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Author: Admin | 2025-04-27
Prior to NMR analysis. Concentrations of total P in the remaining filtrate were determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES), whereas concentrations of molybdate-reactive P (MRP – analogous with inorganic P) was determined using the malachite green method of Ohno and Zibilske (1991). The difference between concentrations of total P via ICP-OES and MRP was taken as molybdate-unreactive P (MUP – analogous with organic P).Sample preparation for NMR analysisPreparation of lyophilised material for solution 31P NMR analysis was based on a modification of the method of Vincent et al. (2013). Briefly, 600 μL of 0.25 M NaOH +0.05 M Na2-EDTA solution was added to each centrifuge tube, placed on a vortex mixer for 2 min, and then centrifuged at 5000 rpm for 20 min. A 500 μL aliquot of the supernatant was transferred to a 1.5 mL microcentrifuge tube, and then spiked with a 50 μL aliquot of 32.3 mM methylenediphosphonic acid (MDP: Sigma-Aldrich, M9508) and 75 μL of sodium deuteroxide solution 40 wt.% in D2O (NaOD: Sigma-Aldrich, 372,072). The microcentrifuge tube was then vortexed for 10 s and transferred to a 5 mm NMR tube prior to NMR analysis.In addition, the NMR spectra in the Year 26 soil and both native forest soils (UB and AB) exhibited considerable line broadening for all peaks: probably due to high sample viscosity. The extraction and preparation of these soils was repeated as previously described, except additional NaOH-EDTA solution was added to the lyophilised material until spectral quality was improved (Cade-Menun and Liu 2014). Samples from site year 26, UB and AB required 1.5, 1.2 and 0.9 mL of NaOH-EDTA solution, respectively.Solution 31P NMR analysisSolution 31P NMR spectroscopy was carried out using a Bruker Avance III HD 500 NMR spectrometer (Bruker Corporation; Billerica, MA) at the combined NMR facility of the Laboratory
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