X-ray absorption near edge structure spectroscopy reveals phosphate minerals at surface and agronomic sampling depths in agricultural Ultisols saturated with legacy phosphorus.

Legacy phosphorus (P) soils have received excessive P inputs from historic manure and fertilizer applications and present unique management challenges for protecting water quality as soil P saturation leads to increased soluble P to waterways. We used P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the dominant P minerals in four representative legacy P soils under conventional till and no-till management in Maryland, USA. Various measures of extractable soil P, including water-extractable P (20.6-54.1 mg kg at 1:10 soil-to-water ratio; 52.7-132.2 mg kg at 1:100 soil-to-water ratio), plant available P extracted with Mehlich 3 (692-1139 mg kg), and Mehlich 3P saturation ratio (0.54-1.37), were above the environmental threshold values, suggesting the accumulation of legacy P in soils. The quantification of dominant P minerals may provide insights into the potential of legacy P soils to contribute to P release for crop use and soluble P losses. Linear combination fits of XANES spectra identified the presence of four phosphate mineral groups, consisting of (i) calcium-phosphate minerals (11-59%) in the form of fluorapatite, β-tricalcium phosphate, and brushite, followed by (ii) iron-phosphate minerals (12-49%) in the form of ludlamite, heterosite, P sorbed to ferrihydrite, and amorphous iron phosphates, (iii) aluminum-phosphate minerals (15-33%) in the form of wavellite and P sorbed to aluminum hydroxide, and (iv) other phosphate minerals (5-35%) in the form of copper-phosphate (cornetite, 5-18%) and manganese-phosphate (hureaulite, 25-35%). Organic P consisting of phytic acid was found in most soils (13-24%) and was more pronounced in the surface layer of no-till (21-24%) than in tilled (16%) fields. Of the P forms identified with XANES, we conclude that P sorbed to Fe and Al, and Ca-P in the form of brushite and β-tricalcium phosphate will likely readily contribute to the soil WEP pool as the soil solution P is depleted by crop uptake and lost via runoff and leaching.