TOWARDS A DIRECT, NON‐DESTRUCTIVE METHOD OF CHARACTERISING SOIL HUMIC SUBSTANCES USING ¹³C N.M.R.

Abstract

The structure of a humic acid from a New Zealand pasture topsoil has been investigated by conventional solution I3C n.m.r. spectroscopy and by the cross polarisation technique. Relaxation results showed that a pulse spacing of about 0.2 s was optimuni for n.m.r. of the humic acid dissolved in 0.5 M aqueous NaOH. Aromatic rings accounted for 26 per cent of the total carbon. Cross polarisation n.m.r. is a viable method for obtaining 13C spectra of solid humic acids, although aromaticity cannot be estimated as accurately as for coals.

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

“…This can bias n.m.r. results against detection of carboxylic and amide signals, which are suppressed by rapid pulsing (Newman et al, 1980). This leads to underestimation of the total carbon signal, and consequently overestimation of fa; but the contribution from carboxylic and amide groups is so small that the errors introduced in this way are no larger than the experimental uncertainty.…”

13C-LABELLING FOR NMR STUDIES OF SOILS: CPMAS NMR OBSERVATION OF 13C-ACETATE TRANSFORMATION IN A MINERAL SOIL

“…(Hatcher 1980a,b, l98la,b;Newman et al . 1980;Preston and Ripmeester 1982;Pres-ton and Schnitzer, in prep.…”

ATTEMPTS TO IMPROVE SOLID STATE 13C NMR SPECTRA OF WHOLE MINERAL SOILS

“…Considerable progress has been made during the past 10 yr in characterizing soil organic matter (SOM) by l3C nuclear magnetic resonance (NMR) spectroscopy (Hatcher et al 1981a, b;Newman et al 1980; Ripmeester 1982 Preston and Schnitzer 1984;Schnitzer and Preston 1986 However, the separation takes much longer (for detailed particle size fractionation see Day (1965)) and the isolates do not yield as high a concentration of OM as the fine sand fraction (see data for C in Table 2); this is especially true for isolates of clay-size fractions.…”