The degree of quantitation achieved in the solid state 13C nuclear magnetic resonance (NMR) spectra of a number of organic materials, an HF-treated soil, and a whole soil, was determined for both cross-polarisation (CP) and Bloch decay (BD) techniques using spin-counting experiments. The percentage of potential 13C NMR signal, which was actually observed (Cobs) was in the range 79–107% for the BD technique and in the range 29–103% for the CP technique. A number of materials, including cellulose, pectin, lignin, and palmitic acid gave quantitative spectra using both CP and BD. A second group, including chitin, collagen, and the HF-treated soil gave quantitative BD spectra, but significantly diminished CP spectra (Cobs-CP=66–75%). A third group including charcoal, a commercial humic acid, and the whole soil gave significantly diminished BD spectra (Cobs-BD=79–87%) and severely diminished CP spectra (Cobs-CP=29–35%). Signal losses in the CP spectra were attributed to rapid relaxation rates (short T1ρH) and/or slow magnetisation build-up rates (long TCH). The spin dynamics of the CP experiment were studied and a new method for correcting for differences in T1ρH between the sample and the reference in CP spin-counting experiments was developed. Signal produced by the Kel-F rotor end-caps was significant for the BD spectra and a correction for the end-cap spectrum was required prior to spin counting. The low 1H content of the fluorinated Kel-F polymer ensured that the contribution of end-caps to the CP spectra was insignificant. The effects of paramagnetic cations on quantitation in solid state NMR spectra was investigated by doping model compounds with paramagnetic impurities. Three mechanisms, which bring about signal loss and operate on three different length scales, were identified. The magnitude of the signal loss brought about by a paramagnetic material was shown to be dependent on both the type of cation involved and on the intimacy of contact with the organic matrix. Chemically bound paramagnetic cations were shown to result in large signal losses in the CP spectrum, whereas paramagnetic salts in a physical mixture with an organic material affected both CP and BD spectra equally.

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a 14CO2-labelled atmosphere for 30 days, defoliated and then grown in a 13CO2-labelled atmosphere for 19 days. Allocation of 14C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of 14C and 13C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (13C) exceeded that of C assimilated before defoliation (14C).

Aggregates are formed by Joining of structural units of different sizes in a hierarchical order. During wetting, the aggregates may either disintegrate completely (slaking) or remain intact with only loosening at the points of weakness (mellowing). This study investigated the effect of saline-sodic solutions with sodium adsorption ratio (SAR) of 5 and 20 and electrical conductivity (EC) of 0.1 to 4 dS m(-1) after a number of wetting-drying cycles on soil mellowing, A Vertic Palexeralf with a coefficient of linear extensibility (COLE) value of 0.15 and a Typic Haploxeralf with COLE of 0.05 were studied. Using a Ca solution, the degree of aggregation in both soils was improved about two times in >50 and 20- to 50-mu m aggregates compared with the original soils, The values for the Vertic Palexeralf were three times those of the Typic Haploxeralf. In contrast, sodic solutions led to the collapse of aggregates, Mellowing ratios of aggregates changed after 10 wetting-drying cycles and were approximate to 0.6 to 0.7 for SAR 5 and 0.8 to 1 for SAR 20 for the Vertic Palexeralf; for the Typic Haploxeralf they were 0.7 to 0.8 and 0.9 to 1.2 for SARs 5 and 20, respectively, Mellowing ratios between 0.9 and 1 were obtained for the Vertic Paleoxeralf minicores using solutions of SAR 20 and EC 0.1 dS m(-1). The mellowing ratio was 0.2 when a solution of SAR 5 and EC 4 dS m(-1) was used. In soils where aggregates collapsed without hierarchical breakdown, mellowing ratios were always higher, indicating structural degradation.

A simple densimetric method for the separation of free and occluded particulate organic materials was developed and applied to five virgin soils. The free organic matter was isolated by suspending the soil in sodium polytungstate solution (d = 1.6 Mg m-3) and decanting the light material. The remaining soil was disaggregated by sonification for liberation of occluded organic materials. The free light fraction consisted of large, undecomposed or partly decomposed root and plant fragments. This fraction comprised 0.59-4.34% of soil dry weight and accounted for 6.9-31.3% and 5.9-22.1% of total soil carbon and nitrogen respectively. Identifiable components of the occluded fraction were small particles of incompletely decomposed organic residues, pollen grains, particles of plant tissue such as lignin coils and phytoliths. This fraction comprised 0.69-1.81% of soil dry weight and represented 9.2-17.5% and 6.2-14.1% of the total soil carbon and nitrogen. The proportion of soil organic carbon recovered as the occluded fraction was high in soils with high clay contents. The chemical composition of occluded and free organic materials was investigated by solid-state 13C CP/MAS NMR spectroscopy. Despite the differences in soils, environmental conditions and vegetation, the organic structure of the free light fraction was similar in four of the five soils. This fraction consisted of 55-63% O-alkyl C, 18-25% alkyl C, 14-18% aromatic C, and 5-7% carbonyl C. In the other soil, this fraction showed a higher proportion of alkyl C (31%) and lower O-alkyl C (46%). Most of the differences between soils were associated with organic materials contained in the occluded light fraction. The differences in chemical structure between the occluded light fraction and free light fractions were similar in all examined soils. The NMR data showed that the proportion of O-alkyl C was lower and alkyl C higher in the occluded light fractions than in the free light fractions. The proportion of aromatic and carbonyl carbon was higher in the occluded fractions of three soils while the percentage of these two types of carbon remained unchanged in the two other soils. It is considered that the occluded organic matter is an old pool of carbon that has been accreted within aggregates during decades of root growth and it is that pool which is lost due to cultivation.

Samples were obtained from the same red-brown earth: (a) in an undisturbed state, (b) after 60 years of an exploitive wheat-fallow rotation and (c) after 40 years under a fertilized mixed grass-legume pasture. Organic materials were concentrated in various fractions which enabled comparative chemical composition of the organic materials in the three soils by 13C CPMAS n.m.r. spectroscopy. Despite more than twofold differences in the organic carbon content of the soils, the chemistry of the organic matter in the soils was similar, particularly organic matter associated with clay fractions. Most of the differences detected were associated with plant debris in particles > 20 �m which contained most of the aromatic carbon. The results indicate a rapid disappearance of phenolic-carbon which originates in lignins. The composition of sodium hydroxide extracts reflects quite well the composition of the organic matter in the soil. It is concluded that in a particular soil type, changes in amounts and nature of added photosynthate do not change the composition of the organic matter which is controlled by the microbial biomass and interactions of the biomass and its decomposition products with the soil matrix. Implications of this conclusion for the turnover of organic carbon in soil and stability of soil structure are discussed.

Size and density fractions separated from a red-brown earth have been analysed by high-resolution solidstate 13C nuclear magnetic resonance (n.m.r.) spectroscopy. Resonances from O-alkyl carbon (mainly carbohydrate) predominated in the spectra of the largest (250-2000 m) fractions, whereas alkyl carbon resonances (mainly polymethylene) predominated in the spectra of clay fractions. Paramagnetics, mainly Fe3+, were found to influence the amount and type of carbon seen in 13C solid-state n.m.r. spectra of clay fractions containing more than several per cent iron oxides. Removal of iron oxides by reduction with dithionite allowed aromatics, carboxyls and carbohydrates to be detected by 13C n.m.r.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructural analysis of geochemical samples by solid-state nuclear magnetic resonance spectrometry. Role of paramagnetic materialAnthony M. Vassallo, Michael A. Wilson, Philip J. Collin, J. Malcolm. Oades, Angela G. Waters, and Ronald L. MalcolmCite this: Anal. Chem. 1987, 59, 4, 558–562Publication Date (Print):February 15, 1987Publication History Published online1 May 2002Published inissue 15 February 1987https://doi.org/10.1021/ac00131a005Request reuse permissionsArticle Views160Altmetric-Citations65LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (650 KB) Get e-Alertsclose Get e-Alerts

After eight years decomposition of 14C, 15N-labelled legume (Medicago littoralis) material previously mixed into topsoils (0-10 cm) at four field sites in South Australia, residual organic 14C and 15N to 30 cm depth accounted for respectively 11-13% of input 14C, and 31-38% of input 15N. About 90% of the residual organic 14C and 70-80% of the residual l15N was recovered in topsoils. For sites in similar rainfall areas, soils of heavier texture retained slightly greater amounts of 14C and15N-labelled residues. Throughout the eight-year experimental period, the rates of decline of residual organic 14C and 15N exceeded those of native soil organic C and N. A comparison of the decline of organic 14C in topsoils, averaged for the four South Australian sites, with the average decline reported for 14C-labelled plant residues in soils at English and Nigerian field sites, suggests that net decomposition rates doubled approximately for an 8-9C rise in mean annual air temperatures. Microbial biomass 14C and 15N of topsoils with time accounted for decreasing proportions of total biomass C and N, and of residual organic I4C and I5N. The relatively greater retention after eight years of biomass 14C and 15N in soils of heavier texture is consistent with the concept that the net decay of C and N in soils is dependent upon the turnover of biomass C and N, and that decay rates are decreased in soils which have the greater capacity to protect decomposer populations.