Phosphorus speciation in a long-term manure-amended soil profile – Evidence from wet chemical extraction, 31P-NMR and P K-edge XANES spectroscopy

Abstract

Long-term application of manure can lead to an enrichment of phosphorus (P) in agricultural soils. To which extent this P leaches into drainage systems and thereby potentially contributing to eutrophication of surface waters, depends on the distribution and speciation of P present in the soil. In this study the P speciation and related soil characteristics were investigated for a sandy loam soil that had been receiving manure for >40 years. A combination of solution-state 31P-NMR, P K-edge XANES and wet chemical extractions was applied.Topsoil P contents were with 42 mmol kg−1 more than twice as high as in the sub soil (14–18 mmol kg−1). Linear combination fitting (LCF) of P K-edge XANES spectra indicated that P accumulation in the topsoil occurred predominantly via adsorption of inorganic P to Fe/Al hydroxide surfaces, resulting in substantial P saturation of these mineral phases. Up to 76% of the topsoil P was associated with Al or Fe. The organic P content was low (5–10%), but an additional important topsoil P pool identified with XANES-LCF was amorphous calcium phosphate (30%), which may have been added with manure or formed in situ. Towards the deeper subsoil the relative portion of primary apatite increased progressively and reached 80% of total P at a depth of 70 to 80 cm. These results highlight the importance of Fe and Al mineral phases for P retention in long-term manure-amended soils also under non-acidic conditions.