Objective This study aimed to clarify the effects of biochar-based fertilizer on improving nutrient supply in saline paddy soil and explore its applicability in coastal saline rice production areas, with the ultimate goal of ensuring high and stable rice yields in coastal saline fields through the application of the biochar-based fertilizer.

Methods Three-year continuous field experiments were conducted in the coastal saline paddy field in the Liaohe River Estuary, using "Yanfeng 47" as the test rice variety. Three fertilization treatments were set up: no nitrogen fertilizer (PK, applied P₂O₅ 90 kg hm⁻² and K₂O 75 kg hm⁻²), conventional chemical fertilizers (NPK, applied N 270 kg hm⁻², P₂O₅ 90 kg hm⁻², and K₂O 75 kg hm⁻²), and the biochar-based fertilizer (BF, with equivalent nutrient levels as the NPK treatment). The effects of application of the biochar-based fertilizer on fertility of saline paddy soil, nitrogen accumulation of plant and rice yield were analyzed.

Results Compared with conventional chemical fertilization, continuous application of the biochar-based fertilizer for three years had no significant effect on soil bulk density, porosity, pH, and available potassium of paddy soil. However, soil organic matter, alkaline hydrolysis nitrogen, and available phosphorus contents significantly increased by 22.88%, 17.76%, and 39.50%, respectively. Additionally, soil acid phosphatase, invertase, and catalase activities were significantly higher in the BF treatment than in the NPK treatment. Application of the biochar-based fertilizer led to an increase in the number of effective panicles, grains per panicle, and seed setting rate of rice. Notably, rice yield of the BF treatment was enhanced by 6.92% compared to the NPK treatment (P < 0.05), and the nitrogen use efficiency of fertilizer increased from 42.58% to 53.56% (P < 0.01).

Conclusion Applying the biochar-based fertilizer in coastal saline paddy field is beneficial for improving soil available nutrients and soil enzyme activities, thereby promoting rice yield increase. It is an effective measure to achieve sustainably high and stable rice yield of coastal saline paddy fields.