Chemically Modified Nitrogen-Enriched Eucalyptus Biochar Improves Nitrogen Availability and Wheat Productivity in Low-Fertility Sandy Soil

Biochar has strong potential to improve soil nutrient retention and enhance nitrogen (N) use efficiency in cropping systems; however, its role as an engineered slow-release N delivery system remains insufficiently quantified. This study evaluated the effects of ammonium-enriched eucalypt biochar—either pristine (untreated) or chemically modified with HNO 3 and Na 2 CO 3 —on N release, wheat ( Triticum aestivum L. cv. Scepter) performance, and N use efficiency in low-fertility sandy soil. Two pot experiments were conducted. In the first experiment (Exp. 1) (“N rate” experiment), four N application rates (0, 20, 80, and 160 mg N kg -1 soil) were tested at a constant biochar rate. In the second experiment (Exp.2) (“biochar rate” experiment), three application rates (0, 1, and 3 g kg -1 soil) of each biochar type were evaluated at a constant N rate. Plants were harvested at tillering (Zadoks 29) and maturity (Zadoks 92). In Exp. 1 increasing N supply significantly increased spike number, grain dry weight, and nitrogen use efficiency with more than a twofold increase observed across both biochar treatments at 160 mg N kg -1 relative to the 0 N treatment. In Exp. 2, chemically modified N-enriched biochar consistently outperformed pristine biochar at equivalent application rates, significantly doubled spike number, grain yield, and nitrogen use efficiency at 3 g Bc kg -1 . The improved performance of modified biochar is attributed to enhanced nitrogen retention and controlled release, reflecting a shift from physical adsorption to charge-mediated nutrient regulation. Overall, chemically modified ammonium-enriched eucalypt biochar functions as an effective slow-release N fertilizer, reducing nitrogen losses while improving nitrogen use efficiency and wheat productivity in low-fertility sandy soils. Ammonium-enriched eucalypt biochar improved soil N retention and plant N recovery reducing fertilizer requirements suggest more efficient nutrient management and support the sustainable intensification of marginal sandy agroecosystems.