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

Abstract

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.

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

“…S2). Such patterns have been observed in many previous calibration studies (Brooks et al, 2010;Evaristo et al, 2015;Sprenger et al, 2016Sprenger et al, , 2017Wang et al, 2017;Benettin et al, 2018;Barbeta et al, 2019;Penna and Van Meerveld, 2019;Liu et al, 2021aLiu et al, , 2022. The slopes of the LMWLs were lower in July (6.79) than in May (7.04) and September (6.85) but were not significantly different (ANCOVA test: df = 0.47, F = 2.49, p = 0.11 > 0.05).…”

“…δ 18 O leaf and δ 2 H leaf values are influenced first by a plant's source water (mainly water taken up by roots from the soil; Cernusak et al, 2016;Barbour et al, 2017;Munksgaard et al, 2017;Liu et al, 2022) and second by the enrichment associated with transpiration (Munksgaard et al, 2017). Soil water for terrestrial plants generally originates from local precipitation, and precipitation isotopes vary spatially and temporally, being subject to controls including temperature, altitude, latitude, distance from the coast, and amount of precipitation (Bowen, 2010;Bowen and Good, 2015;Cernusak et al, 2016).…”

“…These studies rested substantially on the assumption that no isotopic fractionation of δ 18 O and δ 2 H values occurs during water uptake by plant roots (Dawson and Ehleringer, 1991;Ehleringer and Dawson, 1992;Chen et al, 2020), except in saline or xeric environments (Lin and Sternberg, 1993;Ellsworth and Williams, 2007). Some recent studies showed, however, that the occurrence of isotopic fractionation during root water uptake was probably more common than previously thought, especially with respect to δ 2 H values (Zhao et al, 2016;Wang et al, 2017;Barbeta et al, 2019;Poca et al, 2019;Liu et al, 2021aLiu et al, , 2022.…”

Water isotopic composition traces source and dynamics of water supply in a semi‐arid agricultural landscape

“…The stable isotopic composition of oxygen and hydrogen in water (i.e., δ 18 O and δ 2 H values) is initially imparted by the source of precipitation and subsequently altered by evaporation and mixing. Evaporation occurs in water reservoirs exposed to radiative forcing, such as open waters (Bowen et al, 2018, 2019; Brooks et al, 2014; Jasechko et al, 2013; Jinzhao Liu et al, 2022) and shallow soil depths (ca. <30 cm; Liu et al, 2021; Martin et al, 2018; Oerter & Bowen, 2019; Sprenger et al, 2016, 2017).…”

“…Soil development is shaped by landform geomorphic and stratigraphic controls (e.g., bedrock, topography) that in turn dictates overall soil water holding capacity (Schaetzl & Thompson, 2015;Sigler et al, 2020;Vidon & Hill, 2004). Soil texture and vertical structure influence water movement and storage, therefore affecting the isotopic composition of 'mobile water' and 'bulk water' in soils (Adams et al, 2020;Liu & Guo, 2022;Sprenger & Allen, 2020). In addition, physical transformation of surface horizons by cultivation and replacement of native perennial vegetation with annual crops transforms soil hydrological processes, which influence groundwater recharge and evapotranspiration in irrigated and non-irrigated systems (Scanlon et al, 2005;Sullivan et al, 2022).…”