Soil potassium adsorption and speciation dynamics with associated clay microstructural changes revealed by synchrotron X-ray microscopy

This study examined potassium (K) adsorption under five distinct soil management practices to address key knowledge gaps related to its speciation transformations and microstructural changes in clay minerals. The research evaluated K adsorption across soils with varying physicochemical properties, examined changes in its chemical speciation, and analyzed clay flocculation microstructures using transmission X-ray microscopy. Five soil types were examined: non-fertilized soil, long-term K-fertilized soil, alkaline soil, red soil, and forest soil, treated with K solutions at concentrations of 25, 50, 100, 250, and 500 mg K L^{- 1}. The Langmuir isotherm model demonstrated an excellent fit for K adsorption (r² = 0.99), indicating a monolayer adsorption mechanism. Higher K adsorption rates (11-66%) were observed at lower concentrations (25-100 mg K L^{- 1}), while rates decreased (2-12%) at higher concentrations (250-500 mg K L^{- 1}). Adsorbed K underwent transformation into various speciation forms with exchangeable fractions the most dominant. Multiple regression analysis revealed that soil electrical conductivity (EC), available phosphorus, and cation exchange capacity (CEC) were key parameters influencing K adsorption rates. Also, transmission X-ray microscopy showed significant microstructural changes in clay flocculation after K adsorption. Overall, this research contributes to a deeper understanding of K dynamics in soils and has implications for developing sustainable soil management strategies.