Biochar and nitrogen effects on nitrate leaching, physiological performance, and rhizome quality of turmeric (Curcuma longa L.) in sandy soil

Abstract Background Nitrogen fertilization in sandy soils often results reducing nitrogen use efficiency and posing environmental risks. Biochar has been proposed as a soil amendment to improve nutrient retention and crop quality. However, the combined effect of biochar and graded nitrogen rates on the physiology and rhizome quality of turmeric (curcuma longa L.) remain poorly understand. Methods A factorial pot experiment was conducted using four nitrogen rates (0, 70, 150, and 250 kg N ha⁻ 1 ) and two biochar rates (0 and 15 t ha⁻ 1 ) in a greenhouse. Coffee husk biochar was applied, and measurements were taken for nitrate leaching, physiological traits (chlorophyll, relative water content, stomatal characteristics), and rhizome phytochemical quality (phenolics, flavonoids, curcuminoids, oleoresin, and antioxidant activities. Result Nitrogen and Biochar and their interaction significantly influenced physiological traits and nitrate leaching ( p < 0.001). Without biochar, nitrate loss increased from 360 mg N pot⁻ 1 at N0 to 2906.67 mg N pot⁻ 1 at N250. Biochar reduced losses ton 180–280 mg N pot⁻ 1 across all nitrogen levels. The combination of moderate nitrogen and biochar (N150B15), produced the highest chlorophyll (41.99 SPAD)), relative water content (92.61%), phenolics (39.65 mg GAE g⁻ 1 ), flavonoids (16.22 mg CE g⁻ 1 ), curcuminoids (61.67 mg CUE g⁻ 1 ), oleoresin (61.89%), and antioxidant activity (75.03% 2,2 diphenyl-1-picrylhydrazyl activity, and 45.56 mg AAE g⁻ 1 ferric reducing antioxidant power). Conclusion Moderate nitrogen combined with biochar (N150B15) improved turmeric physiology and rhizome phytochemical quality while reducing nitrate leaching in sandy soils. The findings support biochar as suitable nutrient management strategy, though field-scale validation with diverse biochar types and rates is required before broader recommendations can be made.