Predictable Oxygen Isotope Exchange Between Plant Lipids and Environmental Water: Implications for Ecosystem Water Balance Reconstruction

Plain Language Summary In this study, we present the first evidence for predictable exchange of oxygen isotopes between water and lipid compounds. Using laboratory incubations with aliphatic alcohols, hexadecanol and eicosanol, and bulk soil lipid extracts in isotopically enriched water for 160days, we determined the magnitude and direction of exchange rates for bulk and compound-specific lipid extracts. Our data show that O-18 ratios of long-chain aliphatic lipids that persist in hydrophobic portions of soil organic matter integrate the signature of plant water O-18 values, which can be used to reconstruct hydrologic shifts in terrestrial systems. For bulk lipid extracts, equilibrium was reached, indicating that 22% of its oxygen content is exchangeable with a half-life of 0.13years. Incubations with the same bulk lipid extracts in contact with iron oxyhydroxide minerals showed no difference in exchange rates, although the exchangeable fraction decreases to 19% of the total. This result suggests that mineral surfaces can inhibit oxygen exchange for some oxygen-containing functional groups. In contrast, pure compounds showed stable oxygen isotope signatures with no exchange under the same conditions. Taken together, these findings represent a significant development in the mechanistic understanding and application of oxygen isotopes in plant- and soil-derived lipids, toward a path for the use of lipid extracts in reconstructions of ecosystem water balance. This work reports on the fundamental chemistry of compounds deposited into soil by plants. These compounds are interesting to us as measurements of their isotopic composition reveal water availability and stress of plants that produced them. Thus, from these compounds, it is possible to gain understanding of water regime shifts over time and space in an ecosystem. However, this is only possible given some assumptions, primarily that these compounds retain their chemical and isotopic composition long after deposition. This research reports new data that show that some compounds of interest meet this assumption for practical applications paving the way for a new way to study how natural systems respond to changes in environmental water.