Drip Emitter-Line Layout Modulates Spatial Coupling Between Phenolic Load and Biological Activity and Improves Soil Biochemical Status in Young Apple–Soybean Alley-Cropping Systems

In semi-humid loess regions, young apple–soybean alley-cropping is constrained by water competition and phenolic accumulation, but how emitter-line layout reorganizes soil biochemical environment remains unclear. This study evaluated whether locally used management systems differed in the spatial coupling of phenolic load, biological activity, and productivity. A two-year field experiment compared rainfed apple monoculture, rainfed soybean monoculture, rainfed apple–soybean alley-cropping, and drip-irrigated alley-cropping with one emitter line per soybean row (DL1), per two rows (DL2), or per three rows. Drip-irrigated monoculture was not included because monocultures are generally rainfed locally owing to lower water demand relative to regional precipitation. Soil phenolics, enzyme activities, microbial abundances, yield, and WUE were assessed using spatial profiling, PLS-SEM, SHAP-based model interpretation, and a network-informed soil quality index (SQI). Drip-irrigated alley-cropping, particularly DL1 and DL2, was associated with lower phenolic load and higher biological activity than rainfed systems. Under DL treatments, phenolics were concentrated mainly in the 40–60 cm layer, whereas enzymes and microorganisms remained enriched in the 0–20 cm layer and shifted horizontally away from the tree row, indicating reduced spatial overlap between chemical constraints and biological activity. Productivity was more closely associated with microbial abundance under rainfed conditions but with enzyme activity under drip irrigation. The network-informed SQI discriminated treatments more clearly than conventional indices. DL1 had the highest SQI, whereas DL2 achieved the highest productivity. Among locally relevant systems, DL2 provides a practicable emitter-line layout that balances soil biochemical improvement with crop productivity.