Deep vertical rotary tillage and planting density enhance salt leaching and water productivity via root-mediated processes in saline cotton fields

Deep vertical rotary tillage (DVRT) with medium density maximizes root plasticity and stress resilience. DVRT–density synergy boosts water–nitrogen use under saline-alkaline conditions. DVRT enables soil remediation with effective pH and salinity reduction. DVRT–density strategy boosts yield by >50% and improves resource efficiency. Optimizing root architecture is vital for enhancing crop stress tolerance and resource use efficiency. This study examined the interactive effects of deep vertical rotary tillage (DVRT) and planting density on cotton root morphology, water–nitrogen utilization, and soil quality in arid saline-alkali fields. A two-year field experiment was conducted in Xinjiang, China, using cultivar ‘Xinluzhong 56’ under a split-plot design. The main plots included four tillage regimes: conventional plow tillage at 20 cm (D0) and DVRT at 20 cm (D1), 40 cm (D2), and 60 cm (D3). The subplots consisted of three planting densities (R1, 17.6×10 4 ; R2, 21.1×10 4 ; R3, 26.4×10 4 plants ha -1 ). Root traits were significantly improved with increasing DVRT depth. The D3R2 combination was identified as the optimal treatment, significantly increasing root surface area, volume, and diameter by 45.15, 34.85, and 48.05%, respectively, compared with the conventional practice (D0R2). However, excessive density (R3) offset these benefits, resulting in an 11.49% reduction in root volume in D3R3 compared with D3R2. DVRT promoted deeper roots, with 38.54% of root length density in the 40–60 cm layer under D3 versus 19.24% under D0. In 2022, D3R2 increased shoot and root biomass (124.10 and 9.30 g/plant) and root activity (69.92% compared to D0R2). This treatment increased yield (50.33% over two years), water productivity (56.61–62.62%), and partial factor productivity (50.03–50.63%). DVRT also reduced topsoil pH and achieved a desalination efficiency of 55.15%. Path modeling showed that yield gains mainly stemmed from root deepening and soil amelioration, whereas density enhanced root biomass and activity. DVRT at 60 cm with 21.1×10 4 plants ha -1 is recommended to maximize cotton yield and resource use in saline-alkali soils.