Modeling nitrate and potassium leaching in contrasting soil textures and rice husk biochar

Rice husk biochar was applied at 0, 10, 20, and 30 Mg ha⁻¹ to three soil textures (loamy sand, loam, and clay loam) to evaluate its effects on nitrate (NO₃⁻) and potassium (K⁺) leaching under saturated column conditions. HYDRUS-1D inverse modelling and STANMOD were used to estimate solute transport parameters from breakthrough curves (BTCs) for nitrate and potassium. Biochar effects depended strongly on soil texture and application rate. The 10 Mg ha⁻¹ rate had no significant effect on water flux, whereas higher rates increased flow and reduced bulk density. Moderate biochar applications (10–20 Mg ha − 1 ) improved nutrient retention. In loamy sand, 10 Mg ha⁻¹ biochar reduced cumulative nitrate leaching by 28%, and in clay loam, 10 and 20 Mg ha⁻¹ reduced nitrate leaching by 21% and 33%, respectively. Similarly, for potassium, 10–20 Mg ha⁻¹ biochar reduced K⁺ leaching by 13–33% across the soils. In contrast, the highest rate (30 Mg ha⁻¹) often led to earlier breakthrough and increased leached fractions. Biochar amendments also influenced solute transport parameters, with the hydrodynamic dispersion coefficients for NO₃⁻ and K⁺ increasing by up to two to four times at moderate application rates but declining at the highest rate. Inverse modeling with HYDRUS-1D captured the overall leaching trends well (NRMSE approximately 16–24%, index of agreement d between 0.96 and 0.97), though it slightly underpredicted cumulative nitrate leaching (by 9%). Biochar rates of 10 Mg ha⁻¹ for loamy sand and 20 Mg ha⁻¹ for loam and clay loam were the most effective rates. These findings show that the effect of biochar on nutrient transport is rate- and texture-dependent and that excessive application may reduce its retention benefits.