Stable oxygen isotopes reveal distinct water use patterns of two Haloxylon species in the Gurbantonggut Desert

Haloxylon ammodendron and Haloxylon persicum are the dominant species in the Gurbantonggut Desert, China, with the former grows at inter-dune lowland and the later grows at the sand dune. This study aims to investigate the dynamics of water usage of the two species and their responses to the soil water fluctuations resulting from summer precipitation. Oxygen isotope ratios (delta O-18) were measured for xylem water, soil water in different soil layers (0-300 cm), precipitation water and groundwater. Four potential water sources were identified: shallow (0-40 cm), middle (40-100 cm) and deep soil water (100-300 cm), as well as groundwater. The water sources used by the two species were calculated using the IsoSource model. When the upper soil water was abundant in early spring, H. ammodendron mainly used shallow soil water while H. persicum mainly used middle soil water; when the upper soil water was depleted in summer, H. ammodendron mainly used groundwater while H. persicum mainly used deep soil water. Both species were conservative in using shallow soil water following the relatively large summer precipitations. The two Haloxylon species had distinct water use patterns during the growing season, which reflects their adaptations to their specific habitats in this water-limited desert environment. Large precipitations in dry summer hardly altered their water use patterns, which mean that these species are rather conservative in water use strategy.