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

Study region:Xindiangou Watershed in Suide, the Loess Plateau, ChinaStudy focus: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. New hydrologic insights for the region: 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.

Automated summary

This study investigates whether old-aged vegetation continues to use deep soil water (DSW) under dried soil layers (DSLs) in the Loess Plateau in China. The results show that old-aged P. tabuliformis utilizes DSW in the 17-22 m layer, while old-aged C. korshinskii does not. Furthermore, the depletion of DSW is significantly correlated with clay content.