on soil-surface CO2 flux and the implications for NEE measurements under elevated CO2
The effects of chamber pressurization on soil-surface CO2 flux and the implications for NEE measurements under elevated CO2
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DOI：
10.1046/j.1365-2486.1999.00218.x
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发表时间：
1999-03-01
影响因子：
11.6
通讯作者：
Field, CB
中科院分区：
文献类型：
--
作者：
Lund, CP;Riley, WJ;Field, CB
土壤和生态系统痕量气体通量的测量通常采用动态室技术。虽然与这种方法相关的腔室压力异常是已知的误差来源，但它们的影响还没有得到充分的表征。在这项研究中，我们使用土壤气体交换实验的结果和土壤二氧化碳运移模型来表征室内压力对一年生加州草原土壤二氧化碳外流的影响。实验数据表明，当室内压力大于环境压力时，土壤表面的CO2通量随室内压力的增加呈非线性函数下降；对于较干燥的土壤，这种下降幅度更大。在干燥土壤中，0.5pa的表压使测得的土壤二氧化碳流出量比环境压力下的对照测量减少了约70%。土壤二氧化碳运移模型的结果表明，在高于环境压力的情况下对通量腔加压，通过在土壤充满空气的孔隙中产生向下的空气流，有效地将二氧化碳从土壤中冲洗出来。这种平流气流减少了土壤-大气界面上的二氧化碳浓度梯度，导致进入燃烧室顶部空间的扩散通量较小。模拟还表明，扩散通量的减少是燃烧室压力、土壤湿度、土壤质地、土壤CO2产生的深度分布和燃烧室直径的函数。这些结果强调了在解释动态燃烧室痕量气体流量测量时需要谨慎的必要性。在二氧化碳浓度升高的情况下，经常观察到的生态系统净碳吸收增加的一部分可能是由于加压对土壤二氧化碳外流的影响而产生的人工产物。
Soil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas-exchange experiments and a soil CO2 transport model to characterize the effects of chamber pressure on soil CO2 efflux in an annual California grassland. For greater than ambient chamber pressures, experimental data show that soil-surface CO2 flux decreases as a nonlinear function of increasing chamber pressure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO2 efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO2 transport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO2 from the soil by generating a downward now of air through the soil air-filled pore space. This advective flow of air reduces the CO2 concentration gradient across the soil-atmosphere interface, resulting in a smaller diffusive flux into the chamber head space. Simulations also show that the reduction in diffusive flux is a function of chamber pressure, soil moisture, soil texture, the depth distribution of soil CO2 generation, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portion of the frequently observed increase in net ecosystem carbon uptake under elevated CO2 may be an artifact resulting from the impact of chamber pressurization on soil CO2 efflux.