For many countries, the reclamation of coastal saline–alkali land is of great significance for ensuring food security. However, systematic research on the effects of leaching irrigation and biochar application on the “soil–crop” system in coastal areas remains scarce. This study investigated the effects of three irrigation quotas (8, 16, and 24 mm per application, applied once every 10 days) and four biochar rates (0, 3, 5, 7 t·ha⁻¹, from composite biomass) on soil salinity, available nutrients, and yield and quality of sunflower in coastal saline–alkali soils of Jiangsu, China. The experiment used a completely randomized block design with three replications. The results showed that soil salinity under all treatments exhibited a consistent decline–rebound–decline pattern over the growing season, with the lowest values occurring around 55 days after sowing and peaking around 95 days. The combined application of water and biochar enhanced salt reduction, with the highest irrigation and biochar levels achieving the greatest desalination effect. Biochar application increased soil available phosphorus and potassium contents, with average increases of 19.6% and 13.3% under the highest biochar rate compared with the control, while available nitrogen showed no clear response to any treatment. Crop yield responded nonlinearly to water and biochar inputs: under low irrigation, yield increased continuously with biochar addition, whereas under medium or high irrigation, yield plateaued at the moderate biochar rate (5 t·ha⁻¹), with no further gains from additional biochar or water. Regarding seed quality, linoleic acid content generally increased with higher water and biochar levels, oleic acid decreased, and crude fat remained unaffected. Based on a balanced consideration of yield, quality, salt reduction, and water conservation, the combination of 16 mm irrigation per application and 5 t·ha⁻¹ biochar is recommended as the optimal amelioration strategy for coastal saline–alkali land.

Reclaiming coastal saline–alkali land is important for food security, yet little is known about how irrigation and biochar jointly affect the fate of fertilizer nitrogen in these soils. Using ¹⁵N isotope tracing, this field experiment tracked the distribution and recovery of labeled fertilizer in a soil–sunflower system under three irrigation quotas (8, 16, and 24 mm per event, applied every 10 days) and four biochar rates (0, 3, 5, and 7 t·ha⁻¹). After harvest, approximately 73% of residual ¹⁵N remained in the 0–40 cm topsoil, with organic-bound N as the dominant fraction (72–74%). Mineral ¹⁵N increased with soil depth, indicating downward movement with water flow. Within sunflower plants, labeled N accumulation followed the order flower head > stem > leaf > root, with heads containing 11–12 times more ¹⁵N than roots, confirming active transport to reproductive organs. Overall ¹⁵N use efficiency ranged from 18.8% to 24.9% across treatments. Increasing biochar rate enhanced ¹⁵NUE by up to 28.2% under the same irrigation regime, whereas raising irrigation from 16 mm to 24 mm reduced ¹⁵NUE by 3.2–3.8%. Mass balance analysis showed that moderate irrigation (16 mm) combined with high biochar (7 t·ha⁻¹) achieved the highest plant ¹⁵N recovery (24.9%), maintained 70.0% of labeled N in soil, and limited unaccounted ¹⁵N to only 5.1%. These findings demonstrate that integrated water–biochar management can optimize fertilizer N retention and crop uptake in coastal saline–alkali soils, providing a scientific basis for precision fertilization in these degraded lands.