Effects of Biochar and Nitrogen Co-Application on Growth, Yield, and Quality of Water Spinach in Saline Soil

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.