Soil compaction mediates root–nutrient coupling associated with wheat yield response to depth-specific fertilization under contrasting long-term tillage systems

Aims Depth-specific fertilization (DSF) has been proposed to reduce nutrient stratification in conservation tillage by relocating fertilizer; however, long-term no-tillage often develops subsurface compaction that restricts rooting and nutrient capture. We investigated whether DSF responses depend on tillage legacy and examined soil physical, biological, and root mechanisms regulating winter wheat yield. Methods A long-term split-plot experiment compared moldboard ploughing (MC) and no-tillage (NC) with fertilizer placements: conventional surface inorganic fertilizer, shallow placement at 0–10 cm (MC-10, NC-10), and deep placement at 15–25 cm (MC-25, NC-25; 50% IF + 50% pig manure). Soil properties (0–40 cm), root distribution (0–60 cm), antioxidant enzyme activities, and wheat yield were evaluated. Results NC exhibited higher bulk density and penetration resistance than MC, leading to strong nutrient stratification and restricted root penetration into deeper soil layers. Nutrient stratification remained higher under NC-10 and NC-25 than under MC-10 and MC-25 despite depth-specific fertilization. Although NC increased surface (0–10 cm) biological activity, indicated by higher microbial biomass C and dissolved organic C, these gains did not improve root–nutrient coupling or grain yield. In contrast, MC created a more root-permissive soil environment, promoted greater root proliferation across the soil profile, and enhanced root antioxidant enzyme activities. As a result, MC-10 achieved the highest grain yield (7909 kg ha⁻¹). Multivariate analyses showed stronger coupling among nutrients, roots, and yield under MC than under NC. Conclusions DSF must be combined with soil compaction–alleviation practices to achieve stable yield benefits under long-term conservation tillage systems.