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Estimating photosynthetic 13C discrimination in terrestrial CO_2 exchange from canopy to regional scales

机译：估算从冠层到区域尺度的陆地CO_2交换中的光合作用13C歧视

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摘要

We determined δ~(13)C values associated with canopy gross and net CO_2 fluxes from four U.S. sites sampled between 2001 and 2002. Annual mean, flux-weighted δ~(13)C values of net ecosystem CO_2 exchange (NEE) were estimated for four contrasting ecosystems (three forests and one grassland) by analyzing daytime flask measurements collected at the top of canopies. Combining δ~(13)C values associated with respiratory and net (respiration minus photosynthesis) fluxes, we demonstrate a method for estimating whole-canopy photosynthetic discrimination against ~(13)C (Δ_A) in terrestrial ecosystems directly from field measurements. This experimental approach offers the possibility of examining interannual variability in Δ_A from ecosystem δ~(13)C measurements. Our estimated δ~(13)C values associated with photosynthetic fluxes are in agreement with those measured from foliar organic matter for C_3 forests, and are within the range bounded by C_3 and C_4 grasses in a tallgrass prairie. The δ~(13)C associated with NEE fluxes at our C_3 forest sites ranges between -27.1 ± 2.7 and -28.3 ± 2.5‰, and is -22.6 ± 4.0‰ at the prairie site. These estimates differ from a previous study, particularly for C_3 ecosystems at comparable latitudes. Sensitivity analyses indicate that our estimates of δ~(13)C values of net CO_2 fluxes are robust with respect to measurement errors, but can vary depending on the selection of background atmospheric values. Other factors (e.g., drought and sampling footprint) that might have influenced our δ~(13)C measurements and calculations of Δ_A are also discussed. Our measurement-based analyses are particularly useful when both latitudinal and longitudinal variations in Δ_A are to be considered in the global inversion modeling studies.
机译：我们确定了2001年至2002年间从四个美国采样点获得的与冠层总通量和净CO_2通量有关的δ〜（13）C值。估算了生态系统净CO_2交换量（NEE）的年均通量加权通量δ〜（13）C值通过分析在树冠顶部收集的日间烧瓶测量值，可以得出四个对比鲜明的生态系统（三个森林和一个草原）的信息。结合与呼吸通量和净通量（呼吸减去光合作用）相关的δ〜（13）C值，我们展示了一种直接通过野外测量估算陆地生态系统中〜（13）C（Δ_A）的全冠层光合作用的方法。这种实验方法提供了从生态系统δ〜（13）C测量值检查Δ_A年际变化的可能性。我们估计的与光合作用通量相关的δ〜（13）C值与C_3森林的叶有机质测得的值一致，并且在高草草原上以C_3和C_4草为界。在我们的C_3林场，与NEE通量相关的δ〜（13）C在-27.1±2.7和-28.3±2.5‰之间，在草原场为-22.6±4.0‰。这些估计与以前的研究不同，特别是在纬度相当的C_3生态系统中。敏感性分析表明，我们对净CO_2通量的δ〜（13）C值的估计相对于测量误差是稳健的，但可以根据背景大气值的选择而变化。还讨论了可能影响我们的δ〜（13）C测量值和Δ_A计算的其他因素（例如干旱和采样足迹）。当在整体反演模型研究中考虑Δ_A的纬度和纵向变化时，我们基于测量的分析特别有用。

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