No abstract

Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy is an excellent tool with which to study soil organic P, allowing quantitative, comparative analysis of P forms. However, for 31P NMR to be tative, all peaks must be completely visible, and in their correct relative proportions. There must be no line broadening, and adequate delay times must be used to avoid saturation of peaks. The objective of this study was to examine the effects of extractants on delay times and peak saturation. Two samples (a forest litter and a mineral soil sample) and three extractants (0.25 M NaOH, NaOH plus Chelex (Bio-Rad Laboratories, Hercules, CA), and NaOH plus EDTA) were used to determine the differences in the concentration of P and cations solubilized by each extractant, and to measure spin-lattice (T1) relaxation times of P peaks in each extract. For both soil and litter, NaOH-Chelex extracted the lowest concentrations of P. For the litter sample, T1 values were short for all extractants due to the high Fe concentration remaining after extraction. For the soil sample, there were noticeable differences among the extractants. The NaOH-Chelex sample had less Fe and Mn remaining in solution after extraction than the other extractants, and the longest delay times used in the study, 6.4 s, were not long enough for quantitative analysis. Delay times of 1.5 to 2 s for the NaOH and NaOH-EDTA were adequate. Line broadening was highest in the NaOH extracts, which had the highest concentration of Fe. On the basis of these results, recommendations for future analyses of soil and litter samples by solution 31P NMR spectroscopy include: careful selection of an extractant; measurement of paramagnetic ions extracted with P; use of appropriate delay times and the minimum number of scans; and measurement of T1 values whenever possible.

The effect of soluble organics from O1 and O2 litter of seven overstory and understory forest species on dissolution of Al, Fe, Mg, and Mn from two forest soils was investigated. Litter was leached in absence and presence of soil under controlled laboratory conditions, and the leachates were analyzed. In absence of soil, different litters varied widely in their ability to release dissolved organic carbon (DOC) and metals. No significant correlations (p > 0.1) between protons consumed and DOC, Al, Mg, and Mn released from litter were found. In the presence of soils there was a direct relationship between organics released from litters and Al and Fe dissolved from soils but not for the other two metals. The predominant class of organics found in all litter leachates were acids and neutrals, and their contents were directly proportional to total organics dissolved. Thus, hydrophobic plus hydrophilic acid, base, and neutral content of litter leachates corresponded to mean values of 49.6, 8.8, and 41.1%, respectively, for all litters. Total acid content was composed of 30.6% hydrophobics and 18.5% hydrophilics. A good correlation was obtained between soil dissolved Al and hydrophobic acids (r = 0.91, p < 0.001), Al and Fe, and hydrophilic acids (r = 0.93, r = 0.85, p < 0.001, respectively). Polyphenolics constituted from 10 to 23% of the DOC content of both O1 and O2 litter extracts. Polyphenolic content was highly correlated with dissolved Al (r = 0.94, p < 0.001) and with dissolved Fe (r = 0.80, p < 0.001). Five aliphatic acids and six aromatic acids were detected and quantified. Free‐organic acids with high chelating abilities such as oxalic, malic, gallic, and protocatechuic acids made up 60 to 80% of the identified free‐acid content of six litter leachates. Significant correlation was obtained between total free‐organic acid content and dissolved Al (r = 0.87, p < 0.001) and Fe (r = 0.82, p < 0.001) from soils.

Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy is an excellent tool with which to study soil organic P, allowing quantitative, comparative analysis of P forms. However, for 31P NMR to be tative, all peaks must be completely visible, and in their correct relative proportions. There must be no line broadening, and adequate delay times must be used to avoid saturation of peaks. The objective of this study was to examine the effects of extractants on delay times and peak saturation. Two samples (a forest litter and a mineral soil sample) and three extractants (0.25 M NaOH, NaOH plus Chelex (Bio-Rad Laboratories, Hercules, CA), and NaOH plus EDTA) were used to determine the differences in the concentration of P and cations solubilized by each extractant, and to measure spin-lattice (T1) relaxation times of P peaks in each extract. For both soil and litter, NaOH-Chelex extracted the lowest concentrations of P. For the litter sample, T1 values were short for all extractants due to the high Fe concentration remaining after extraction. For the soil sample, there were noticeable differences among the extractants. The NaOH-Chelex sample had less Fe and Mn remaining in solution after extraction than the other extractants, and the longest delay times used in the study, 6.4 s, were not long enough for quantitative analysis. Delay times of 1.5 to 2 s for the NaOH and NaOH-EDTA were adequate. Line broadening was highest in the NaOH extracts, which had the highest concentration of Fe. On the basis of these results, recommendations for future analyses of soil and litter samples by solution 31P NMR spectroscopy include: careful selection of an extractant; measurement of paramagnetic ions extracted with P; use of appropriate delay times and the minimum number of scans; and measurement of T1 values whenever possible.

No abstract