Dynamics of water usage in Haloxylon ammodendron in the southern edge of the Gurbantünggüt Desert

Aims Plant water use is an important aspect of plant-water relations in desert regions, and is vital in understanding the adaptation of desert species to arid environment. Haloxylon ammodendron is a dominant plant species in the Gurbantünggüt Desert, China. Its water use pattern has an important effect on water balance of the local ecosystem and plant community composition. This study aims to investigate the dynamics of water usage in H. ammodendron and its response to soil water fluctuations resulting from precipitation pulses.Methods Oxygen stable isotope ratios (δ ¹⁸O) were measured for xylem water, soil water in different soil layers (0-300 cm depth), precipitation and groundwater. Four potential water sources were classified: shallow (0-40 cm), middle (40-100 cm) and deep soil water (100-300 cm), and groundwater. The possible ranges of potential water sources used by H. ammodendron were calculated using the IsoSource model. Important findings Main water sources for H. ammodendron shifted from the shallow soil water in April to the groundwater during May to September. In April, the contributions of shallow soil water were in the range of 62%-95%, and the possible ranges of middle and deep soil water and groundwater were 0-8%, 0-15% and 0-38%, respectively. However, during May to September, the contribution of shallow soil water decreased drastically while that of groundwater increased rapidly. Contributions of groundwater were in the range of 68%-100%. Haloxylon ammodendron responded differently to two similar precipitation pulses occurred in different months. After the 6.7 mm precipitation pulse on May 22, the usage of soil water increased from 9.8% prior to the event to maximum at 40.4% one day after rainfall (May 23), while the usage of groundwater decreased from 83%-98% to 42%-81%. After 7 mm precipitation pulse on August 31, the usage of soil water was almost unchanged and the usage of groundwater was still up to 71%-98%, implying that roots of H. ammodendron in the upper soil layer are inactive due to the long-lasting low soil water content. Hence, H. ammodendron seemed to be insensitive to the August precipitation pulse. The shallow soil water recharged by snowmelt and precipitation in spring and groundwater are two important water sources for H. ammodendron. Dynamics of water usage in H. ammodendron reflects its adaptation to this water-limiting desert environment.