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Sequential chemical extraction is a common analytical approach used to separate soil P into operationally defined fractions based on solubility in increasingly strong extractants. However, there are some concerns that the relationship between these operationally defined pools and the true chemical speciation of P may not always be correct. This study was conducted to compare the speciation of P in soils amended with either biosolids or an inorganic fertilizer as using a combination of sequential chemical extraction and synchrotron X-ray absorption nearedge structure (XANES) spectroscopy on the solid-phase residues after each extraction step. The combined results from sequential chemical extraction and XANES analysis indicate that although sequential extraction steps may remove the same broad class of P from soil, the solubility and precise chemical speciation of that P may be quite different. For example, P K-edge XANES analysis determined that calcium phosphates are removed by the HCl extraction step, but this takes the form of a poorly crystalline dicalcium phosphate in biosolid-amended plots and apatite-type calcium phosphate in the commercially fertilized plots.

Chemical form of phosphorus (P) in soils influences both plant accessibility and solute transport of P. Soil P speciation receiving inorganic and organic fertilizers is extensively studied to address a range of agronomic and environmental concerns. It is known inorganic P sources react over long time scales in soils, but relatively few studies have focused upon long-term P speciation changes in soils receiving organic amendments. This study was conducted to address this gap by providing detailed information on the speciation of P in agricultural soils from short-(2 yr) and long-term (11 yr) applications of liquid hog manure (LHM) and solid cattle manure (SCM) made at a field research site. Chemical speciation was performed with a combination of laboratory-based soil test P extraction methods and solid-state synchrotron-based techniques. There was clear evidence that initial phosphate minerals in the manures rapidly transform into new phases after short-term soil application. The X-ray absorption near edge spectroscopy results for SCM soil samples were consistent with high solubility (likely Mg-substituted) dicalcium phosphate minerals and phosphate (PO 4) adsorption species present after short-term period, whereas long-term SCM application resulted in transformation of soil phosphate into more crystalline calcium phosphate minerals. In contrast, phosphate speciation after short term of LHM application revealed predominantly magnesium phosphate and adsorbed P compounds. Soils under long-term LHM amendment still had phosphate speciation dominated by poorly crystalline dicalcium phosphate minerals. This suggests that the manure form plays a strong role not only in short-term plant availability but also in long-term P speciation.

There is limited information on how manure and inorganic fertilizer application in a concentrated band impacts the solubility, mobility, and transformation of the phosphate compounds in Canadian prairie soils. A combination of resin membrane probes, sequential chemical extraction, and synchrotron based techniques were used to reveal the spatial distribution and chemical speciation of soil phosphorus (P) when inorganic fertilizer P or manure is placed in the soil in a band under field conditions. This study was performed at Central Butte and Dixon, SK, Canada. Monoammonium phosphate blended with urea was applied at a rate of 54 kg N ha~i and 12 kg P ha~^ at Central Butte (Brown Chernozem) and solid cattle manure was applied at rate of 60 t ha~^ yr~^ at Dixon (Black Chernozem). Fertilizer application increased the total and labile P supply at the center of the band and was mainly confined to a relatively small area within about 5 cm of band regardless of slope position. The X-ray absorption near edge structure (XANES) spectroscopy directly identified different forms of soil P, while chemical extraction results provided indirect support that there are differences in the forms of P in the band center as related to in the landscape position and fertilization type. The XANES analysis results showed that different P retention mechanisms take place depending on both landscape position and fertilizer type. For the inorganic fertilizer application made in calcareous upslope positions, a significant amount of apatite and adsorbed P species were observed at center and near-band and only apatite was present at large distances from the band; adsorbed P species dominated at all distances from the band at downslope positions. The solid cattle manure applied soil contained dicalcium phosphate (DCP) and organic P at center and near-band at both slope positions but adsorbed P and calcium phosphate further away at upslope positions and adsorbed P further away at downslope positions. The XANES results also indicated that the movement of organic P was limited as this species was always localized near the band.Abbreviations: Ca-P, calcium phosphates; DCP, dicalcium phosphate; HAP, hydroxyapatite; HCl-P, phosphorus extracted with 1 M HCl; H2SO4-P, phosphorus extracted with concentrated H2SO4; LC, linear combination; MAP, monoammonium phosphate; NaH-CO3-P, phosphorus extracted with 0.5 M NaHCOj; NaOH-P, phosphorus extracted with 0.1 M NaOH; PRS, plant root simulator; SCM, solid cattle manure; SXRMB, soft X-ray micro-characterization beamline; XANES, X-ray absorption near edge structure.

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