title: Assessing crop residue phosphorus speciation using chemical fractionation and solution 31P nuclear magnetic resonance spectroscopy.
abstract: At physiological maturity, nutrients in crop residues can be released to the soil where they are incorporated into different labile and non-labile pools while the remainder is retained within the residue itself. The chemical speciation of phosphorus (P) in crop residues is an important determinant of the fate of this P. In this study, we used chemical fractionation and (31)P nuclear magnetic resonance (NMR) spectroscopy, first separately and then together, to evaluate the P speciation of mature oat (Avena sativa) residue. Two water extracts (one employing shaking and the other sonication) and two acid extracts (0.2N perchloric acid and 10% trichloroacetic acid) of these residues contained similar concentrations of orthophosphate (molybdate-reactive P determined by colorimetry) as NaOH-EDTA extracts of whole plant material subsequently analysed by solution (31)P NMR spectroscopy. However, solution (31)P NMR analysis of the extracts and residues isolated during the water/acid extractions indicated that this similarity resulted from a fortuitous coincidence as the orthophosphate concentration in the water/acid extracts was increased by the hydrolysis of pyrophosphate and organic P forms while at the same time there was incomplete extraction of orthophosphate. Confirmation of this was the absence of pyrophosphate in both water and acid fractions (it was detected in the whole plant material) and the finding that speciation of organic P in the fractions differed from that in the whole plant material. Evidence for incomplete extraction of orthophosphate was the finding that most of the residual P in the crop residues following water/acid extractions was detected as orthophosphate using (31)P NMR. Two methods for isolating and quantifying phospholipid P were also tested, based on solubility in ethanol:ether and ethanol:ether:chloroform. While these methods were selective and appeared to extract only phospholipid P, they did not extract all phospholipid P, as some was detected by NMR in the crop residue after extraction. These results highlight the need for careful interpretation of results from chemical fractionation, as separation can be compromised by incomplete recovery and side reactions. This study also highlights the benefits of employing a technique that can simultaneously detect multiple P species (solution (31)P NMR) in combination with chemical fractionation.
authors.affiliation: School of Agriculture, Food and Wine and Waite Research Institute, The University of Adelaide, PMB 1, Waite Campus, Glen Osmond 5064, SA, Australia.
authors.forename: Sarah R
authors.initials: SR
authors.lastname: Noack
authors.affiliation: School of Agriculture, Food and Wine and Waite Research Institute, The University of Adelaide, PMB 1, Waite Campus, Glen Osmond 5064, SA, Australia. Electronic address: ronald.smernik@adelaide.edu.au.
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Smernik
authors.affiliation: CSIRO Sustainable Agriculture Flagship, CSIRO Ecosystem Sciences, PMB 2, Glen Osmond 5064, SA, Australia.
authors.forename: Therese M
authors.initials: TM
authors.lastname: McBeath
authors.affiliation: Grains Innovation Park, Department of Environment & Primary Industries, PMB 260, Horsham 3400, VIC, Australia/; LaTrobe University, Department of Agricultural Sciences, Bundoora 3086, VIC, Australia.
authors.forename: Roger D
authors.initials: RD
authors.lastname: Armstrong
authors.affiliation: School of Agriculture, Food and Wine and Waite Research Institute, The University of Adelaide, PMB 1, Waite Campus, Glen Osmond 5064, SA, Australia; CSIRO Sustainable Agriculture Flagship, CSIRO Land and Water, PMB 2, Glen Osmond 5064, SA, Australia.
authors.forename: Mike J
authors.initials: MJ
authors.lastname: McLaughlin
mesh_terms: D018554:Avena / Q000737:chemistry; D005591:Chemical Fractionation / Q000379:methods*; D018556:Crops, Agricultural / Q000737:chemistry*; D004492:Edetic Acid / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010472:Perchlorates / Q000737:chemistry; D010710:Phosphates / Q000032:analysis / Q000302:isolation & purification; D010743:Phospholipids / Q000032:analysis / Q000302:isolation & purification; D010758:Phosphorus / Q000032:analysis* / Q000302:isolation & purification; D010759:Phosphorus Isotopes; D015203:Reproducibility of Results; D012972:Sodium Hydroxide / Q000737:chemistry; D012996:Solutions / Q000737:chemistry; D014238:Trichloroacetic Acid / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D010472:Perchlorates; D010710:Phosphates; D010743:Phospholipids; D010759:Phosphorus Isotopes; D012996:Solutions; D014867:Water; D010758:Phosphorus; D012972:Sodium Hydroxide; D014238:Trichloroacetic Acid; D004492:Edetic Acid; C576518:Perchloric Acid
keywords: Chemical fractionation; Crop residues; Speciation