Mulching duration and irrigation regulate spatiotemporal soil hydrothermal dynamics and root niche differentiation in an alley cropping system

Optimizing soil hydrothermal dynamics is critical for sustaining agroforestry productivity in semi-arid regions, yet the interactive effects of plastic mulching and irrigation on root distribution and crop performance remain unclear under interannual climatic variability. A three-year field experiment was conducted from 2021 to 2023 in a young apple–soybean alley cropping system on the Loess Plateau, using three irrigation limits (50%, 65%, and 80% of field capacity; W1–W3) and two mulching durations (half-period mulching and full-period mulching; M1–M2). High-frequency in situ monitoring and structural equation modeling (SEM) were used to examine the statistical links among soil hydrothermal conditions, plant morpho-physiological traits, soybean grain yield and water use efficiency (WUE). Results revealed that mulching duration and irrigation reshaped spatiotemporal soil hydrothermal patterns and were closely associated with root niche responses in the alley cropping system. Soil water depletion was strongest near the tree–crop interface, while film mulching generally buffered near-surface temperature fluctuations. Compared with full-period mulching, half-period mulching increased the responsiveness of soil temperature to soil moisture variation, indicating a more dynamic soil water–temperature relationship after film removal. In association with these altered hydrothermal conditions, W2M1 was linked to partial root spatial differentiation, with the Pianka overlap index decreasing from 0.88 in 2021–0.77 in 2023. Treatment performance shifted with climatic conditions. W3M2 exploited yield potential in normal years, but its advantage weakened under drought stress. In contrast, W2M1 showed greater resilience, achieving the highest dry-year yield and the highest three-year mean soybean yield. W2M1 also showed clear advantages in water productivity: its multi-year mean WUE exceeded the overall treatment average by 36.5%, and its dry-year WUE was 128.5% higher than the mean of all full-period mulching treatments. SEM further indicated statistical pathways linking soil hydrothermal conditions and root traits with soybean yield and WUE. Overall, these findings support a dynamic irrigation–mulching strategy rather than a fixed high-input approach: W3M2 may exploit yield potential in water sufficient years, whereas W2M1 provides a more robust option for balancing soil hydrothermal stability, root spatial differentiation, yield stability, and water productivity under climatically variable conditions.