Seasonal variation in water uptake patterns of three plant species based on stable isotopes in the semi-arid Loess Plateau

- Seasonal variations of water uptake pattern were determined by dual stable isotopes (δD and δ18O) and MixSIAR model.
- Soil water in the 0-120 cm depth contributed 75-80% to the total water uptake in the growing season.
- Vitex negundo displayed larger degree of ecological plasticity to switch water between shallow and deep soil layers.
- Functionally dimorphic root systems were related to flexible water uptake pattern.

Water is a limiting factor and significant driving force for ecosystem processes in arid and semi-arid areas. Knowledge of plant water uptake pattern is indispensable for understanding soil-plant interactions and species coexistence. The 'Grain for Green' project that started in 1999 in the Loess Plateau of China has led to large scale vegetation change. However, little is known about the water uptake patterns of the main plant species that inhabit in this region. In this study, the seasonal variations in water uptake patterns of three representative plant species, Stipa bungeana, Artemisia gmelinii and Vitex negundo, that are widely distributed in the semi-arid area of the Loess Plateau, were identified by using dual stable isotopes of δ2H and δ18O in plant and soil water coupled with a Bayesian mixing model MixSIAR. The soil water at the 0-120 cm depth contributed 79.54 ± 6.05% and 79.94 ± 8.81% of the total water uptake of S. bungeana and A. gmelinii, respectively, in the growing season. The 0-40 cm soil contributed the most water in July (74.20 ± 15.20%), and the largest proportion of water (33.10 ± 15.20%) was derived from 120-300 cm soils in August for A. gmelinii. However, V. negundo obtained water predominantly from surface soil horizons (0-40 cm) and then switched to deep soil layers (120-300 cm) as the season progressed. This suggested that V. negundo has a greater degree of ecological plasticity as it could explore water sources from deeper soils as the water stress increased. This capacity can mainly be attributed to its functionally dimorphic root system. V. negundo may have a competitive advantage when encountering short-term drought. The ecological plasticity of plant water use needs to be considered in plant species selection and ecological management and restoration of the arid and semi-arid ecosystems in the Loess Plateau.

204