A novel method for analysing phosphorus-bound stable oxygen isotope ratios in groundwater ecosystems

磷酸盐（PO4）在地下水生态系统中的作用和命运直到最近才成为科学关注的焦点。与通常的假设相反，由于生物地球化学过程，地下水可以在局部保持相当高浓度的PO4。研究这些过程的一种方法是评估固体（即含水层基质）和水相（即地下水）中的PO4库，并监测其随时间的变化。近年来，分析PO4稳定氧同位素组成（δ18OPO4）在实际确定的PO4池中成为一种新的有前途的工具。通过将δ18OPO4分析与同位素标记实验相结合，我们能够监测微生物循环以及控制地下水中PO4命运的非生物过程。尽管这种方法产生了无数可能的实验可能性，但迄今为止，地下水系统中还没有经过严格测试的分析方案，可以将顺序提取与δ18OPO4分析相结合。目前，现有的土壤和沉积物δ18OPO4分析方案主要集中在样品制备上，目的是去除提取的PO4中的有机物和其他杂质。缺少的是对潜在信号变化和各自提取步骤选择性的联合测试。这种严格的方法测试很重要，因为含水层材料和地下水可能表现出广泛的物理化学性质（例如有机质含量、碳酸钙含量、普遍氧化还原状态和pH值），这可能会干扰同位素分析和信号解释。该项目的目标是开发和验证一种方法，以可靠地确定和解释地下水系统中固体（即含水层物质）和水（即地下水）PO4池中的δ18OO4信号。拟议的测试方案将允许评估是否存在以及在多大程度上存在对高浓度有机物、碳酸钙和从含铁还原条件的含水层中获得的样品的适用性的限制。该工具对于同位素标记和孵育实验非常有用，而且对于确定地下水和含水层物质中的天然同位素丰度也非常有用。总之，该项目将提供一种新的方法方法，对追踪地下水系统中PO4的生物地球化学循环和命运非常有用。

The role and fate of phosphate (PO4) in groundwater ecosystems has only recently come into scientific focus. In contrast to common assumptions, groundwater can locally maintain considerable concentrations of PO4 because of biogeochemical processes. One approach to study these processes is the assessment of PO4 pools in the solid (i.e. aquifer matrix) and aqueous phase (i.e. groundwater) and to monitor changes therein over time. Lately, the analysis of the stable oxygen isotope composition of PO4 (δ18OPO4) in operationally defined PO4 pools emerged as a novel and promising tool. By combining δ18OPO4 analysis of operationally defined PO4 pools with isotopic labelling experiments, we are able to monitor microbial cycling as well as abiotic processes that control in concert the fate of PO4 in groundwater. Despite the countless possible experimental possibilities arising from this approach, no rigorously tested analytical protocol exists to date for groundwater systems combining sequential extractions with δ18OPO4 analysis. At present, available δ18OPO4 analysis protocols for soil and sediments focus on sample preparation aiming to eliminate organic matter and other impurities from extracted PO4. What is missing is a combined testing of potential signal alterations and selectivity of the respective extraction steps. Such a rigorous method testing is important as aquifer materials and groundwater may exhibit a wide range of physicochemical properties (e.g. contents of organic matter, calcium carbonates, prevailing redox state and pH) that could interfere with the isotopic analysis and signal interpretation. The objective of this project is to develop and validate a methodological approach to reliably determine and interpret δ18OO4 signals in solid (i.e. aquifer material) and aqueous (i.e. groundwater) PO4 pools in groundwater systems. The proposed test scheme will allow to evaluate if and to which extent limitations exist regarding the applicability for samples with a high concentration of organic matter, calcium carbonates, and samples obtained from aquifers with iron-reducing conditions. This tool will be highly useful for isotopic labelling and incubation experiments, but also for the determination of natural isotopic abundances in groundwater and aquifer materials. In summary, the project will provide a novel methodological approach that will be highly useful to trace the biogeochemical cycling and fate of PO4 in groundwater systems.