Saline soil represents an important reserve of cultivated land in China, yet poor soil conditions and low-nitrogen use efficiency constrain crop production. Biochar has been widely applied to improve soil properties; however, its interactive effects with nitrogen fertilization in saline soils remain unclear. A pot experiment using coastal saline soil collected from the northern Jiangsu province was conducted to evaluate the combined effects of biochar (0%, 4%, and 8% w·w⁻¹) and nitrogen fertilizer (0, 150, and 200 mg·kg⁻¹) on the growth performance, photosynthetic indices, yield, quality, and nitrogen use efficiency of water spinach (Ipomoea aquatica Forssk.). Moderate biochar application significantly improved vegetative growth of water spinach, as indicated by higher plant height, stem diameter, leaf area index, and SPAD values. In addition, biochar substantially enhanced photosynthetic performance, dry matter accumulation, and yield, whereas excessive biochar or nitrogen application generally inhibited plant performance. The combined application of 4% biochar with 150 mg·kg⁻¹ nitrogen consistently produced the highest yield and nitrogen partial factor productivity, while simultaneously increasing soluble protein, soluble sugar, and vitamin C contents and reducing nitrite accumulation. These research results demonstrated a clear synergistic interaction between biochar and nitrogen fertilization. In coastal saline soils, reducing the usage of nitrogen fertilizer moderately and adding approximately 4% of biochar is an effective strategy.

To address freshwater scarcity in agriculture, the use of brackish and reclaimed water for alternate irrigation has emerged as a viable alternative. This study evaluated four biochars (rice husk, peanut shell, rice straw, and wheat straw, applied at 2%) and three silicon fertilizers (Lang-Si (S1), Nayou-Si (S2), and sodium metasilicate pentahydrate (S3)) as amendments for sandy loam soil (Lang-Si, Nayou-Si, foliar spray at 1000× dilution; sodium metasilicate pentahydrate, foliar spray at 150 mg∙L⁻¹). Their effects on soil salinity, physicochemical properties, and microbial community structure were assessed under alternate irrigation with brackish and reclaimed water. Alternate irrigation reduced soil electrical conductivity and increased total phosphorus (TP) content compared to single-source irrigation. The effects of amendments varied by type. Biochars improved soil fertility and reduced salinity: peanut shell biochar decreased EC by 15.5%; rice husk biochar increased total nitrogen (TN), TP, and organic matter (OM) by 11.8%, 8.2%, and 10.1%, respectively; and wheat straw biochar elevated subsurface soil TN and OM by 14.1% and 40.0%. Straw-derived biochars and sodium metasilicate pentahydrate maintained higher bacterial α-diversity (Shannon index ≥ 6.67). These effects corresponded with the nutrient adsorption capacity of biochars and the ionic stress alleviation by soluble silicon. The correlation analysis identified OM, TN, TP, and EC as the key drivers shifting the microbial community. Straw-derived biochars and sodium metasilicate pentahydrate are suitable amendments for alternate irrigation systems. These materials balance salinity control, fertility improvement, and microbial conservation, offering practical options for sustainable use of brackish and reclaimed water in agriculture.