The Annual Dynamics of the Water Source of an Ammopiptanthus mongolicus Community in the Gobi Desert of the Mongolian Plateau

The Gobi Desert is one of the harsh terrestrial ecosystems distributed on the Mongolian Plateau and northwest China. Water is the key restricting environmental factor for the Gobi Desert plants’ growth. Exploring the annual dynamic of water sources for the vulnerable plants in the Gobi Desert helps to understand their adaptation to the arid climate and is fundamental for their conservation, e.g., the vulnerable plant Ammopiptanthus mongolicus (Maxim. ex Kom.) S. H. Cheng. The water source of the dominant and companion shrubs in a Gobi Desert A. mongolicus community was determined by comparing the δD and δ18O values of their xylem water and different layers of soil water using the MixSIAR model from spring to autumn over two years. The results showed that A. mongolicus mainly utilized 50–150 cm of middle and deep soil water. However, it also used 10 or 25 cm of surface soil water after heavy rains in the early spring and moderate rains in the autumn of 2021 and after heavy rains in the summer of 2022. Three companion shrubs (Nitraria sphaerocarpa Maxim, Caragana korshinskii Kom, and Convolvulus tragacanthoides Turcz.) had similar main water sources, competing for relatively reliable deep soil water with the dominant A. mongolicus during droughts in 2021 and 2022. Moreover, A. mongolicus utilized more deep soil water in 2021 with less rain. However, C. tragacanthoides used more shallow soil water during the growing season of 2022 with more rain. Therefore, four xerophytic shrubs in the A. mongolicus community utilized soil water in different layers based on their ability to adapt to the annual fluctuation of rain in the Gobi Desert.