Abstract
Nitrogen (N) leaching from coarse-textured soils poses a significant environmental threat due to high permeability and inherently low nutrient exchange capacity, leading to groundwater degradation and diminished nitrogen use efficiency (NUE). Despite its potential, the depth-resolved and rate-dependent mechanisms governing the interaction between biochar amendments and inorganic N fertilizers remain inadequately characterized. This study evaluated the efficacy of corncob-derived biochar, in synergy with urea fertilizer, on the retention and leaching dynamics of ammonium (NH4+) and nitrate (NO3-) in sandy loam soil. A soil column experiment was conducted under controlled greenhouse conditions across six treatments: an unamended control, urea-only fertilizer, and urea combined with corncob biochar at application rates of 1%-4% (w/w). Biochar amendment significantly changed fundamental soil physicochemical properties, including soil pH, cation exchange capacity (CEC), and water retention capacity. Our results demonstrated that a 3% biochar application rate maximized NH4+ stabilization in subsoil layers, primarily driven by improved CEC. The 4% biochar treatment demonstrated the highest concentration of NO₃⁻ in the soil profile, while an increase in nitrate leaching was also noted. Principal Component Analysis (PCA) elucidated robust correlations between biochar-induced alterations (CEC, pH, and exchangeable cations) and N mobility, highlighting the divergent mechanisms regulating NO3- behavior. While biochar effectively immobilized NH4+, its influence on NO3- leaching was highly sensitive to application rates. These findings provide critical, depth-specific evidence for optimizing biochar-fertilizer formulations, emphasizing that tailored application rates are essential to balance nitrogen stabilization against leaching risks.