Investigating phosphate sorption reactions in acid soils through solution, NMR, and L- and K-edge XANES analyses

Abstract The plant availability of phosphorus (P) is decreased in acid soils through increased P sorption onto solid-phase binding sites. This is a significant problem in Victoria, Australia. This research utilised a suite of complementary techniques, including classical solution analysis, nuclear magnetic resonance (NMR) and synchrotron-based X-ray technologies (X-ray Absorption Near Edge Structure, XANES; micro X-ray fluorescence spectroscopy, µ-XRF) to investigate the forms of sorbed P in acid soils. The addition of carboxylic acids modified sorption of applied P, with oxalic acid being most effective in competing with P for binding sites. Solid state 31 P NMR showed that inorganic P, as PO 4 was the dominant species present in the bulk soil, while XANES provided information on reactions occurring at (P L-edge XANES) or near (P K-edge XANES) the soil surface. For example, the addition of oxalic acid resulted in dissolution of surface-bound Al, exposing previously occluded P. Key Words