Water use patterns differed notably with season and slope aspect for Caragana korshinskii on the Loess Plateau of China

Flexible water use patterns are of great importance for the survival of vegetation in dryland regions. Our study aimed to investigate the influences of season and slope aspect on the water use patterns of Caragana korshinskii on typical shady and sunny slopes on the Loess Plateau in China. We collected xylem water, soil water from the 0-200 cm soil layers, and precipitation in May, July, September and November 2018 and measured their hydrogen (delta D) and oxygen (delta O-18) isotope compositions. The root distribution of C. korshinskii in the 0-200 cm soil layer on both slopes was also investigated. The 0-20, 20-80, and 80-200 cm soil layers were identified as three potential water sources for C. korshinskii, and their proportional contributions to the plant water supply were calculated using a linear mixed model. The results showed that C. korshinskii on the sunny slope absorbed 89.3% of their water from the 0-20 cm soil layer in May but mainly utilized water from the 0-20 and 20-80 cm soil layers in both July and September, and the total contribution percentages of these layers were 84.5% and 89.2%, respectively. However, C. korshinskii on the shady slope extracted water evenly from all three soil layers in May. Thereafter, the plants absorbed water from the 20-80 and 80-200 cm soil layers in both July and September, and the total contribution percentages of these layers were 96.0% and 91.8%, respectively. In November, C. korshinskii absorbed 84.8% and 78.0% of their water from the 80-200 cm soil layer on the shady and sunny slopes, respectively. The generally shallower soil water use of C. korshinskii on the sunny slope than on the shady slope may be due to the larger fine-root (d < 2 mm) length density in the 0-80 cm soil layer on the sunny slope (1.12 +/- 0.28 mm/cm(3)) than on the shady slope (0.57 +/- 0.16 mm/mm(3)) (P < 0.05). This study indicates that C. korshinskii on both shady and sunny slopes seasonally switched their water sources from different soil water layers during the growing season. However, due to their reliance on soil water at the 0-20 cm depth, which is supplied by unstable rainfall, C. korshinskii on sunny slopes may be more vulnerable to future climatic warming and drying on the Loess Plateau. Our findings provide valuable information related to optimal water management in the afforested shrublands in this region.