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草地是陆地生态系统的重要组成部分,具有重要的生产和生态功能[1]。世界草地面积约占全球陆地面积的40%[2],它不仅是畜牧业重要的生产基地,且在调节气候、保持水土、改良土壤和维持生物多样性等方面发挥着巨大作用[1-3]。我国天然草地面积约在349×104392×104 km2,约占国土总面积的40%以上[1, 4]。近半个世纪以来,受全球气候变化及人类活动影响,我国草地面临水资源短缺、植被退化、生态功能和生产力持续下降等严峻的生态问题[3]。
水分是干旱半干旱地区植被生长和生态恢复的主要限制因子。目前,全世界80%的草地位于干旱半干旱地区,该地区具有降水稀少、蒸发量大、水资源储量低等特点[5]。降水格局变化首先对土壤水分分布有影响[2],进而影响草地植物个体和生态系统水分利用效率[6],最终影响草地生态系统水循环过程及生产力水平[7]。
前人对草地生态系统水文过程的研究,主要集中于运用传统水文学方法研究其水文过程的某一环节,如植被层对降水的截留和分配[8-9]、植被蒸散[10]、土壤水分蒸发[11]及土壤水分入渗[12]等方面,且缺乏将草地生态系统各水体作为一个整体进行综合研究并定量化;而稳定同位素技术具有较高的灵敏度与准确性,可将生态系统水循环过程(包括从大气降水到地表水、土壤水、地下水、植物水和蒸发水等整个迁移、转化与分配过程)作为一个整体来研究,系统和定量地阐明其过程与机制[13-14]。
本文综述了碳氢氧稳定同位素在草地生态系统水循环研究中的国内外进展,并展望其未来的应用前景,对我国草地资源保护、科学利用以及退化草地生态系统恢复等具有重要的指导意义。
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稳定同位素是指某元素中不发生或极不容易发生放射性衰变的同位素。天然存在于水分子中的氢有1H(氕)和D(氘)共2种稳定同位素,氧有16O、17O和18O共3种稳定同位素;天然碳有12C和13C共2种稳定同位素。不同环境条件下,各水体(包括植物水)氢氧稳定同位素和植物组织中碳稳定同位素组成不同,因此,可通过分析其微小变化,定量研究陆地生态系统水循环过程以及各水体转化关系。
由于稳定同位素在自然界含量极低,用绝对量表示其同位素的差异比较困难。因此,国际上规定使用相对量即待测样品的同位素比值(Rsample)与一标准物质的同位素比值(Rstandard)作比较,比较结果称为样品的同位素值(δ值),其定义为:
$ \delta X\left( ‰ \right) = ({R_{{\rm{sample}}}}/{R_{{\rm{standard}}}} - 1) \times 1000‰ $
式中:Rsample是样品中元素的重轻同位素丰度比(如D/H,18O/16O,13C/12C);Rstandard是国际通用标准物的重轻同位素丰度之比(氢、氧稳定同位素采用V-SMOW标准)。
碳氢氧稳定同位素在草地生态系统水循环研究中的应用
Application of Carbon, Hydrogen and Oxygen Stable Isotope on Hydrologic Cycle of Grassland Ecosystem: A Review
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摘要: 碳氢氧稳定同位素是存在于天然水体和植物组织中的良好的示踪剂,具有较高的灵敏度与准确性,可系统和定量地阐明草地生态系统水循环过程及各水体的转化关系、植物水分利用策略以及植被对全球变化的响应机制等。本文概述了稳定同位素的基本概念和原理,总结和分析了草地生态系统水循环的研究方法和现状,重点探讨和综述了氢氧稳定同位素技术在草地生态系统水循环过程(包括大气降水、地表水、土壤水、地下水、植物水、蒸发水等)以及碳稳定同位素技术在植物水分利用效率中应用的国内外研究进展,分析了其存在的问题,展望了这一领域未来的研究方向和应用前景,为我国草地资源保护、合理利用及退化草地生态系统恢复等提供理论依据。Abstract: Grassland is a major component and important resource of terrestrial ecosystem and has important production and ecological functions. However, the grassland in China has faced with severe ecological problems such as water resource shortage and vegetation degradation due to global climate change and long-term irrational utilization, which poses a serious threat to the stability of ecosystem. Therefore, quantitatively studying the hydrologic cycle of grassland ecosystem provides important application value and significant guidance for the protection and sustainable development of grassland ecosystem. Stable isotopes of carbon, hydrogen and oxygen are natural tracers in water body and plant tissue, with high sensitivity and accuracy. Stable isotopes of hydrogen and oxygen can be used to systematically and quantitatively clarify the hydrologic cycle and processes of grassland ecosystem, the conversion relationship between water bodies, the water use strategy of plants and the response mechanism of vegetation to global climate change. In this article, the basic concept and principle of stable isotopes are reviewed. The research methods for examining the hydrologic cycle of grassland ecosystems and their application are summarized and analyzed. Progresses in the application of hydrogen and oxygen stable isotopes in the hydrologic process of grassland ecosystem are extensively discussed, including atmospheric precipitation, runoff, soil water, groundwater, water source for plants, and evaporation, and the application of carbon stable isotope in plant water use efficiency. The problems and future researches of stable isotope techniques in the study of hydrologic cycle are discussed and analyzed.
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