Phosphorus speciation and micro-scale spatial distribution in North-American temperateagricultural soils from micro X-ray fluorescence and X-ray absorption near-edge spectroscopy
Plant and Soil
Camille Rivard 1,*
Bruno Lanson2,3
Marine Cotte1,4
(1)European Synchrotron Radiation Facility (ESRF), F-38000 Grenoble, France.
(2)Univ. Grenoble Alpes, ISTerre, F-38041 Grenoble, France.
(3)CNRS, ISTerre, F-38041 Grenoble, France.
(4)CNRS, LAMS, F-75005 Paris, France
* Corresponding author. E-mail:
Extraction and fractionation of dissolved organic matter (DOM) from Carex and Sphagnum peat
The following protocol is adapted from Thurman and Malcolm (1981).
- Weigh ~200g of peat and put them in a 5L beaker with ~4L of deionized water (resistivity > 18MΩ.cm-1);
- Stir continuously and adjust pH to 8 with a 1M NaOH solution;
- Once pH is stable, centrifuge the suspension at 7600 rpm for 15 minutes (Beckman AvantiJ20-XP centrifuge with a JLA 8.1 6×1L rotor);
- Supernatant is freeze-dried to obtain the dissolved organic matter (DOM) which contains both humic and fulvic acids, together with a minor amount of clays.
- To separate humic and fulvic acid fractions from the bulk DOM, adjust the supernatant pH from 8 to 2 by using a 1M HCl solution;
- Pass this pH2 suspension over a column packed with Amberlite® XAD-8 resin previously prepared at pH2. The resin will retain humic and fulvic acids only;
- Rinse the column with a 0.1M HCl solution and eluate the column with a 0.1M NaOH solution;
- Acidify the sodiceluate to pH1 with a 2M HCl solution and wait for 24hrs to precipitate humic acids whereas fulvic acids remain in solution;
- Centrifuge the solution to separate humic and fulvic acids (in the pellet and supernatant, respectively)
- Rinse the humic acid pellet with deionized water, solubilize it in a 0.1M NaOH solution, and pass it through a cation exchange resin (Dowex® 50WX8/100-200 mesh). The humic acid fraction is obtained by freeze-drying the obtained solution;
- Pass the initial supernatant that contains the fulvic acid fraction through a column packed with Amberlite® XAD-8 resin, rinse the column with deionized water, eluate the column with a 0.1M NaOH solution and pass the eluate through a cation exchange resin (Dowex® 1x8). The fulvic acid fraction is obtained by freeze-drying the resulting solution.
P adsorption protocol on mineral and organic (DOM) references
The following P adsorption protocol is adapted from Edzwald et al. (1976),Oh et al. (1999), andViolante and Pigna (2002).
- Disperse the sorbent in a NaCl 0.05M (mineral) or 0.01M (DOM) solutionand shake for 12hrs;
- Adjust pH to 5;
- Keep pH constant for 3hrs before adding P to the suspension as 10mL of a 0.01M Na-phosphate (Na3PO4, 12H2O) solution (pH 11). Keep pH constant (pH 5) during P addition with a 0.1M HCl solution;
- Shake the suspension for an additional 12hrs;
- For mineral sorbents, centrifuge the suspension 15000 g for 15min, rinse the pellet with a 105M HCl solution to remove non-adsorbed phosphates, centrifuge at 30000 g for 15min, and freeze-dry the pellet;
- For DOM, filter the suspension at 1000Da under Ar pressure on an Amicon® ultrafiltration cell. Stop filtration with ~10mL left, and freeze-dry the residue and the remaining suspension.
Bibliography
Edzwald JK, Toensing DC, Leung MC-Y (1976) Phosphate adsorption reactions with clay minerals. Environ Sci Technol 10:485-490
Oh YM, Hesterberg DL, Nelson PV (1999) Comparison of phosphate adsorption on clay minerals for soilless root media. Commun Soil Sci Plant Anal 30:747-756
Thurman EM, Malcolm RL (1981) Preparative isolation of aquatic humic substances. Environ Sci Technol 15:463-466 doi:10.1021/es00086a012
Violante A, Pigna M (2002) Competitive sorption of arsenate and phosphate on different clay minerals and soils. Soil Sci Soc Am J 66:1788-1796
Fig. ESM1X-ray diffraction patterns collected on randomly oriented powders of bulk soil samples and identification of the major mineral components
Fig. ESM2 P K-edge XANES spectra collected on dissolved organic matter (DOM) from Sphagnum peat before (solid green line) and after (dotted black line) interaction with phosphate (a). Zoom on the position of the white line for the previous two spectra and for a selection of spectra collected on bulk DOM from Sphagnum and Carex peats and on specific fractions thereof (b). FA and HA stand for the fulvic and humic acid fractions, respectively. 60 quick-scan spectra were systematically summed up except for Carex (20 spectra collected)
Fig. ESM3 X-ray fluorescence maps of CC/U (a) and CC/MPS (b) collected at 2.25 and 7.4 keV and showing P distributions as gray-scale images (light represents high counts, darklow counts) and P-Si-Al, P-Na-Mg and P-Ca-Fe as Red-Green-Blue (RGB) images. The areas mapped are 300 × 120 and 400 × 120 µm2 for CC/U and CC/MPS, respectively. The P-rich area at the bottom right of CC/MPS map is a root section.Ellipses and dotted line delimit areas where the X-ray detector saturates due to high concentration of Ca and Fe, respectively. As a result, the concentration of P is artificially increased in these areas.Graphics at the bottom display Al-P correlation for all the pixels of the map and selected areas
Fig. ESM4 Average P K-edge XANES spectrum of the Morrow plots bulk soil (dots) and linear combination fits (solid gray line) with the contribution of phosphate sorbed onto Al oxide (top) and onto clay (bottom). The inset zooms on the pre-edge region. DOM is the bulk DOM from Sphagnum peat