An improved isotopic method for partitioning net ecosystem–atmosphere CO2 exchange
An improved isotopic method for partitioning net ecosystem–atmosphere CO2 exchange
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DOI：
10.1016/j.agrformet.2015.09.009
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发表时间：
2015-12
影响因子：
6.2
通讯作者：
R. Wehr;S. Saleska
中科院分区：
文献类型：
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作者：
R. Wehr;S. Saleska
稳定碳同位素可用于将CO2的净生态系统-大气交换（NEE）划分为光合和呼吸组分，但由于仪器和理论的限制，该方法尚未被广泛采用。在这里，最近改进的仪器的动机，我们扩展理论的同位素通量分区，包括光呼吸，叶片白天的“暗”呼吸，和其他改进，在一个通用的，但实际的配方，所有以前的配方可以推导出简化的近似。我们使用一个完整的生长季节的同位素涡度协方差通量数据从温带落叶林来证明该方法，量化其不确定性，并确定与以前公布的配方相关的偏差。我们发现，当以高精度（100 s积分时间为0.02‰ RMSE）获得CO2的δ 13 C时，分区通量的统计不确定性与NEE本身相当-即，尽可能的好。可评估的系统不确定性为生态系统生产总值（GEP）的±17%，主要是由于羧化同位素分馏的不确定性。此外，目前无法量化的系统的不确定性与治疗冠层作为一个单一的“大叶”。这两个来源的系统不确定性可以大大减少可行的支持叶级测量。我们的扩展理论纠正了以前的同位素方法中的系统偏差，包括由于忽略光呼吸而高估（13%）GEP。这种分配决定了光合作用的同位素特征，我们发现光合作用的同位素特征在-24 ‰和-28 ‰之间季节性变化，因此生态系统碳输入和输出之间的同位素不平衡在大部分生长季节保持稳定在-0.5 ‰左右。同位素分区的关键优势，标准的，基于回归的分区是，它使控制生态系统规模的光合和呼吸通量出现从观察，而不必假设的功能关系，环境driversa先验。作为一个例子，我们展示了如何同位素分区揭示了某些大的变化，在白天NEE所造成的高和低呼吸通量的区域之间的通量塔采样足迹的变化，发现无法通过标准分区。出于这个原因，同位素分区可以比标准分区更精确地量化NEE上的环境控制。
Stable carbon isotopes can be used to partition the net ecosystem–atmosphere exchange (NEE) of carbon dioxide (CO2) into its photosynthetic and respiratory components, but the method has not been generally adopted due to instrumental and theoretical limitations. Here, motivated by recently improved instrumentation, we extend the theory of isotopic flux partitioning to include photorespiration, foliar daytime ‘dark’ respiration, and other refinements, arriving at a general yet practical formulation from which all previous formulations can be derived as simplifying approximations. We use a full growing season of isotopic eddy covariance flux data from a temperate deciduous forest to demonstrate the method, quantify its uncertainties, and determine biases associated with previously published formulations. We find that whenδ13C of CO2is acquired with high precision (0.02‰ RMSE for 100 s integration times), the statistical uncertainty in the partitioned fluxes is comparable to that in NEE itself—i.e., as good as practicably possible. Assessable systematic uncertainty is ±17% of gross ecosystem production (GEP), due mostly to uncertainty in the isotopic fractionation by carboxylation. Additional, currently unquantifiable systematic uncertainty is associated with treating the canopy as a single “big leaf”. Both sources of systematic uncertainty could be greatly reduced by feasible supporting leaf-level measurements. Our extended theory corrects systematic biases in previous isotopic approaches, including overestimation (by 13%) of GEP due to the omission of photorespiration. The partitioning determines the isotopic signature of photosynthesis, which we find to vary seasonally between −24 and −28‰ such that the isotopic disequilibrium between ecosystem carbon input and output remains stable at approximately −0.5‰ through most of the growing season. The key advantage of isotopic partitioning over standard, regression-based partitioning is that it enables controls on the ecosystem-scale photosynthetic and respiratory fluxes to emerge from observations, without having to assume functional relations to environmental driversa priori. As an example, we show how isotopic partitioning reveals certain large variations in daytime NEE to be caused by shifts in the flux tower sampling footprint between regions of high and low respiratory flux—a finding unobtainable by standard partitioning. For this reason, isotopic partitioning can be more precise than standard partitioning for quantifying environmental controls on NEE.