Nitrogen-Enriched Nanobiochar Enhances Spinach Growth via Improved Nitrogen Retention and Uptake Mechanisms

The increasing demand for sustainable agriculture requires innovative strategies to enhance nitrogen use efficiency while minimizing environmental losses associated with conventional fertilizers. This study aimed to develop and compare ammonium chloride- and ammonium nitrate-modified nanobiochar as controlled-release nitrogen carriers and to elucidate their effects on nitrogen retention, soil properties, and physiological nitrogen utilization in spinach (Spinacia oleracea L.). Nitrogen-modified nanobiochar was synthesized using ammonium chloride (NB-AC) and ammonium nitrate (NB-AN) at three nitrogen rates (0.03, 0.06, and 0.12 g N g−1 NB) and applied to soil at 1% (w/w). Soil properties, nutrient dynamics, and plant growth and physiological traits were analyzed after 15 and 30 days. Nitrogen modification significantly improved soil nitrogen retention and nutrient availability compared with unmodified nanobiochar. The highest nitrogen loading treatments (NB-AC3 and NB-AN3) notably improved spinach growth, photosynthetic efficiency, pigment content, nitrogen metabolism enzymatic activities, and accumulation of key metabolites (soluble sugars, flavonoids). Nitrogen-release assessments indicated a pronounced controlled-release with reduced nitrogen leaching and greater retention, particularly under NB-AN3. Overall, this study demonstrates that nitrogen-modified nanobiochar functions as an effective nitrogen carrier that enhances nitrogen utilization and growth. These findings provide mechanistic insights into its potential as a sustainable alternative to conventional nitrogen fertilizers.