Drip Fertigation Enhances Nitrogen Uptake and Improves Winter Wheat Yield and Stability Across Planting Densities

Drip fertigation (DF) is increasingly adopted to improve winter wheat productivity, yet its interactions with planting density (PD) and the underlying source–sink mechanisms remain insufficiently quantified. Here, we evaluated winter wheat performance under two water–nitrogen (N) regimes—conventional management (CM) and DF—across a wide PD gradient (100–800 seeds m−2) during two growing seasons. Grain yield, yield components, population traits, dry matter production, source–sink indices, canopy N status, N uptake and N-use efficiencies were assessed. Across seasons, DF increased grain yield by 15.4–20.8% relative to CM. Yield exhibited a quadratic response to PD under both regimes; however, DF shifted the optimal PD upward (456–487 seeds m−2) compared with CM (377–378 seeds m−2) and sustained near-maximum yields over a broader PD range. DF improved population productivity by increasing productive stem percentage and grains per ear, resulting in greater grain number per m2 (sink size). DF also strengthened source capacity during grain filling: post-anthesis dry matter production increased by 15.5–17.6% and strongly associated with yield (r2 ≥ 0.819). Source–sink analysis suggested that DF was associated with more density treatments showing simultaneously high grain number and high post-anthesis dry matter accumulation, a pattern consistent with a broader high-yield density range. Enhanced N acquisition, especially after anthesis, may have contributed to this response. DF increased N nutrition index at anthesis and markedly increased post-anthesis N uptake by 47.7–49.5%, thereby raising total N uptake at maturity and grain N accumulation. DF improved fertilizer-N recovery efficiency and agronomic efficiency by 33.9–42.3% and 26.7–30.9%, respectively. Collectively, DF improved N uptake and source–sink coordination, enabling high yield and reduced yield penalties when planting density deviated from the optimum.