Characteristics of Dried Soil Layers Under Apple Orchards of Different Ages and Their Applications in Soil Water Managements on the Loess Plateau of China

Negative soil water balance (i.e., water input < water output) can lead to soil desiccation and subsequently the occurrence of a dried soil layer (DSL). The DSLs are generally studied at a specific sampling depth (e.g., 500 cm), and the actual extent of DSLs remains unknown due to the challenge of collecting deep soil samples. To investigate the characteristics of actual DSLs under different ages of apple orchards and ascertain the optimal age of apple orchards for avoiding/controlling the formation of DSLs, soil samples were collected to a depth of 1 800 cm under apple orchards of different ages in Changwu on the Loess Plateau of China. As the ages increased, soil water content (SWC) and mean SWC in DSLs showed an overall decreasing trend, whereas while DSL thickness and the quantity of water deficit (QWD) in DSLs demonstrated an increasing trend. The DSL was the thickest (1 600 cm) under the 17-year-old orchard, the forming velocity of DSL thickness was the highest at the apple tree growth stage of 9-17 years (168 cm year^-1), and the highest increasing velocity of QWD (-181 mm year^-1) was also observed at this stage. The thickness of DSL was significantly correlated with growth age and root depth of apple trees (r > 0.88), whereas the QWD and mean SWC in DSLs were found to have no correlation with them. The optimal age of apple orchards for avoiding/controlling the formation of DSLs was about 9 years. This information provided pertinent references for the management of deep water resources by controlling the growth age of plants.