Vertical variation in shallow and deep soil moisture in an apple orchard in the loess hilly–gully area of north China

Deep soil (>2 m) moisture is relatively stable water that supports tree growth and water management in the loess hilly–gully area of China. Characterizing the spatial variation of deep soil moisture is important for water management in apple orchards. Using a space-for-time substitution approach, we analysed the spatial variation of soil moisture content (θ) in 0–8 m soil profiles in a dry year (2015, precipitation: 392 mm) in rainfed apple (Malus pumila Mill.) orchards of various ages (mainly established in 2009, 2006, 2003, 1997 and 1994). The soil moisture surveys revealed: (a) four soil layers according to soil moisture distribution characteristics at 0–8.0 m depth: shallow rapidly changing layer (0–0.6 m), rainfall redistribution layer (0.6–2.0 m), transition layer (2.0–4.0 m) and stable layer (4.0–8.0 m); (b) the most critical zone for water consumption is the 0.6–2.0 m soil layer, with the lowest θ in the orchards, while water in the 4.0–8.0 m soil layer remained stable; and (c) significant (p <.01) positive correlations between θs in the 0.6–2.0 m, 2.0–4.0 m and 4.0–8.0 m layers. In addition, the θs showed a significant (p <.01) negative relationship with orchard age in the deep soil layers (>2 m), and especially in the 0.6–2.0 m soil layer. Therefore, effective water management through various water regulation measures at the 0.6–2.0 cm soil layer is essential for the sustainable development of apple orchards and reducing drought stress on the Loess Plateau and, potentially, other semi-arid orchards.