Optimizing biochar interlayer thickness reshapes soil water-salt trajectory and drives microbial community assembly to enhance sunflower productivity in saline-alkali soils

Soil salinization severely threatens global agriculture. Cultivating oil-type sunflower ( Helianthus annuus L.) on saline-alkali marginal lands is crucial for regional oil resource security due to its robust salt tolerance, offering a dual advantage of marginal soil rehabilitation and sustainable oilseed production. However, maintaining stable crop productivity remains a significant challenge under severe salinity conditions unless soil-zone engineering strategies are precisely optimized. This two-year field experiment in Ningxia, China, evaluated maize straw-derived biochar interlayers of 3 cm (S3), 5 cm (S5), and 10 cm (S10) installed at 20 cm depth alongside a non-biochar control (CK). Results demonstrated that biochar interlayers significantly reshaped soil profile physicochemical properties. In the topsoil, the S10 treatment achieved the most effective salt interception, reducing total dissolved solids and sodium adsorption ratio by 79.4% and 58.0%, respectively, relative to CK. In the subsoil, the S5 treatment significantly elevated soil organic carbon to 16.79 g kg⁻¹ and increased available nutrients, successfully constructing a localized reservoir. Furthermore, the S5 treatment produced the most stable microbial response, relatively enriching Basidiomycota (13.46%) and Mortierellomycota (9.99%). Consequently, the subsoil soil quality index (SQI) under the S5 treatment was the highest, significantly exceeding all other treatments. Driven by these micro-environmental improvements, sunflower grain yields under the S3, S5, and S10 treatments increased by 28.5%, 38.7%, and 29.3% compared to CK. Importantly, this work shows that interlayer thickness directly changes the balance: while a thicker layer excels at surface salt blocking, a moderate 5 cm interlayer offers the optimal balance by activating the subsoil micro-environment to maximize grain yield, estimated oil yield, and economic value. Our findings identify a structurally optimized 5 cm biochar interlayer as a practical engineering solution for sustainable industrial oilseed production on marginal lands without competing with food crops.