Oxygen isotope signatures of plant and soil organic matter

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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).