利用稳定氢氧同位素定量区分白刺水分来源的方法比较

水是影响植物分布的重要生态因子之一,对植物水源的研究有助于在全球变化背景下了解植物的时空分布格局.根据同位素质量守恒,利用稳定氢氧同位素可以确定植物水分来源,相关的方法也不断改进.利用三源线性混合模型、多源线性混合模型、吸水深度模型以及动态模型分别对格尔木白刺(Nitraria Tangutorum)的水分来源进行了对比研究,发现格尔木白刺主要吸收利用50-100 cm处的土壤水及地下水.在研究方法上,各模型都有自己的应用范围和局限:三源线性混合模型一般只能在植物吸收的水分来源不超过3个的情况下运行；多源线性混合模型弥补了三源线性混合模型的不足,可以同时比较多种来源水各自对白刺的贡献率及贡献范围；吸水深度模型弥补了混合模型中不能计算白刺对土壤水的平均吸水深度的缺陷；动态模型则会为未来降水格局变化对植物的时空分布的影响研究起很大作用.针对不同的适用范围,模型的选择及综合应用会更广泛.但是,该技术还存在一些不足,需要结合测定土水势,富氘水的示踪等方法来弥补.

Comparison of the methods using stable hydrogen and oxygen isotope to distinguish the water source of Nitraria Tangutorum

Water is one of the most important factors affecting vegetation distribution in terrestrial ecosystems, especially in arid region, water will be the key factor to restrict growth of plant speies. Plant species have different availability to absorb water from different sources. Water sources of plant species directly determined the patters of species distribution. Therefore, quantifying water use of dominant species from certain sources are critical important to define and predict spatio-temporal distribution patterns of vegetation under global changes. Stable isotopes of oxygen and hydrogen were considered to be a good tools and have been used to identify the source of water, because different sources of water possess different oxygen or hydrogen isotope signatures. Until now, several approaches have been developed to quantify the contribution of different water sources to plants based on the isotopic mass-balance principal. In this study, we used four different models to estimate the sources of water used by Nitraria Tangutorum, the dominant species in an alpine desert steppe in Golmud on the Tibetan Plateau. Furthermore, the advantages and disadvantages of the four models were compared. The four models were as follows: a three-compartment linear mixing model, a multiple sources linear mixing model, a model of mean depth of water uptake by plants and a dynamic model. Results showed that N.tangutorum mainly used the below-ground water which concentrated in the 50 to 100 cm depth (mostly from the depths of 70 to 100 cm). The four models we used in this study have their own range of application and limitations. The three-compartment linear mixing model was used only in the situation that only less than three water sources were available. If more than three water sources were available for plants, the water sources should be reduced within three sources by removing those with less contribution to plant growth. The multiple sources mixed-linear model can remedy the defects of the three compartment linear mixing model. Therefore it can be used to quantify more than three water sources as well as the range of each source. However, either of the two models can not be used to calculate the mean depth of the water from which N. tangutorum can take up. However, the model of mean depth of water can solve this problem. For the dynamic model, control experiments are often needed. This study ultimately failed to use this model because of the rainfall time and the precipitation. Nonetheless, control experiment and the dynamic model should play a great role in predicting spatio-temporal distribution of dominant plant species under future precipitation patterns in this region. By comparison, the multiple sources mixed-linear model and the model of mean depth of water can work very well only with hydrogen or oxygen isotope. The result obtained by the oxygen isotope should be more precise, because hydrogen isotope fractionation should be more easily influenced by environmental factors. In view of the different application scopes, selection of models should be crucial and need to be pay more attention. Stable isotope method is a promising approach, although there are some defects in distinguishing the source of water of the plant species using stable isotopic hydrogen and oxygen. It is suggested that stable isotopic measurement on hydrogen and oxygen should be in combination with measurement on soil water potential and deuterium in different water components.