Biochar and nitrification inhibitors were demonstrated to be effective materials for enhancing soil quality and fertilizer utilization. However, the combined effects on soil modification, nitrogen transformation and storage, and crop yield in soybean-wheat crop system of saline alkaline remain ambiguous. A field experiment was conducted, including five treatments: CK: no nitrogen fertilizer applied, N: formulated urea, ND: urea + dicyandiamide (DCD, 2 %), NB: urea + biochar (1 %, 30 t ha(-1)), NBD: urea + DCD (2 %) +biochar (1 %). The results indicated biochar and DCD synergistically enhanced saline-alkali soil fertility and crop productivity. Biochar elevated SOM, while DCD enhanced nitrogen conservation. Their combination (NBD) significantly improved structural stability of soil aggregates, promoting the transformation of < 0.053 mm particles to 0.25-0.5 mm micro-aggregates (increased 10-20 %) and increasing MWD/GMD. Microorganisms immobilized nitrogen was significantly higher in soybean season than in wheat season, especially NBD treatment. NBD promoted the microbial transform inorganic-N into organic-N, enhanced soil's N nutrient supply capacity, and promoted the storage and renewal of soil organic nitrogen (SON). DCD had a stronger effect on regulating SON components than biochar, predominantly affecting the fungal community structure and alpha-diversity, promoting soil N supply, but the effect was not significant after combination. Active SON showed significant positive correlations with soil protease, urease, and nitrate reductase, and biochar enhanced the organic N mineralization, particularly by augmenting the activity of the rate-limiting enzyme-protease, increased the nitrogen supply during the wheat season. The yield, stem weight and above-ground biomass were significantly increased with biochar and DCD, and the total annual yield increased by 15.5-17.1 %, and the fertilizer use efficiency was significantly improved (44 % similar to 70 % and 77 similar to 86 %) in the soybean and wheat seasons, respectively. The combined of biochar and DCD, via the "inhibition of nitrifying bacteria + activation of fungal mineralization" dual pathway, mitigates nitrogen loss, fosters SON mineralization, and synergistically enhances N retention and plant effectiveness. The study offers empirical evidence and theoretical underpinnings for the effective management of saline-alkali land.