2.2. Field sampling
We collected the samples, concluding the plant and soil, from each of the two experimental sites during the re-green season, growing season, and withering season in 2021 (20 May, 15 June, 16 July, 16 August, 7 September, and 12 October). Three replicates per plant species were randomly chosen on each sampling date in the two sites, and we collected a total of 72 xylem water samples. The lignified stem sections ofP. squamosa were sampled at a height of 10 cm above the soil surface. In order to avoid the evaporative gas exchange in the bark tissue leading to isotopically enriched water, we removed the outer bark and phloem on the stem to separate the xylem for isotope analysis (Dawson&Ehleringer, 1993; Martín-Gómez et al., 2017). Three soil cores were extracted for soil samples at 0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, 20–30 cm, 30–40 cm, 40–50 cm, 50–60 cm, 60–80 cm, and 80–100 cm within a 2 m radius around the selected plants. A total number of 360 soil samples were collected for isotopic analysis. All plant and soil samples were sealed in 20 ml glass vials with parafilm, and stored in a refrigerator at −10 °C until water extraction was performed using cryogenic vacuum distillation.
Precipitation and groundwater were sampled concurrently with soil water and plant water from May to October. The event-based precipitation samples were collected using a polyethylene bottle at the herder’s home, 2 km away from our sites during the study period. Groundwater samples were collected once a week from a well 2 km away from our sites. The well had a depth of 2 m, and was commonly used for groundwater monitoring. A total of 162 and 72 samples from precipitation and groundwater, respectively, were all transferred into clean polyethylene bottles and stored in a refrigerator at −10 °C until water extraction was performed using cryogenic vacuum distillation.
The soil water content (SWC) was determined using an automatic soil moisture monitoring system (CR800; Campbell, USA) with sensors installed at depths of 5 cm, 10 cm, 15 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 80 cm, and 100 cm below the soil surface. Precipitation measurements were collected using a precipitation gauge (52,203, RM Young, USA) at a height of 0.5 m. Temperatures were obtained from a meteorological station (Molis 520; Vaisala, Finland). All data were recorded every 30 min.