Controls on leaf water hydrogen and oxygen isotopes: A local investigation across seasons and altitude

The stable oxygen ( 18Oleaf) and hydrogen ( 2Hleaf) isotopes of leaf water act as a bridge that connects the hydroclimate to plant-derived organic matter. However, it remains unclear whether the source water (i.e., twig water, soil water, and precipitation) or meteorological parameters (i.e., temperature, relative humidity, and precipitation) are the dominant controls on 18Oleaf and 2Hleaf. Here, we reported a seasonal analysis of 18Oleaf and 2Hleaf together with isotopes from potential source waters and meteorological parameters along an elevation transect on the Chinese Loess Plateau. We found that 2Hleaf values were more closely correlated with source water isotopes than 18Oleaf values, whereas 18Oleaf and 2Hleaf values were similarly correlated with meteorological parameters along the elevation transect. Dual-isotope analysis showed that the 18Oleaf and 2Hleaf values were closely associated because of their similar altitudinal and seasonal responses, generating a well-defined isotope line relative to the local meteoric water line (LMWL). We also compared the measured 18Oleaf and 2Hleaf values with values predicted by the Craig-Gordon model and found no significant differences between them. We demonstrate that the first-order control on 18Oleaf and 2Hleaf values was the source water, and the second-order control was the enrichment associated with biochemical and environmental factors on the Loess Plateau.