Use of alkaline soil extracts for ¹³C n.m.r. characterization of humic substances

Abstract

Previous solution I3C n.m.r. studies of soil organic matter have been confined to isolated humic or fulvic acid fractions. A study of 10 crude alkaline extracts from five New Zealand topsoils has now shown that such purification is unnecessary. Spectra were not noticeably broadened by co-extracted iron in concentrations up t o 2 mg ~m -~. Optimized pulse parameters were similar to those reported for a solution of a purified humic acid. Relaxation results support a generalized model for humic material, in which aromatic structures, carbohydrates, amino acids and polymethylene chains are linked together to form flexible macromolecules. Relative contributions from each component varied from soil to soil.

13C NMR of humic substances: pH and solvent effects

“…The spectra of the podzol B FA (Preston & Schnitzer, 1984), the Bainsville HA (Preston & Blackwell, 1985;Preston & Schnitzer, 1984), and the p-benzoquinone polymers (Preston et al, 1982) have been previously described. The features of the Suwanee FA spectra are similar to those of the podzol B FA, the Bainsville HA, and to other published spectra of humic materials (Hatcher et al, 1981;Newman et al, 1980;Newman & Tate, 1984;Schnitzer & Preston, 1986;Skjemstad et al, 1983;Verheyen et al, 1982;Wilson et al, 1981), and therefore only features relevant to this study will be discussed. The distributions of relative intensities ofthe spectral regions for different solution conditions are given in Tables 3-5.…”

“…Unfortunately, even at high fields, quantitative spectra of humic and fulvic acids (HA,FA) with good S/N still require very long accumulation times. The conditions for quantitative spectra are inverse-gated decoupling for suppression of the nOe (nuclear Overhauser effect), with delay times (decoupler off) reported from 0.5 to 10 s (Newman et al, 1980;Newman & Tate, 1984;Preston & Blackwell, 1985;Schnitzer & Preston, 1986;Verheyen et al, 1982;Wilson et al, 1981). With 50-100 mg/ml, 10 mm sample tubes and a 2 s delay, 100 000 or more scans may be required (40-70 h).…”

“…The development of high-field superconducting magnets and of Fourier transform techniques has made possible the application of 13C NMR spectroscopy to humic substances in solution (Hatcher et al, 1981;Newman et al, 1980;Newman & Tate, 1984;Preston & Blackwell, 1985;Preston & Schnitzer, 1984;Schnitzer & Preston, 1986;Skjemstad et al, 1983;Verheyen et al, 1982;Wilson et al, 1981). Samples are usually run in basic solution (0.5 or 1 M NaOH in H,O/D,O mixtures) to obtain maximum solubility.…”

Chemical structure of terrestrial humus materials formed from different vegetation characterized by solid‐state 13C NMR with CP‐MAS techniques

“…These originate mainly from carbohydrates. This finding is not in accordance with the results of Ogner (1985), Schnitzer & Khan (1978) and Wilson et al (1987), but is in agreement with Arshad et al (1988( ), Hempfling et al (1987 and Newman & Tate (1984).…”

“…Methods available for isolation and separation of humic substances may alter their chemical structure (Newman et al, 1980;Newman & Tate, 1984;Lentz et al, 1977;. Arshad et al ( 1988), Newman & Tate (1984), demonstrated the adequacy of NMR in studies of the chemical structure of humus. Solid-state I3C NMR, therefore, was used in the present work to study the main functional groups in whole soil samples.…”