Abstract:

Objective: Seasonal drought is a significant factor limiting tree growth, and high-intensity water use by trees can easily lead to soil dry layer formation. This study investigates the characteristics of soil water movement in Robinia pseudoacacia plantations in the loess region of western Shanxi under simulated drought conditions, aiming to provide theoretical support for optimized vegetation allocation in this area. Method: A 20-year-old R. pseudoacacia plantation in the western Loess Plateau was selected for an in-situ controlled experiment with three gradients: control (natural conditions), dry (50% precipitation reduction), and extremely dry (100% precipitation reduction). Soil water content (SWC) and soil water potential (SWP) were measured at 9:00 on the day after rainfall events and on typical sunny days using the oven-drying method and soil tensiometers, respectively. The zero flux plane (ZFP) method was applied to analyze soil water transport patterns under different drought gradients. Result: 1) In the control plot, torrential rain events increased the soil moisture content in the 0–60 cm layer, with the increase diminishing with soil depth. The post-rain moisture content in the 0–10 cm layer reached 32.42%±8.63%, rate of increase: 60.4%. Moderate and light rainfall only transiently affected shallow SWC, while deep soil layers (>60 cm) showed no response. Dry treatment exhibited limited SWC increases in shallow layers (only after torrential rainfall, peaking at 12.25% ± 3.51%), and extremely dry treatment showed no SWC changes. 2) After torrential rainfall, SWP in the shallow layer (15–30 cm) of the normal treatment significantly increased by 25 kPa, showing a strong positive correlation with rainfall (r = 0.781). Moderate and heavy rainfall had minimal effects. Dry treatment showed weaker SWP responses (r = 0.710), while extremely dry treatment exhibited no significant correlation. 3) The distribution of zero flux planes (ZFPs) varied with different drought treatments. In the normal plot, precipitation significantly influenced the ZFP position: after torrential rainfall, water transport in the normal treatment shifted from upward evaporation to downward infiltration, with ZFP descending to 60 cm. In dry treatment, water primarily migrated to deeper layers (45–60 cm). Extremely dry treatment maintained continuous evaporation in the surface layer (0–30 cm) and upward water movement in deep layers (80–100 cm), with stable ZFP positions. Under moderate and light rainfall, the overall direction of soil water movement across all gradients remained unchanged before and after the rainfall. Conclusion: Soil water dynamics in R. pseudoacacia plantations in the Loess Plateau of western Shanxi Province exhibit significant vertical stratification. The variability of SWC and SWP decreases with soil depth and drought intensity. Soil water transport direction is jointly influenced by depth and drought severity. Torrential rainfall drives shallow soil water recharge, but as drought intensifies and shallow water becomes insufficient, R. pseudoacacia roots absorb deeper soil water to meet transpiration demands. This study reveals the characteristics of soil water movement in R. pseudoacacia plantations under drought conditions, providing theoretical support for optimized vegetation allocation in the region.

Key words: soil water content, soil water potential, drought stress, Robinia pseudoacacia plantations, Loess Plateau of western Shanxi Province