## Materials and Methods
### Experimental Design  
The water gradient zone is 8 km long, north of the Aqikesu River, and perpendicular to the riparian zone [55]. The climatic conditions in the gradient zone are relatively uniform, and there is little difference in topography and altitude. In August 2013, three 5 km long transects were established on the water gradient zone at intervals of 5 km [56]. According to the groundwater level and vegetation development, the gradient belt from southwest to northeast can be divided into riverbank habitat, transitional zone habitat, and desert margin habitat [19]. The riverbank habitat is located at the southern end of the gradient zone (0-1.50 km from the riparian zone), and the groundwater table is relatively shallow due to its proximity to the channel. The main plant species are _Populus euphratica, Halomodendron halodendron_, and _Phragmites australis_, and the soil nutrients and biomass are higher than in other regions [57]. The transitional zone habitat is located in the middle of the gradient zone (1.50-4.50 km from the riparian zone), and suitable water content maintains the symbiosis of 17 plants, including _Reaumuria soongorica_ ([PERSON].) [PERSON] and _Aalropus pungens_ (M. [PERSON].) [PERSON]. The desert margin habitat is at the northernmost end of the gradient zone (4.50-8.00 km from the riparian zone). The soil particle size is large, the organic matter content (1.35 g kg\({}^{-1}\)) is low, and the soil water content (1.04%) is also low, making plant survival challenging. The species growing in this area mainly include _Haloxylon ammodendron_, _Nitraria tangutorum_, and _Sueda salsa_[19].  
In mid-July 2018, one of the three replicate transects was selected, and a 50 m \(\times\) 50 m plot was established in each of the three habitats of this transect, namely plot 1, plot 2, and plot 3 (Figure 1). The nested sampling design method was used to systematically divide each plot according to the sampling scales of 10 m \(\times\) 10 m and 5 m \(\times\) 5 m, respectively [48; 49]. For the vegetation survey, Rs monitoring, soil sampling, and plant sample collection, 25 grids were evenly set at each scale. First, the 50 m \(\times\) 50 m plot was equally spaced into 25 grids of 10 m \(\times\) 10 m. Then, the vertices of each grid were marked and fixed with PVC pipes as corner stakes, and the midpoint of four boundary lines of each 10 m \(\times\) 10 m quadrat was measured by a ruler, and each quadrat was further divided into four grids of 5 m \(\times\) 5 m. Finally, a total of 150 grids (150 grids = 25 grids \(\times\) two sampling scales \(\times\) three plots) were established in the three plots. The latitude, longitude, altitude, and other geographic information for each sample plot were recorded by handheld GPS, and each grid was numbered. Due to the negative effects of large-scale destructive soil sampling and expensive labor and time costs, it was impossible to test in all transects [58]. In addition, according to the findings of the previous research group, there were no significant differences in plant composition and soil properties among the three replicate transects (Table S1) [56]. Therefore, we hypothesized that our experimental design could reflect the response of Rs to habitat changes (water gradients) at different spatial scales. This non-repeated experimental design method has been widely used to study the spatial variability of Rs on the fine scale [58; 59; 60; 61].