Quantifying the Fate of 15N-Labeled Fertilizer in a Soil–Sunflower System as Affected by Irrigation and Biochar Management on 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.