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

The stable oxygen (δ18Oleaf) and hydrogen (δ2Hleaf) 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 δ18Oleaf and δ2Hleaf. Here, we reported a seasonal analysis of δ18Oleaf and δ2Hleaf together with isotopes from potential source waters and meteorological parameters along an elevation transect on the Chinese Loess Plateau. We found that δ2Hleaf values were more closely correlated with source water isotopes than δ18Oleaf values, whereas δ18Oleaf and δ2Hleaf values were similarly correlated with meteorological parameters along the elevation transect. Dual-isotope analysis showed that the δ18Oleaf and δ2Hleaf 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 δ18Oleaf and δ2Hleaf values with values predicted by the Craig–Gordon model and found no significant differences between them. We demonstrate that the first-order control on δ18Oleaf and δ2Hleaf values was the source water, and the second-order control was the enrichment associated with biochemical and environmental factors on the Loess Plateau.