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

Abstract. The stable oxygen (δ¹⁸O_leaf) and hydrogen (δ²H_leaf) isotopes of leaf water act as a bridge that connects 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 δ¹⁸O_leaf and δ²H_leaf. Here, we reported seasonal analysis of δ¹⁸O_leaf and δ²H_leaf together with isotopes from potential source waters and meteorological parameters along an elevation transect on the Chinese Loess Plateau. We found that δ²H_leaf values were more closely correlated with source water isotopes than δ¹⁸O_leaf values, whereas δ¹⁸O_leaf and δ²H_leaf values were similarly correlated with meteorological parameters. Dual-isotope analysis showed that the δ¹⁸O_leaf and δ²H_leaf values were closely correlated because of their similar altitudinal and seasonal responses, and so generated a well-defined isotope line relative to the local meteoric water line (LMWL). We also compared the measured δ¹⁸O_leaf and δ²H_leaf values with predicted values by the Craig-Gordon model, and found no significant differences between them. We demonstrate that the first-order control on δ¹⁸O_leaf and δ²H_leaf values was the source water, and the second-order control was the enrichment associated with biochemical and environmental factors.