Optimizing rotary tillage depth improves soil water–salt dynamics and crop growth in a drip-irrigated saline wasteland

Efficient root-zone water–salt regulation is essential for improving irrigation effectiveness in drip-irrigated saline wasteland, but the appropriate rotary tillage depth remains unclear. A two-year field experiment in Xinjiang, China compared traditional ploughing at 20 cm (CT) with rotary tillage at 20, 40 and 60 cm (F1, F2 and F3). Soil physical properties, soil water storage, soil salt storage, stage-specific changes in soil salt storage (ΔSSS), Suaeda salsa growth, salt uptake, irrigation water productivity and net profit were evaluated. Compared with CT, F2 and F3 reduced soil bulk density and improved root-zone soil water conditions. During the non-growing season, F3 limited surface salt accumulation, with 0–20 cm ΔSSS values of 9.82–10.69 t ha⁻¹ compared with 16.86–20.65 t ha⁻¹ under CT. During the rapid-leaching period, F2 and F3 enhanced surface salt reduction in 2022, while in 2023 they showed a non-significant tendency toward greater salt reduction in the 20–40 cm layer. Based on total irrigation input, F3 produced the highest two-year mean harvestable forage dry biomass at the mid-growth silage-harvest stage (10.70 Mg ha⁻¹) and biomass-based irrigation water productivity (1.55 kg m⁻³). However, F2 achieved the highest net economic irrigation water productivity (1.38 RMB m⁻³) and net profit (9548.27 RMB ha⁻¹), indicating that the additional biomass gain under 60 cm rotary tillage did not fully offset its higher mechanical operation cost. These results indicate functional differentiation between rotary tillage depths: 60 cm favored deeper-profile water–salt regulation and marketable forage dry matter production, whereas 40 cm provided the best agronomic–economic compromise for productive use of drip-irrigated saline wasteland.