Subsoil tillage boosts sorghum yield in saline-alkali soils by alleviating constraints on root growth through integrated soil improvement.

To enhance crop productivity in saline-alkali soils, it is essential to overcome soil constraints that limit root growth. This study used sorghum as the test crop and conducted a field experiment in the Hetao Irrigation District of Inner Mongolia to compare subsoiling tillage (ST) with conventional tillage (CT). We evaluated soil physical structure, salt transport, microbial activity, and root morphology, and applied multivariate statistics and structural equation modeling to elucidate the mechanisms through which ST promotes crop growth. ST significantly improved the root-zone soil environment. Bulk density in the 10-40 cm layer decreased by 0.08-0.23 g cm-3, and the three-phase ratio was optimized. Total salt content in the 0-60 cm layer decreased by 30.34%, with reductions in major salt ions (HCO3-, Cl-, Na+). Improved soil structure and alleviated salt stress enhanced microbial activity, with bacterial and actinomycete counts increasing by 96.4% and 394.4%, respectively, at maturity. The optimized rhizosphere supported morphological reconstruction of sorghum roots, with root length, root surface area, and root dry weight increasing by 20.1%, 54.7%, and 100.2%, respectively, compared to CT. Multivariate analysis showed clear separation between ST and CT in principal component analysis (PCA) space, and Random Forest modeling identified HCO3-, bacterial counts, and root traits as key factors influencing yield. Structural equation modeling revealed the causal pathway "soil improvement-root expansion-yield increase." ST increased the R2 of soil physicochemical and biological properties for root traits from 0.38 to 0.65, enhanced the contribution of root traits to yield by 83%, and mitigated the negative effect of salinity. Ultimately, ST increased grain yield by 7.0% and raised the harvest index from 0.49 to 0.56 (P < 0.05). This study demonstrates that subsoiling tillage, by improving the physical, chemical, and biological properties of root-zone soil, systematically optimizes the root habitat and converts saline-alkali stress into a yield advantage, offering a mechanism-based agronomic strategy for arid saline-alkali regions.