利用原位连续测定水汽δ18O值和Keeling Plot方法区分麦田蒸散组分

 利用稳定同位素技术和Keeling Plot方法可以有效分割地表蒸散量, 进而加深对陆地生态系统水循环的理解。该研究通过原位连续测定麦田的水汽同位素数据, 评价Keeling Plot方法在分割地表蒸散中的应用, 并揭示华北冬小麦(Triticum aestivum)蒸腾在总蒸散中的比例。实验于2008年3–5月在中国科学院栾城农业生态站进行, 利用国际上先进的H₂¹⁸O、HD¹⁶O激光痕量气体分析仪(TDLAS)为基础构建的大气水汽¹⁸O/¹⁶O和D/H同位素比原位连续观测系统, 同时利用涡度相关技术、真空抽提技术、同位素质谱仪技术, 获取了必要的数据。研究分析了一天中不同时间段的连续的大气水汽δ¹⁸O与水汽浓度倒数拟合Keeling Plot曲线的差异和可能的原因。结果显示, 中午时段的拟合结果较好, 这也暗示中午时段蒸腾速率高时最可能满足植物蒸腾的同位素稳定态假设。进一步的分析发现植物蒸腾的同位素稳定态并不总是成立, 尤其是水分胁迫下进入成熟期的小麦, 其蒸腾水汽同位素一般处于非稳定态。利用同位素分割结果显示, 生长盛期麦田94%–99%的蒸散来源于植物蒸腾。

Partitioning wheat field evapotranspiration using Keeling Plot method and continuous atmospheric vapor δ18O data

Aims Stable isotopes technique and Keeling Plot relationship offer great promise for partitioning evapotranspi-ration (ET), which can help us better understand the hydrologic cycle within terrestrial ecosystems. Our objectives are to evaluate the Keeling Plot method in ET partitioning using in situ continuous δ¹⁸O data and find the frac-tional contribution of crop transpiration to total ET in a winter wheat (Triticum aestivum) field. Methods Field experiments were conducted at Luancheng Agro-ecology Station, Chinese Academy of Sciences. A hydrogen and oxygen isotopes in situ measurement system based on Tunable Diode Laser Absorption Spectros-copy (TDLAS) was used to obtain the continuous atmospheric vapor δ¹⁸O data. Other measurements were made with the eddy covariance technique, cryogenic vacuum distillation and stable isotope ratio mass spectrometry. Important findings An analysis on the Keeling Plot relationships based on data from different time intervals in one daytime showed that the Keeling Plot would be better when using the midday time interval data to build Keeling Plot, which inferred that the plant transpiration isotopic steady-state (ISS) can be more easily obtained during midday when plant transpiration flux is generally largest. ISS was not always satisfied in field conditions, especially when mature wheat suffered from water stress. Using isotopic partitioning, we estimated transpiration contributed roughly 94%–99% to the total ET during the field measurement period, which indicated plant transpi-ration dominated local ET.