SOLID-STATE CP/MAS C-13 NMR ANALYSIS OF PARTICLE-SIZE AND DENSITY FRACTIONS OF A SOIL INCUBATED WITH UNIFORMLY LABELED C-13-GLUCOSE

A soil incubated for 34 days in the absence (control) and presence (treated) of uniformly labelled 13C-glucose was dispersed using an ultrasonic probe and fractionated by sedimentation inwater and a polytungstate solution of density 2-0 Mg m-3. The residual substrate carbonwas concentrated in the clay and light fractions. Solid state CP/MAS 13C n.m.r. (crosspolarization/magic angle spinning 13C nuclear magnetic resonance) spectroscopy was usedto characterize the chemical structure of the native soil organic carbon and the residualsubstrate carbon in the fractions of the control and treated soils. To obtain quantitativeresults it was essential to determine the spin lattice relaxation times in the rotating frame, TipH, of the individual carbon types in the spectra as the relaxation behaviour of the nativeorganic materials in the clay fraction was substantially different from that of the residualsubstrate carbon. After correcting for TipH effects, a significant linear relationship existedbetween the signal intensity and 13C content of the samples. This enabled the content, expressed in ¼mol 13C g-1 fraction, of each type of carbon in the fractions to be calculated.The residual substrate carbon was found to accumulate in predominantly alkyl and O-alkylstructures in both fractions. However, significant amounts of acetal and carboxyl carbon werealso observed in the clay fraction. Little if any aromatic or phenolic carbon was synthesizedby the soil microorganisms utilizing substrate carbon. Dipolar dephasing CP/MAS 13C n.m.r.experiments were also performed and allowed the proportion of each type of carbon whichwas protonated and nonprotonated to be estimated. Essentially all of the O-alkyl and acetalcarbon, 25-40% of the aromatic carbon and 66-80% of the alkyl carbon was protonated inthe fractions isolated from the treated soil. © 1990, CSIRO. All rights reserved.