Oxygen isotope signatures of plant and soil organic matter

要解释稳定的同位素比值以阐明环境过程，需要对环境中每种元素的同位素体系有深刻的了解，这些体系已经收集了几种元素，如土壤中的碳(C)和氮(N)。我们以前的项目揭示了相对于碳和氮，氢(H)同位素比率对环境驱动因素的响应存在根本差异。土壤中的氧(O)同位素系统受到的关注较少。氧同位素体系类似于氢的同位素体系，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。碳和氮同位素系统与氧有相似之处，因为所有这些元素都在胞外酶的催化下发生生化反应，这可能与动力学同位素分馏有关。由于在迄今稀少的文献中提出了有机质中氧同位素比率与气候的联系，对氧同位素系统的更好的理解可能提供一种新的工具，可以用来检测生态系统中被忽视的微妙的气候变化影响。我们的总体目标是探索土壤有机质中非交换性O的18O值在生态背景下的意义。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)通过微生物和胞外酶活性确定环境水中O并入OM的动力学，(Iii)评估OM分解是否与O同位素分馏有关，如果是，则量化净表观同位素分馏，(Iv)评估降水的18O值与森林土壤有机层非交换性O库之间的关系，以及(V)调查整体有机质中非交换性O的18O值是否与选定的生物标记化合物的相关性。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，以及(B)将开发一种基于提取氧阴离子和通过消声破坏OM的新方法。我们将选择最可靠的方法，并在受控条件下进行实验，以便能够追踪和量化环境水中O和H的掺入以及相关的同位素分馏(WP2)。此外，还将进行实地研究，研究自然有机质中可交换的O和H的程度，以及环境水中O和H在C、N、O和H系统中的并入和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，以指示与气候有关的过程变化(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).