Oxygen isotope signatures of plant and soil organic matter

解释稳定同位素比率以阐明环境过程需要对环境中每种元素的同位素系统有深刻的了解，这些同位素系统已经收集了几种元素，例如土壤中的碳(C)和氮(N)。我们之前的项目揭示了相对于C和n，土壤中氢(H)同位素比率对环境驱动因素的响应存在根本差异。O同位素系统与H类似，因为这两种元素都以易交换和不可交换的形式存在，而只有后者具有有意义的信号。C和N同位素系统与O同位素系统有相似之处，因为所有这些元素都经过细胞外酶催化的生化反应，这些反应可能与动力学同位素分馏有关。由于有机质中O同位素比率与气候之间的联系在迄今为止的文献中很少，因此对O同位素系统的进一步了解可能提供一种新的工具，可用于检测生态系统中微妙的气候变化影响，否则这些影响可能被忽视。我们的总体目标是探索在生态背景下土壤有机质（SOM）中不可交换性O的<s:1> 0 - 18值的意义。我们计划(i)量化植物凋落物和土壤有机质中交换性O的比例，（ii）通过微生物和细胞外酶活性确定环境水O进入土壤有机质的动力学，（iii）评估土壤有机质分解是否与O同位素分异有关，如果是，则量化净表观同位素分异，（iv）评估降水的<s:1> 18O值与森林土壤有机层的非交换性O库之间的关系。(v)研究大块SOM中非交换性O的<s:1> 18O值是否与选定的生物标志物化合物相关。作为先决条件（WP1），需要控制无机O的影响。因此，我们将(a)测试为H同位素建立的脱矿方法是否可以转移到O同位素上，(b)将开发一种基于氧阴离子提取和通过消声破坏OM的新方法。我们将选择最可靠的方法，在可控条件下进行实验，以追踪和量化环境水O和H的掺入以及相关的同位素分馏（WP2）。这将得到实地研究的补充，实地研究将研究自然OM中可交换的O和H的程度以及环境水O和H的加入和C、N、O和H系统中的同位素分馏的中期后果，并探索O同位素系统与H和/或其他元素的潜力，以表明与气候相关的过程变化（WP3）。

The interpretation of stable isotope ratios to elucidate environmental processes requires a profound knowledge of the isotope system of each element in the environment which has already been gathered for several elements e.g., carbon (C) and nitrogen (N) in soil. Our previous projects revealed fundamental differences in the response of the hydrogen (H) isotope ratios to environmental drivers relative to C and N. The oxygen (O) isotope system in soil has received less attention. The O isotope system is similar to that of H because both elements occur in an easily exchangeable and a nonexchangeable fraction while only the latter bears a meaningful signal. The C and N isotope systems share similarities with that of O because all these elements undergo biochemical reactions catalyzed by extracellular enzymes, which are potentially associated with kinetic isotope fractionation. Because of the proposed link of O isotope ratios in organic matter with climate in the hitherto scarce literature, an improved understanding of the O isotope system might provide a new tool that could be used to detect subtle climate change effects in ecosystems that are otherwise overlooked. Our overarching aim is to explore the meaning of the 18O values of nonexchangeable O in soil organic matter (SOM) in an ecological context. We plan to (i) quantify the proportion of exchangeable O in plant litter and SOM, (ii) determine the kinetics of ambient-water O incorporation into OM by microbial and extracellular enzyme activity, (iii) assess whether OM decomposition is related with an O isotope fractionation and if so quantify the net apparent isotope fractionation, (iv) evaluate the relationship between the 18O values of precipitation and the nonexchangeable O pool of forest soil organic layers, and (v) investigate whether the 18O values of nonexchangeable O in the bulk SOM correlate with those of selected biomarker compounds. As a prerequisite (WP1), the influence of inorganic O needs to be controlled for. We will therefore (a) test if a demineralization method, which was established for H isotopes can be transferred to O isotopes and (b) will develop a novel method based on extractions of oxyanions and destruction of the OM via muffling. We will select the most reliable method and conduct experiments under controlled conditions to be able to trace and quantify the ambient-water O and H incorporation and the associated isotope fractionation (WP2). This will be complemented by a field study where the extent of exchangeable O and H in natural OM and the medium-term consequences of ambient-water O and H incorporation and isotope fractionation in the C, N, O, and H systems will be studied and the potential of the O isotope system together with that of H and/or other elements to indicate climate-related process changes explored (WP3).