Ecohydrological separation in a pair catchments covered with natural grassland and planted forestland on the Chinese Loess Plateau: Evidence from a one‐year stable isotope observation

Plantation and natural restoration has been practised on the Chinese Loess Plateau (CLP). However, how vegetation restorations affect ecohydrological processes remains unclear. Here, we investigated δ18O and δ2H values in rainwater/snow, soil mobile and less‐mobile water, and root/stem and leaf water under different vegetation covers in two neighbouring catchments (non‐woods from DZG‐Grassland, trees and understory from YJG‐Forestland) over 1‐year period. Soil mobile and less‐mobile water showed significantly distinct δ18O and δ2H values across months, but isotopic offsets between plant stems/roots and soil mobile and less‐mobile water (Δ18OM and Δ18OLM or Δ2HM and Δ2HLM) were consistent across vegetation types. These results supported ecohydrological separation, with a dynamic exchange between soil mobile and less‐mobile water on the CLP. The widespread isotopic offsets existed in the contrasted catchments, challenging the prior assumption of no isotope fractionation during root water uptake in the most terrestrial plants. Difference in leaf water isotopes was non‐significant across vegetation types, indicating that leaf water transpiration varied less significantly with vegetation types. Comparisons of the isotopic ecohydrology between forestland and grassland will help elucidate the hydrological cycle on the CLP.