Reference spectra of important adsorbed organic and inorganic phosphate binding forms for soil P speciation using synchrotron-basedK-edge XANES spectroscopy

Abstract

Direct speciation of soil phosphorus (P) by linear combination fitting (LCF) of P K-edge XANES spectra requires a standard set of spectra representing all major P species supposed to be present in the investigated soil. Here, available spectra of free- and cation-bound inositol hexakisphosphate (IHP), representing organic P, and of Fe, Al and Ca phosphate minerals are supplemented with spectra of adsorbed P binding forms. First, various soil constituents assumed to be potentially relevant for P sorption were compared with respect to their retention efficiency for orthophosphate and IHP at P levels typical for soils. Then, P K-edge XANES spectra for orthophosphate and IHP retained by the most relevant constituents were acquired. The spectra were compared with each other as well as with spectra of Ca, Al or Fe orthophosphate and IHP precipitates. Orthophosphate and IHP were retained particularly efficiently by ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated soil organic matter (SOM), but far less efficiently by hematite, Ca-saturated montmorillonite and Ca-saturated SOM. P retention by dolomite was negligible. Calcite retained a large portion of the applied IHP, but no orthophosphate. The respective P K-edge XANES spectra of orthophosphate and IHP adsorbed to ferrihydrite, boehmite, Al-saturated montmorillonite and Al-saturated SOM differ from each other. They also are different from the spectra of amorphous FePO4, amorphous or crystalline AlPO4, Ca phosphates and free IHP. Inclusion of reference spectra of orthophosphate as well as IHP adsorbed to P-retaining soil minerals in addition to spectra of free or cation-bound IHP, AlPO4, FePO4 and Ca phosphate minerals in linear combination fitting exercises results in improved fit quality and a more realistic soil P speciation. A standard set of P K-edge XANES spectra of the most relevant adsorbed P binding forms in soils is presented.

Bone char effects on soil: sequential fractionations and XANES spectroscopy

“…Our LCF also assigned Ca phytate in BC-BC plus samples, which seems to be controversial as a component of animal bone materials. The P K-edge spectrum of Ca phytate is very similar to other Ca-bound P compounds with a distinct white line and lack of a pre-edge feature, although it is likely distinguishable due to the specific shape of white-line tailing and the absence of a post-edge signal at 2164 eV (Prietzel et al, 2016). Moreover, some inaccuracies in LCF estimations have to be considered because of (1) uncertainty in the speciation of organic P forms by K-edge XANES, (2) lack of reference compounds representing all P forms in BCs and (3) smaller Ca phytate proportions than the proposed 10 to 15 % of P t as a detection limit for reliable XANES fittings (Beauchemin et al, 2003).…”

“…All XANES spectral data were baseline corrected in the pre-edge region between 2115 and 2145 eV and normalized in the post-edge region of 2190-2215 eV. The same ranges were used for the reference P K-edge XANES spectra to achieve consistency in the following fitting analysis (Prietzel et al, 2016). To achieve the best compatible set of references with each specified sample spectrum, LCF analysis was performed in the energy range between −20 eV and +30 eV relative to the E 0 using the combinatorics function of ATHENA software to attain all possible binary, ternary and at most quaternary combinations between all 19 P reference spectra.…”

Spatial Patterns of Phosphorus Fractions in Soils of Temperate Forest Ecosystems with Silicate Parent Material

“…Pedogenic Al and Fe oxyhydroxides form by 15 weathering of primary minerals in early stages of pedogenesis (Torn et al, 1997) and phosphate absorption to these minerals increases with decreasing pH (Goldberg and Sposito, 1984). It has often been reported that Al and Fe oxyhydroxides have a high capacity to adsorb P org (e.g., Celi et al, 1999;Yan et al, 2014;Prietzel et al, 2016a), as well as P inorg species (e.g., Parfitt, 1989;Violante and Pigna, 2002). At CON and MIT, our results indicate that adsorption of P inorg was negligible, probably due to competing adsorption of P org .…”

“…2). Prietzel et al (2016a) showed in sorption experiments that SOM, in buffered acidic conditions, retained 68 % of P inorg , and 89% of P org , when saturated with Al. In addition, Fe oxyhydroxides adsorbed 88 % of P inorg , and 89% of P org (Prietzel et al, 2016a).…”

“…Prietzel et al (2016a) showed in sorption experiments that SOM, in buffered acidic conditions, retained 68 % of P inorg , and 89% of P org , when saturated with Al. In addition, Fe oxyhydroxides adsorbed 88 % of P inorg , and 89% of P org (Prietzel et al, 2016a). Recent speciation results by Prietzel et al (2016b) showed that total P in the 20 BBR Ah horizon consists of 41 % of amorphous FePO 4 , 8 % P adsorbed by Fe oxyhydroxides and 40 % P adsorbed by Alsaturated SOM.…”