Strategic differences in phosphorus stabilization by alum and dolomite amendments in calcareous and red soils

Surplus phosphorus (P) above agronomic requirements can negatively affect the water status of connected surface and subsurface water bodies. The in situ stabilization of soil P through soil amendment has been recognized as an efficient way to reduce this environmental pressure. However, the mechanism of how P is stabilized during this process and how plant available P is affected are unknown. This can be achieved by sequential chemical extraction and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy investigations. Therefore, in the present study, P-enriched calcareous and red soils were amended with alum, dolomite, and a 1:1 mixture of alum and dolomite (MAD) at a 20 g/kg soil rate, and soil properties and P fractions were measured after a 45-day period. Results showed that alum amendment significantly decreased CaCl₂-P and Olsen-P contents in calcareous and red soils when compared with dolomite. However, dolomite incorporation maintained relatively high P availability and even increased CaCl₂-P and Olsen-P contents by 1.32% and 40.5% in red soil, respectively, compared to control. Amendment with MAD was not as effectively as the alum in P stabilization. Sequential inorganic P extraction indicated that alum dominantly contributed labile P transformed to Al-P in both soils. P K-edge XANES spectroscopy measurements further explained that alum adsorbed phosphate in calcareous soil and precipitated phosphate as AlPO₄ in red soil. Results of P fractionation and Mehlich-3-extracted Ca showed that dolomite mainly adsorbed loosely bound P in calcareous soil and red soil. However, dolomite incorporation in red soil led to Al-P and Fe-P release. The P sorption isotherms showed that dolomite and alum increased soil P sorption maxima and decreased the degree of P saturation (DPS) in both soils, while dolomite declined the Langmuir bonding energy in red soil. Differences in P stabilization by alum and dolomite addition across soil types were closely related to their characteristics, and soil properties changed, especially soil pH.