Water–nitrogen synergy driven by subsoiling increases post–anthesis resource uptake and wheat productivity on the Loess Plateau

Dryland wheat production on the Loess Plateau is constrained by water scarcity and low nitrogen use efficiency. A three-season field experiment (2022–2025) evaluated the combined effects of fallow tillage (subsoiling, SS; no-tillage, NT) and nitrogen rates (0, 90, 135, 180, 225 kg ha⁻¹) in a split-plot design. SS significantly increased soil water storage change during fallow (ΔSWS f ) in the 100–200 cm layer by 37.4 mm, leading to a 101.4% increase in post-anthesis deep soil water use. When combined with optimized N (135–180 kg ha −1 ), SS reduced 0–200 cm nitrate residue compared to NT, whereas excessive N (225 kg ha −1 ) increased deep leaching risk. SS with optimized N also enhanced post-anthesis flag leaf photosynthesis, nitrogen metabolism enzyme activities, and antioxidant capacity, thereby promoting dry matter and nitrogen accumulation, especially in dry seasons. SS with 180 kg N ha −1 in the wet and normal seasons and 135 kg N ha −1 in the dry season achieved the highest grain yield of 7114, 5699, and 5211 kg ha −1 and water productivity of 13.55, 13.39, and 13.04 kg ha −1 mm −1 . SS increased agronomic efficiency, recovery efficiency, and partial factor productivity of N, but these efficiencies declined significantly when N rates exceeded the optimum across all season types. Partial least squares path model identified ΔSWS f as the primary yield driver and flag leaf traits as a key regulator reducing nitrate residue. In conclusion, SS with optimized N rates (135–180 kg ha −1 ) synergistically improves yield, resource efficiency, and environmental sustainability, but excessive N under wet conditions should be avoided to prevent deep nitrate accumulation.