The hydrogen isotope footprint of source water in tree lignin methoxy groups

The stable hydrogen isotope values of tree lignin methoxy groups (δ²H_meth) show a robust relationship with source water hydrogen isotopes (δ²H_SW), enabling the reconstruction of the source water origin using an average hydrogen isotopic fractionation (ε) of around -200 mUr between δ²H_meth and δ²H_SW values (Greule et al., 2021; Keppler et al., 2007). Reconstructed δ²H_SW is currently mainly used for climatic reconstruction of temperature but could also be used to better understand ecohydrological processes such as root water uptake. As the use of lignin methoxy groups as a source water proxy is relatively new, there are still uncertainties regarding additional influences on δ²H_meth interfering the reconstruction of δ²H_SW. Factors such as temporal changes in the isotopic composition of source water, soil moisture, and changes in root system and biomass may influence lignin methoxy fractionation, and a better understanding of these factors is important to improve the application of this proxy.

Here, we analyzed wood samples collected from a dry pine forest in Switzerland, where an extensive irrigation experiment was conducted. The site was divided into eight plots and since 2003 four of these plots received irrigation during the growing season from a nearby channel fed by the Rhone River, doubling the annual precipitation amount in the irrigated (1200mm) compared to the control stands (600 mm). Irrigation water is about 46 ± 9 mUr more depleted in ²H than the soil water, resulting in average δ²H values of -76 vs -68 mUr in irrigated and control soil water (0-10 cm) (Guidi et al., 2023).

We present results of δ²H_meth measurements from four irrigated and four control trees analyzed annually from 1990 to 2023. We observed a significant ²H depletion in the irrigated trees compared to the control trees, supporting the use of this proxy to reconstruct source water changes. By further comparing δ²H_meth values of irrigated and control trees, including root and leaf samples, we gain additional insight into hydrogen isotope fractionation processes in trees, improving our understanding of the influences of biological processes on δ²H_meth. With our study, we hope to contribute to the further development of a new ecohydrological proxy that potentially allows the reconstruction of past variations in root water uptake of plants.

References:

Greule, M., Wieland, A., Keppler, F., 2021. Measurements and applications of δ2H values of wood lignin methoxy groups for paleoclimatic studies. Quat. Sci. Rev. 268, 107107. https://doi.org/10.1016/j.quascirev.2021.107107

Guidi, C., Lehmann, M.M., Meusburger, K., Saurer, M., Vitali, V., Peter, M., Brunner, I., Hagedorn, F., 2023. Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest using a novel hydrogen isotope approach. Soil Biol. Biochem. 184, 109113. https://doi.org/10.1016/j.soilbio.2023.109113

Keppler, F., Harper, D.B., Kalin, R.M., Meier-Augenstein, W., Farmer, N., Davis, S., Schmidt, H.L., Brown, D.M., Hamilton, J.T.G., 2007. Stable hydrogen isotope ratios of lignin methoxyl groups as a paleoclimate proxy and constraint of the geographical origin of wood. New Phytol. 176, 600–609. https://doi.org/10.1111/j.1469-8137.2007.02213.x