Optimizing irrigation and mulching strategies to improve root–water relations, water use efficiency, and yield in apple–soybean alley cropping systems on the Loess Plateau, China

The productivity of young apple-soybean alley cropping systems (ASCS) in the Loess Plateau of northwestern Shanxi, China, is threatened by water scarcity and interspecific competition as fruit trees mature. Research on interspecies water utilization in precision irrigation with lateral drip line layouts is limited. A 3-year study evaluated the effects of three drip emitter spacings (L1: one line per soybean row; L2: one every two rows; L3: one every three rows) combined with mulching (M1) and non-mulching (M0) on soil water content (SWC), root distribution, grain yield (GY), and water-use efficiency (WUE) in ASCS. Additionally, two non-irrigated control treatments were included: CK1 (mulching) and CK0 (non-mulching). The results showed that mulching increased SWC (23.59 %) and root length density (RLD) (49.01 %) in the 0–60 cm layer, accompanied by higher water consumption. However, it alleviated the detrimental water competition caused by drought under non-irrigated (CK1) conditions. The narrow L1 and wide L3 emitter spacings exhibited opposing moisture distribution patterns, both restricting root growth in the intercropping system. Notably, L3 reduced soybean RLD by 56.35 % compared to L1. In contrast, the optimal drip spacing L2 created alternating wet and dry zones, enhancing the soil water potential gradient. This distribution promoted opposing root expansion and mitigated competition for water absorption over the growing seasons. Among non-mulched treatments, L1 achieved the highest WUE and GY, increasing by 60.17 % over other non-mulched treatments. In mulched treatments, L2 maximized GY and WUE, increasing by 56.32 % and 46.78 %, respectively, compared to non-mulched L2 treatment. Therefore, one emitter line per soybean row under non-mulched conditions and one per two rows under mulched conditions in 3–6-year-old ASCS is recommended to optimize root-water distribution and improve productivity.