pH-Dependent Mineral Release and Surface Properties of Cornstraw Biochar: Agronomic Implications

Surface charge and pH-dependent nutrient release properties of cornstraw biochar were examined to elucidate its potential agronomic benefits. Kinetics of element release was characterized by rapid H⁺ consumption and rapid, pH-dependent P, Ca, and Mg release, followed by zero-order H⁺ consumption and mineral dissolution reactions. Initial K release was not pH-dependent, nor was it followed by a zero-order reaction at any pH. Rapid and constant rate P releases were significant, having the potential to substitute substantial proportions of P fertilizer. K releases were also significant and may replace conventional K fertilizers, however, not long-term plant demand. The cation exchange capacity (CEC) of the biochar leached with a mild acidic solution increased linearly from 179 to 888 mmol_c (kg C)⁻¹ over a pH range of 4−8, while the anion exchange capacity of 154 mmol_c (kg C)⁻¹ was constant over the same pH range. Since native soil organic constituents have much higher CEC values (average 2800 mmol_c (kg C)⁻¹ at pH 7), improved soil fertility as a result of enhanced cation retention by the biochar probably will be favorable only in sandy and low organic matter soils, unless surface oxidation during aging significantly increases its CEC.