title: XANES Demonstrates the Release of Calcium Phosphates from Alkaline Vertisols to Moderately Acidified Solution.
abstract: Calcium phosphate (CaP) minerals may comprise the main phosphorus (P) reserve in alkaline soils, with solubility dependent on pH and the concentration of Ca and/or P in solution. Combining several techniques in a novel way, we studied these phenomena by progressively depleting P from suspensions of two soils (low P) using an anion-exchange membrane (AEM) and from a third soil (high P) with AEM together with a cation-exchange membrane. Depletions commenced on untreated soil, then continued as pH was manipulated and maintained at three constant pH levels: the initial pH (pHi) and pH 6.5 and 5.5. Bulk P K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the main forms of inorganic P in each soil were apatite, a second more soluble CaP mineral, and smectite-sorbed P. With moderate depletion of P at pHi or pH 6.5, CaP minerals became more prominent in the spectra compared to sorbed species. The more soluble CaP minerals were depleted at pH 6.5, and all CaP minerals were exhausted at pH 5.5, showing that the CaP species present in these alkaline soils are soluble with decreases of pH in the range achievable by rhizosphere acidification.
authors.affiliation: Agronomy and Soil Science, School of Environmental and Rural Science, University of New England , Armidale, New South Wales 2350, Australia.
authors.forename: Karl O
authors.initials: KO
authors.lastname: Andersson
authors.affiliation: Agronomy and Soil Science, School of Environmental and Rural Science, University of New England , Armidale, New South Wales 2350, Australia.
authors.forename: Matthew K
authors.initials: MK
authors.lastname: Tighe
authors.affiliation: Agronomy and Soil Science, School of Environmental and Rural Science, University of New England , Armidale, New South Wales 2350, Australia.
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Guppy
authors.affiliation: Agronomy and Soil Science, School of Environmental and Rural Science, University of New England , Armidale, New South Wales 2350, Australia.|Hawkesbury Institute for the Environment and the School of Science and Health, University of Western Sydney , LB 1797, Penrith, New South Wales 2751, Australia.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Milham
authors.affiliation: Agronomy and Soil Science, School of Environmental and Rural Science, University of New England , Armidale, New South Wales 2350, Australia.|Group of Plant Nutrition, Institute of Agricultural Sciences, Swiss Federal Institute of Technology (ETH) Zurich , Eschikon 33, CH-8315 Lindau, Switzerland.
authors.forename: Timothy I
authors.initials: TI
authors.lastname: McLaren
authors.affiliation: Schefe Consulting , 59 Sheridan Court, Rutherglen, Victoria 3685, Australia.
authors.forename: Cassandra R
authors.initials: CR
authors.lastname: Schefe
authors.affiliation: Future Industries Institute, University of South Australia , Building X, Mawson Lakes Campus, Mawson Lakes, South Australia 5095, Australia.
authors.forename: Enzo
authors.initials: E
authors.lastname: Lombi
mesh_terms: D002130:Calcium Phosphates / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008567:Membranes, Artificial; D008903:Minerals; D010758:Phosphorus / Q000737:chemistry; D058441:Rhizosphere; D012987:Soil / Q000737:chemistry*; D012996:Solutions; D056928:X-Ray Absorption Spectroscopy
chemical_list: D002130:Calcium Phosphates; D008567:Membranes, Artificial; D008903:Minerals; D012987:Soil; D012996:Solutions; D010758:Phosphorus