Deep soil water use of old-aged vegetation (17- to 36-year stand age) after the formation of dried soil layers based on in situ monitoring

Xindiangou Watershed in Suide, the Loess Plateau, China Deep soil water (DSW) is gradually depleted as vegetation stand age increases, resulting in dried soil layers (DSLs) in the Loess Plateau in China. However, it is unclear whether old-aged vegetation continues to use DSW under DSLs. This study monitored soil water five times (2001, 2005, 2006, 2007, 2020) in the deep profile for three common vegetation types [Pinus tabuliformis Carr. (P. tabuliformis), Caragana korshinskii Kom. (C. korshinskii), Platycladus orientalis L. Franco (P. orientalis)] planted in 1985. In 2020, we measured soil water content, root characteristics, and soil particle composition to investigate DSW status and the influencing factors. Soil water in the 2–10 m soil layer did not significantly differ from 17- to 36-year stand for the three vegetation types (p < 0.05), indicating that old-aged vegetation no longer used DSW with a strong soil desiccation index in 2–10 m. P. tabuliformis (23- to 36-year stand) used DSW in the 17–22 m with an average consumption of 11.6 mm year–1, while old-aged C. korshinskii did not use DSW. DSW under long-term depletion correlated significantly with clay content. This study showed unfavorable DSW resources for water use under old-aged vegetation, with further measures to conduct selective thinning or complementary planting considered to alleviate the DSW deficit.