红树植物叶蜡正构烷烃氢同位素分馏机理研究

Leaf wax n-alkane is one of the powerful biomarkers that can be widely found and preserved for geological time scales. It is ideally suited to record and archive environmental and physiological information. Current conceptual models suggest hydrogen isotope will experience evaporative enrichment of soil water, transpirative enrichment of leaf water, and biosynthetic fractionation during integrating from source water to n-alkanes. For mangrove plants in costal ecosystems, effects of source water salinity lead the fractionation during root water uptake and the enrichment of leaf water to be critical processes shaping the hydrogen isotope composition of leaf wax. In this study, we aim to explore the exact influences of source salinity on hydrogen isotope composition of mangrove plants Aegiceras corniculatum and Kandelia obovata, which have different salt management strategies, by conducting both controlled greenhouse experiment and field experiment. Specifically, we have three issues to be highlighted: (1) to quantify the fractionation during root water uptake and its relation with xylem water potential; (2) to introduce leaf water model to interpret the mechanism of isotope enrichment and effects of plant physiology; (3) to establish a mechanistic model between salinity and hydrogen isotope composition of leaf wax. On the basis of the above, the outcomes of this study can provide theoretical and technological support for employing hydrogen isotope composition of leaf wax n-alkane to reconstruct environment change over time.

叶蜡正构烷烃是一类可在地质体中广泛存在且易于长期保存的生物标志物，其氢同位素组成是对过去环境水文信息和植物生理特征的综合反应。水源水氢同位素逐步整合到植物叶蜡烷烃的过程中，会经历土壤水蒸发富集、叶片水蒸腾富集和合成烷烃时的生化分馏。对滨海红树植物而言，水源水盐度的影响使得其根系吸水分馏和叶片水富集成为影响叶蜡烷烃氢同位素组成的关键环节。本项目以具有不同盐管理方式的红树植物桐花树和秋茄为研究对象，采用控制实验和野外实验相结合的方式，探讨盐度变化对红树植物氢同位素分馏的影响。主要研究内容包括：（1）量化根系吸水分馏效应及其与木质部水势的联系；（2）引入叶片水模型，阐明叶片水氢同位素富集机理及叶片生理的影响；（3）建立盐度影响叶蜡烷烃氢同位素组成形成的机理模型。通过以上研究，为利用沉积物中红树植物叶蜡烷烃研究环境变化提供理论和技术支撑。

水循环是生态系统中重要的物质循环之一，水中氢氧同位素具有示踪、整合和指示的功能特征，是研究生态系统水循环过程机理和生态学效应的重要技术手段，广泛应用于植物水分溯源、生态系统蒸散发及气候重建等研究领域。近年来，不断有研究报道在植物水源水研究中出现的氢同位素贫化现象，但迄今为止未有公认合理的解释，这极大地影响了氢同位素在生态环境诸多领域应用的可靠性。本研究：（1）设计并搭建了一套用于在线测量植物全冠层稳态蒸腾水汽氢氧同位素组成的装置，对9种盐生、旱生和中生植物进行了测量，并与经典的低温真空提取方法进行了比较，判定后者是造成氢同位素贫化的主要原因；（2）植物样品复水实验发现植物样品含水量可以预测真空提取提取过程中氢同位素的贫化程度，从而给出了可行的校正措施；（3）对不同科属木本植物进行的比较实验证实，真空提取过程导致的分馏是一个普适性现象。通过将以上结论应用于植物水源水判别和生态水文分离等案例研究中，发现从氢同位素证据的角度以往相关研究的结论将有待商榷。本研究结果指出，在水源水判别、蒸散发拆分和过去气候水文环境重建等相关研究中植物水氢同位素的使用需谨慎，尤其在采用低温真空提取技术时应配合必要的校正步骤。