P. Buurman

Quantitative aspects of solid-state 13C-NMR spectra of humic substances from soils of volcanic systems

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Cross-Polarisation Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance spectroscopy (CPMAS 13 C-NMR) represents one of the most powerful tools to investigate soil organic matter (SOM) mainly because of its inherent capacity to provide a semi-quantitative evaluation of carbon distribution. A critical parameter during acquisition of CPMAS 13 C-NMR spectra is the contact time required to obtain the cross-polarisation between proton and carbon nuclei. The procedure to evaluate the best contact time for the acquisition of a quantitative CPMAS 13 C-NMR spectrum is to perform Variable Contact Time (VCT) experiments. In this work the structural features of a number of purified humic substances from Italian and Costarican volcanic soils were investigated by CPMAS 13 C-NMR spectroscopy after having performed preliminary VCT experiments. The VCT experiments showed that the average contact times vary according to the origin and chemical structure of the humic material. The optimal contact times (OCT) for nine humic samples were between 250 and 800 μs. These values were different from the time of 1000 μs that is commonly applied as the best average contact time for humic materials. Moreover, by comparing the NMR data to those obtained by elemental analysis (C/H ratio), it appeared that the efficiency of the cross-polarisation between protons and carbons, and hence the contact time, is affected not only by the number of protons, but also by their distribution over the molecules. The evaluation of errors in quantitative estimation of the different carbons revealed that the use of OCT generally reduced by half the loss of signals occurring when the average contact time of 1000 μs is used in CPMAS 13 C-NMR spectra of humic substances.

Quantitative differences in evaluating soil humic substances by liquid- and solid-state 13C-NMR spectroscopy

Abstract We compared the quantitative responses of liquid-state (LS) and solid-state (CPMAS) 13 C-NMR spectroscopy of four different soil humic substances. The intensities of signals for the alkyl carbons (0–40 ppm) were significantly larger in CPMAS than in LS spectra. This difference is in agreement with the pseudo-micellar model of the conformational nature of humic substances. By this view, the hydrophobic interactions holding together the heterogeneous molecules of humic micelles inhibit the molecular motions of the alkyl carbons, thereby enhancing the spin-lattice relaxation times and consequently lowering the sensitivity of liquid-state NMR. Conversely, regardless of their position in the humic conformation, a better estimation of the number of alkyl carbons can be obtained by CPMAS-NMR because of the cross-polarization of hydrogen nuclei in CH 2 and CH 3 groups. The intensity of the 40–110 ppm region is also slightly lower in LS than in CPMAS-NMR spectra, despite the hydrophilicity of the oxidized and peptidic carbons resonating in this chemical shift interval. Their molecular motion may also be reduced by either the formation of intra- and inter-molecular hydrogen bondings due to poorly acidic hydroxyl groups of saccharides, or the degree of conformational rigidity that a pseudo-micellar arrangement confers even to hydrophilic domains. The higher content of aromatic carbons (110–160 ppm) found in the LS spectra was attributed partly to the high degree of substitution of the aromatic ring that slows down cross-polarization in CPMAS experiments and partly to the relative overestimation of this region by LS-NMR due to a lack of signal in the aliphatic interval. The slightly lower content of carboxyl carbons estimated in CPMAS spectra as compared to LS spectra was also attributed to slow cross-polarization. This work shows that the combined use of both NMR techniques is profitable in conformational analysis of humic substances and of dissolved organic matter in general.

Increase in stability against thermal oxidation of soil humic substances as a result of self association

The stability to thermal oxidation of soil humic extracts saturated with H, Na, Ca, or Al, was followed after treatment with relatively polar organic compounds, such as methanol, formic acid, and acetic acid. While thermal characteristics of H-humates did not change upon addition of the polar molecules, thermal decomposition (oxidation) of Na-humates was shifted to much higher temperatures (750–830 � C) than control. Substantially less dramatic was the effect on Ca-humates, whereas hardly any alteration was observed when polar organic compounds were added to Alhumates. These results can be explained by considering the forces that hold humic molecules together. Humic components are strongly bound to each other by hydrogen bonding in H-humates and by electrostatic bridges in Ca- and Alhumates. These binding forces were not overcome by the simple addition of polar organic molecules, and their stability remained generally unchanged. In Na-humates, associations of humic molecules are held together only by non-specific hydrophobic interactions. Our results showed that wetting the relatively more flexible Na-humates with organic solvents slightly less polar than water caused a significant increase in thermal stability. Because most polysaccharide-C has largely disappeared at 400 � C, this thermal behaviour can be explained by the rearrangement of largely hydrophobic humic components in methanol, leading to an increase in association energy. The intensity and reversibility of thermal stabilization indicate that association occurred among relatively small molecules rather than among macromolecules. Also Na-saturated humus of bulk samples showed an increase in stability against oxidation upon addition of methanol. These findings suggest that, counter-ions and amphiphilic organic compounds may affect organic matter stability also in natural soils. # 2002 Elsevier Science Ltd. All rights reserved.

Litter Quality of Quercus Plants Growing in Enhanced Atmospheric CO2 in Mediterranean-Type Ecosystems

The impact of elevated atmospheric CO2 on the litter quality and litter C and N turnover rates was evaluated for two Mediterranean oak species (Quercus cerris L. and Quercus pubescens Willd.). Litter from 30-year-old trees was collected (1) in the vicinity of a CO2-emitting spring with 500±32 ppm CO2 and (2) in a control area with 360±5 ppm CO2, away from the spring. Litter quality was determined by chemical analysis and by tracking CO2 evolution and N mineralization from litter incubated for about 4 months under controlled conditions. Total C, C/N ratio, lignin, cellulose, lignin/N, cellulose/N, and (cellulose+lignin)/N ratios were not affected by elevated CO2. However, total N content of Quercus cerris litter grown in elevated CO2 was about 8% lower than in ambient conditions. Elevated atmospheric CO2 did not depress rates of C decomposition and N mineralization of the litter. Therefore we may expect that effects on litter quality play no role in changing the C balance of these Mediterranean-type ecosystems under future higher levels of atmospheric CO2.

Discussion: Oil seepage or fossil podzol? An Early Oligocene oil seepage at the southern rim of the North Sea Basin, near Leuven (Belgium) by E.D. van Riessen & N. Vandenberghe, Geologie en Mijnbouw 74: 301-312 (1996)

Reactie op het artikel van ED van Riessen en N. Vandenberghe in Geologie en mijnbouw (1996), p 301 ev.