The stable isotopic composition variation characteristics of desert plants and water sources in an artificial revegetation ecosystem in Northwest China

Characterizing the dynamics of plant water availability and the mechanisms through which plants access available water sources are key challenges in ecohydrology, especially in arid and semi-arid regions. The establishment of artificial revegetation of different ages has formed vegetation landscapes with different chronological sequences in the southeast margin of the Tengger Desert. As an important regulatory factor of the Soil Plant Atmosphere Continuum (SPAC) system, changes in vegetation characteristics will affect the water uptake of the entire ecosystem. In this study, the δ²H and δ¹⁸O isotope ratios in precipitation, plant water, soil moisture, and underground water, and the δ¹³C isotope ratio of plant leaves were measured using the stable isotope technique for artificial revegetation used to fix the sand dunes in the Tengger Desert. The purpose of this study was to establish the isotope content and characteristics of potential water sources for plants and leaf carbon isotopes, to clarify whether the soil moisture condition and duration of sand-fixing influenced the isotopic signal, and to identify the main water source for desert plants. The results indicated that air temperature and amounts of precipitation were two important influencing factors in precipitation isotope ratio change. Soil water isotopic ratios were different under different water conditions, and under the same water conditions, the vertical variation in soil water isotopes in a single area and spatial variation at any depth for different revegetation areas were all significant (p < 0.01). Shallow soil waters were enriched in isotope ratios and the value increased with an increase of the sand-fixing duration (as found in older revegetation areas). The δ²H and δ¹⁸O values in Caragana korshinskii leaves were higher than they were in Artemisia ordosica leaves because the former has a higher transpiration rate. The higher δ¹³C value of Artemisia ordosica plant leaves indicated high water use efficiency, which is a result of better adapting to the arid environment compared to Caragana korshinskii. Overall, water uptake of these two shrubs was mainly (>70%) concentrated at a soil depth of 20–150 cm. For Caragana korshinskii plants, the 60–150 cm soil layer was the main water supply layer, while the water contribution layer of Artemisia ordosica plants focused on the 20–100 cm soil layer and the contribution rate of shallow soil water increased with the increase of sand fixation time in older revegetation areas, regardless of the species used for revegetation.