Synergistic Effects of Subsurface Drainage and Root-Zone Oxygenation on Wheat Yield and Ion Homeostasis in Saline Soils with Shallow Groundwater

Shallow groundwater in saline soils creates a self-reinforcing cycle where waterlogging-induced root hypoxia impairs the ATP-dependent sodium exclusion mechanisms that plants rely on for salt tolerance. We conducted a two-year field experiment to test whether subsurface drainage must precede root-zone aeration for oxygen delivery to be effective. The experimental site was located in Heyang County, Weinan City, on the Guanzhong Plain of Shaanxi Province, north-central China—a major alluvial agricultural region representative of shallow-groundwater-induced salinization. The site had saturated paste electrical conductivity of 6.0 dS m−1 and groundwater depth fluctuating between 0.5 and 1.4 m. A randomized complete block design with 2 × 2 factorial arrangement compared four treatments: control (CK), subsurface drainage only (SD), root-zone aeration only (RA), and both interventions combined (SD + RA). Drainage increased air-filled porosity from 5.8% to 13.5%, crossing the 10.2% threshold (95% CI: 9.1–11.3%) where gas-phase continuity emerges according to segmented regression analysis. Without drainage, aeration achieved only 4.2 mg L−1 dissolved oxygen with high spatial variability (CV 12.5%), while the combined treatment reached 6.8 mg L−1 (CV 6.8%). Root ATP content increased by 89% in SD + RA compared to control, accompanied by 56% lower root Na+ and 185% higher K+/Na+ ratio. These physiological changes correlated with 31% higher grain yield (7580 vs. 5798 kg ha−1). The synergy index of 1.40 (95% CI: 1.28–1.52) indicated that combined effects exceeded the sum of individual treatments by 40%. Methane emissions declined by 62%, and the system achieved a 2.9-year payback period with a benefit–cost ratio of 4.08. These results establish drainage as a physical prerequisite for effective oxygenation, providing a mechanistic explanation for the variable performance of aeration systems reported in previous studies.