Biochar (BC) demonstrates considerable potential for reducing nitrogen emissions. However, it frequently exhibits a limited capacity for the adsorption of NO
3−-N, thereby reducing its effectiveness in mitigating N
2O emissions. Nano-biochar (NBC) is attracting attention due to its higher
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Biochar (BC) demonstrates considerable potential for reducing nitrogen emissions. However, it frequently exhibits a limited capacity for the adsorption of NO
3−-N, thereby reducing its effectiveness in mitigating N
2O emissions. Nano-biochar (NBC) is attracting attention due to its higher surface energy, but there is a lack of information on enhancing NO
3−-N adsorption and reducing N
2O emissions. Accordingly, this study conducted batch adsorption experiments for NO
3−-N and simulated N
2O emissions experiments. The NO
3−-N adsorption experiments included two treatments: bulk BC and NBC; the N
2O emissions experiments involved three treatments: a no-biochar control, BC, and NBC. N
2O emissions experiments were incorporated into the soil at mass ratios of 0.3%, 0.6%, 1%, and 3%. The results demonstrate that NBC exhibits nearly twice the NO
3−-N adsorption capacity compared to bulk biochar (BC), with adsorption behavior best described by a physical adsorption model. The enhanced adsorption performance was primarily attributed to NBC’s significantly increased specific surface area, pore volume, abundance of surface acidic functional groups, and higher aromaticity, which collectively strengthened multiple sorption mechanisms, including physical adsorption, electrostatic interactions, π–π interactions, and apparent ion exchange. In addition, NBC application (0.3–3%) reduced cumulative N
2O emissions by 11.60–54.77%, outperforming BC (9.16–32.65%). NBC treatments also increased soil NH
4+-N and NO
3−-N concentrations by 2.4–8.2% and 7.3–59.0%, respectively, indicating improved inorganic N retention. Overall, NBC demonstrated superior efficacy over bulk BC in mitigating N
2O emissions and conserving soil nitrogen, highlighting its promise as a sustainable amendment for integrated nutrient management and greenhouse gas reduction in soil.
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