Analysis on soil moisture energy feature of apple orchards in Weibei area based on van Genuchten model

Abstract: In arid and semi-arid regions, the emergence of dry soil layer can limit apple tree's growth, development and yield. This study investigated the spatio-temporal variability of soil water suction in apple orchards in Weibei area, northwest China. As the growth of fruit tree was restricted by drought and water deficit in Weibei area, water suction dynamics in 0-150 cm soil profile during apple tree growth stage was investigated in the present study. We chose both young (<10 years) and old (>20 years) orchards, as well as cropland (control), which located in the same site conditions. Stratified sampling was performed at different growth stages of apple trees from March to September, and the moisture in 0-150 cm soil profile was measured using the oven-drying method. According to the soil profile constitutions in Weibei orchard, the centrifuge method and vapor balance method was integrated to measure soil water characteristic curves of tested soil based on the soil genetic layer. The van Genuchten model was used to fit the water characteristic curves. Soil moisture in young orchards, old orchards and farmlands was converted to soil water suction based on the van Genuchten model. The drought stress of planting fruit was evaluated with the farmland as control. The results showed that the van Genuchten model could well fit soil moisture characteristic curves in orchard topsoil, farmland arable layer, black loessial soil and loess subsoil in Weibei, and the fitting accuracy reached 0.96. Orchard soil moisture gradually decreased from mid-March to early July (loss phase), followed by an increase till the end of September (recovery phase). Farmland was greatly impacted by drought stress in Weibei area, and the 0-100 cm soil layer was with water suction higher than 3.98 (heavy water stress) from mid-March to early July. Affected by vegetation canopy cover and fruit growth period, the impacts of drought on orchard were smaller than that on farmland, while the impacts on old orchard were greater than that on young orchard. The high water suction for young orchards occurred at 0-20 cm soil layer from mid-March to early May and from late May to early July, and extended to 40-70 cm soil layer from mid-June to mid-July. The high water suction for old orchards occurred at 0-40 cm soil layer from mid-March to the end of April, at 30-100 cm soil layer from the end of May to mid-July and at 0-20 cm soil layer from mid-July to the end of August. In conclusion, high water suction can appear intermittently in different growth stages and different soil depths for apple orchards with different ages. The influences of drought stress on orchards are smaller than that on farmland and during the recovery phase, "seasonal interior water stress" can be restored in apple orchards, and thus, planting fruit is helpful to alleviate drought stress in Weibei area.