Afforestation vegetation uses water from very deep soil layers in the semi-arid Loess Plateau

Investigating the availability of water in very deep soil layers for vegetation is important for assessing the eco-hydrological effects of the thick loess soil and optimizing soil water management and vegetation restoration initiatives. However, the proportion of soil water sourced from the soil layer deeper than 300 cm by typical afforestation species on the Loess Plateau, along with its seasonal variations, remains unclear. In this study, we examined the composition of the water sources used by two typical afforestation plant species (Caragana korshinskii and Medicago sativa) from soil depths of up to 10 m during the growing season, using isotopic analysis. The results showed that soil desiccation occurred in the soil layers of 260-1000 cm and 260-700 cm at the upper and lower slope positions, respectively, for both species plots. Relative stability of the isotopic values in >260 cm soil layers explained a piston flow occurred in these layers. The contribution proportion of soil water in 260-1000 cm was at 41.7% & PLUSMN; 9.0% and 51.3% & PLUSMN; 8.8% for C. korshinskii on upper and lower slope, respectively and was at 67.6% & PLUSMN; 6.7% and 73.7% & PLUSMN; 7.0% for M. sativa on upper and lower slope, respectively. The serious soil desiccation was induced by the dependence on very deep soil water and the slow recharge with the piston flow. Meanwhile, C. korshinskii converted the main water source between 0-260 cm and 260-1000 cm in different seasons. In contrast, M. sativa used water mainly from soil layers deeper than 260 cm in June and increased the utilization proportion of deeper water sources in July and September. The persistent dependence on very deep soil water for M. sativa, induced a deeper desiccation depth as compared to C. korshinskii. Thus, thinning to the optimal leaf area index and converting to natural grassland were recommended for C. korshinskii and M. sativa plots, respectively to restore the depleted soil water and to promote water cycle.