叶片水H218O富集的研究进展

 植物叶片水H₂¹⁸O富集对大气中O₂和CO₂的¹⁸O收支有着重要影响。蒸腾作用使植物叶片水H₂¹⁸O富集, 而植物叶片水H₂¹⁸O富集的程度主 要受大气水汽δ¹⁸O和植物蒸腾水汽δ¹⁸O的影响。过去, 通过引入稳态假设(蒸腾δ¹⁸O等于茎水δ¹⁸O)得到Craig-Gordon模型的闭合形式, 或 将植物整个叶片水δ¹⁸O经过Péclet效应校正后得到植物叶片水δ¹⁸O的富集程度。然而, 在几分钟到几小时的短时间尺度上, 植物叶片蒸腾 δ¹⁸O是变化的, 稳态假设是无法满足的。最近成功地实现了对大气水汽δ¹⁸O和δD的原位连续观测, 观测精度(小时尺度)可达到甚至优于稳定 同位素质谱仪的观测精度。在非破坏性条件下, 高时间分辨率和连续的大气水汽δ¹⁸O和蒸腾δ¹⁸O的动态观测, 将提高植物叶片水H₂¹⁸O富集的 预测能力。该文综述了植物叶片水H₂¹⁸O富集的理论研究的新进展、研究焦点和观测方法所存在的问题, 旨在进一步加深理解植物叶片水H₂¹⁸O 富集的过程及其机制。

RECENT ADVANCES IN H218O ENRICHMENT IN LEAF WATER

There is considerable interest in the use of atmospheric C¹⁸O¹⁶O and ¹⁸O¹⁶O as a tracer for resolving the role of the terrestrial biosphere in the global carbon cycle. Leaf transpiration will result in the enrichment of the heavy H₂¹⁸O isotopes. The δ¹⁸O of leaf water at the evaporating site in the stomatal cavity directly influences the C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges, instead of that of the bulk leaf water. How to best quantify this enrichment effect remains an active area of research. In the past, a closed form of the Craig-Gordon model was obtained by invoking the steady-state assumption (δ¹⁸O of the transpired water is identical to δ18O of the xylem water). For the purpose of verification, the predictions of Craig- Gordon model are compared with δ¹⁸O of the bulk leaf water after appropriate corrections for the Péclet effect. On small time scales of minutes to hours, δ¹⁸O of the transpired water is variable in field conditions, implying that the steady state assumption is invalid. Recently, in-situ δ¹⁸O and δD measurement technology has been developed that has potential for improving our understanding of isotopic exchanges between the Earth’s surface and the atmosphere. The precision of hourly δ ¹⁸O and δD is comparable to the precision of mass spectrometry. It has the potential to improve prediction of δ¹⁸O of leaf water at the evaporating site within the stomatal cavity for the temporal dynamics of atmospheric water vapor δ¹⁸O and the δ¹⁸O of the transpired water, especially if its measurement is made in a non-destructive manner and on a continuous basis. Because the isotopic flux of δ¹⁸O and δD is influenced by a similar set of biological and meteorological variables, simultaneous observations of δ¹⁸O and δD will provide additional constraints on the hydrological and ecological processes of the ecosystem. We review the theory and measurement techniques for the enrichment of H₂¹⁸O in leaves and focus on the recently developed in-situ measurement technology and its potential for improving our understanding of H₂¹⁸O enrichment in leaf water and C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges between the ecosystem and atmosphere.