基于水稳定同位素的地下水型陆地植被识别:研究进展、面临挑战及未来研究展望

Isotopic approaches to identify groundwater dependent terrestrial vegetation:Progress,challenges,and prospects for future research

地下水型陆地植被具有重要的生态服务功能,但正遭受严重威胁,亟需在水资源管理中予以关注和保护.准确识别地下水型陆地植被是其管理和保护的前提.水稳定同位素方法是识别地下水型陆地植被的唯一直接方法,其中:①直接比较法只能获得植物对地下水利用的定性信息,但目前的应用最为广泛;②同位素混合模型能定量评估植物对地下水的依赖性,近期随贝叶斯模型的引入取得了较大进展.当前,植物对地下水利用的时空异质性增加了基于水稳定同位素的地下水型陆地植被的识别难度,还限制了小尺度上的研究成果向大尺度上的拓展;部分植物以间接方式利用地下水,对地下水型陆地植被的识别造成了困扰;根系吸水过程中的同位素分馏和同一植株内木质部水同位素组成的时空异质性常使植物样品的代表性受到质疑;采集到具有代表性的地下水和土壤样品也是当前面临的一个主要挑战.为应对上述挑战,未来应加强3个方面的研究:①研发植物木质部水同位素组成的原位在线连续观测技术,提升基于水稳定同位素的地下水型陆地植被识别的时空分辨率;②借助控制性的同位素标记实验,精细刻画地下水-土壤-植物体系内同位素体的迁移和分馏过程;③将同位素观测与具有物理学意义的生态水文模型相耦合,提高定量评估的分辨率和降低不确定性,探索解决时空异质性和升尺度难题的途径.

As one of the major types of groundwater dependent ecosystems,groundwater dependent terres-trial vegetation(GDTV)has a variety of ecological service functions.Protection of GDTV is an important criterion in sustainable water resources management,particularly when it is being threatened globally by degradation due to the over-exploitation of groundwater and surface water.Identifying GDTV is the first requisite step to managing and protecting it.While GDTV has been identified at large scales using the White method,water balance method,and satellite-based approaches,only stable isotope techniques canprovide direct means for identifying GDTV.The stable isotopes approach can be further divided into direct inference approach and mixing model approach.Though only qualitatively identifying GDTV,the direct in-ference approach is still more prevalent than the mixing model approach.In recent years,with the applica-tion of Bayesian mixing models,progress has been made in quantifying the dependency of GDTV on groundwater by using mixing model approach.However,many conceptual and methodological challenges remain.The first one is how representative an individual tree/plant studies are for larger-scale systems,giv-en the spatial and temporal heterogeneity of groundwater use pattern by GDTV,which also makes it diffi-cult to applying our observational results at small-scale to understand catchment-or landscape-scale phe-nomena.Secondly,it has been found that many plants might use groundwater in an indirect way,which dis-torts the identifying of GDTV.The third main challenge is how the isotopic fractionation at soil-root inter-face and the spatial-temporal variation in xylem isotopic signatures within a plant can inform endmember determination,F urthermore,obtaining representative groundwater and soil water samples is also challeng-ing.In the coming years,efforts should be made towards:(1)developing novel methods for in situ,online,and continuous isotopic measurements in xylem water,to help to gather isotope data at higher temporal and spatial resolutions;(2)conducting extensive isotopic labelling experiments with known boundary condi-tions,to facilitate a more detailed characterization of the flow and fractionation of isotopologues in the groundwater-soil-vegetation system;and(3)developing approaches coupling physically based ecohydro-logical models and isotopic observation,to provide root water uptake profiles with high spatial resolution andlower uncertainty.