Characterization of soil organic matter fractions from grassland and cultivated soils via C content and delta13C signature

Variations in (13)C natural abundance and distribution of total C among five size and density fractions of soil organic matter, water soluble organic C (WSOC) and microbial biomass C (MBC) were investigated in the upper layer (0-20 cm) of a continuous grassland soil (CG, C(3) vegetation), a C(3)-humus soil converted to continuous maize cultivation (CM, C(4) vegetation) and a C(3)-humus soil converted to a rotation of maize cultivation and grassland (R). The amounts of WSOC and MBC were both significantly larger in the CG than in the CM and the R. In the three soils, WSOC was depleted while MBC was enriched in (13)C as compared with whole soil C. The relative contributions to the total C content of C stored in the macro-organic matter and in the size fraction 50-150 microm decreased with decreasing total C contents in the order CG > R > CM, while the relative contribution of C associated with the clay- and silt-sized fraction <50 microm increased. This reflects a greater stability and physical protection against microbial degradation associated with soil disruption (tillage) of the clay- and silt-associated organic C, in relation to the organic C in larger size fractions. The size and density fractions from the CG soil showed significant differences in (13)C enrichment, indicating different degrees of microbial degradation and stability of soil organic C associated with physically different soil organic matter (SOM) fractions. Delta(13)C analysis of the size and density fractions from CM and R soils reflected a decreasing turnover rate of soil organic C with increasing density among the macro-organic matter fractions and with decreasing particle size.     

<Superscript>13</Superscript>C NMR and mass spectrometry of soil organic matter

Liquid state, high resolution ¹³C NMR spectroscopy and mass spectrometry were used to study the composition and structure of soil organic matter (SOM) using soil extracts from two long-term experiments at the Rothamsted Experimental Station. Both one- and two-dimensional NMR techniques were applied. ¹³C NMR sub-spectra of the CH _n (n=0...3) groups, obtained by the Distortionless Enhancement by Polarisation Transfer (DEPT) technique, were used for the elucidation of the qualitative and quantitative composition of humic and fulvic acids in the soils. The chemical structure of SOM was further analysed at the molecular level through Fast Atom Bombardment Mass Spectrometry (FABMS) and Gas Chromatography-Mass Spectrometry (GC/MS). Humic and fulvic extract results were not only compared to each other, but also to the solid state ¹³C NMR results for the complete soil sample.     

TMAH-preparative thermochemolysis for the characterization of organic matter in densimetric fractions of a Mediterranean forest soil

Physical protection is one of the most important ways for stabilization of organic carbon in soils, and in order to properly manage soils as a sink for carbon, it is necessary to know how much organic carbon a given soil could protect and to have information on the molecular composition of this protected organic matter in soil. To this end, we studied individual horizons taken from a soil profile under Quercus rotundifolia stands over calcareous parent material. Horizons were subjected to a sequential extraction using solutions of sodium polytungstate (NaPT) of increasing density (1.6, 1.8 and 2.0) to differentiate five fractions: a free light, extractable without sonication, three occluded (extractable by sonication) and a dense (retained in the dense residue, after sonication). The obtained fractions were analyzed by preparative thermochemolysis followed by gas chromatography–mass spectrometry (GC/MS) in order to get some insight on the molecular composition. The total ion chromatograms obtained for the pyrolysates of both of the densimetric fractions show various series of fatty acids (as their methyl esters), n-alkanols (as their methyl ethers), methylated α,ω-diacids, methylated ω-hydroxyacids, various lignous subunits and permethylated deoxy aldonic acids derived from carbohydrates. The comparison of the distributions of the thermochemolysis products shows that organic carbon in the dense fractions of the deepest horizons were more influenced by a microbial reworking than the others dense fractions from the upper horizons. It is also the case for the occluded fraction 1 of the H horizon even the vegetal part of the organic carbon in that occluded fraction appears to have a non-woody origin. On the other hand, the dense fraction of the H horizon is strongly marked by vegetal origin.     

Chemical structure of soil organic matter

Journal of Thermal Analysis and Calorimetry - Selective preservation belongs among the important stabilization mechanisms of soil organic matter (SOM). Conceptually, it is based on non-covalent...     

Characterization of organic substrates bound to cross-linked polystyrenes by 13C NMR spectroscopy

The ¹³C NMR spectra of a wide variety of organic substrates bound to 2% cross-linked polystyrenes may be obtained routinely, provided the resins can be sufficiently swollen. The ¹³C chemical shifts of polymer-bound trityl alcohol, polymer-bound monotrityl ethers of the symmertrical diols HO (CH₂)_nOH (n=2, 4, 6, 7, 9 and 10), and some related intermediates in the solid phase synthesis of insect pheromones are presented. ¹³C shift additivity correlations, differing little from those in free trityl ethers, are drawn.     

Elucidating molecular level impact of peat fire on soil organic matter by laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry

Analytical and Bioanalytical Chemistry - In this work, laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI–FTICRMS) was used to...     

Application of scanning calorimetry to estimate soil organic matter loss after fires

The severe heating of soil during wildfires and prescribed burns may result in adverse effects on soil fertility due to organic matter loss. No rapid and reliable procedure exists to evaluate soil organic matter (SOM) losses due to heating. Enthalpy of SOM combustion correlates with organic matter content. Quartz is a ubiquitous mineral in soils and has a remarkably constant composition and reversible α–β phase transition at 575 °C. We suggest that SOM content in heated and unheated soils can be compared using the ratio of SOM combustion enthalpy on heating to the β–α quartz transition enthalpy measured on cooling of the same sample. This eliminates the need to dry and weigh the samples, making possible field applications of the proposed method. The feasibility of using the (ΔH _{comb SOM})/(ΔH _β–α Qz) ratio was established with experiments on soil samples heated in the laboratory and the method was then used for evaluation of SOM loss on two pile burn sites at UC Berkeley’s Blodgett Forest Research Station in Georgetown, California.     

13C NMR Spectra of organic peroxides

Conclusions A study has been made of the¹³C NMR spectra of hydroperoxides, peroxides, peroxy esters, and diacyl peroxides.The signal of the carbon atoms of a hydrocarbon radical present in the-position relative to the peroxide group is shifted downfield in comparison with the nonperoxide analog. The replacement of -O- by an -OO-group at a carbonyl carbon produces an upfield shift of the signal from this atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 271–275, February, 1982.The authors wish to express their appreciation to P. P. Chernov for recording the spectra, and to I. I. Chervin for participating in discussion of this work.     

In situ 13C solid-state polarization transfer NMR to follow starch transformations in food

Convenience food products tend to alter their quality and texture while stored. Texture-giving food components are often starch-rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ ¹³C solid-state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments—the time scale of reorientation of the CH-bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing ¹³C solid-state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the ¹³C chemical shift. In conjunction with ¹H solid-state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch-based products and commercial pasta as a function of temperature and storage.     

Distribution of mercury in soil organic matter fractions obtained by dissolution/precipitation method

Sequential multistep procedure, usually used for the fractionation and characterization of soil organic matter was tested for Hg content in the individual steps and fractions. Under general laboratory conditions serious problems have arosen during the attempt in the Hg mass balance calculation. Several sources of Hg contamination were recognized. The most serious was the background concentration of Hg even in research grade chemicals (what is in general not declared) and laboratory air. The work on operational Hg speciation as non-humic bound, humic acid, fulvic acid bound proceeds from the established status-quo on distribution of mercury of soil organic matter.     

Analysis of soil organic matter biomarkers by sequential chemical degradation and gas chromatography-mass spectrometry

Low molecular weight (LMW) biomarkers can be used to trace the source and stage of soil organic matter. However, methods that selectively isolate these groups of compounds are underdeveloped. In this study, we isolate biomarkers by a successive series of extraction and chemical degradation procedures involving solvent extraction (TSE), base (BHY) and acid hydrolysis (AHY), and CuO oxidation (CUO). GC-MS was used to analyze these fractions and the extraction methods were verified by solid-state 13C NMR spectroscopy. The GC-MS response was high for the BHY products (96%), intermediate for the TSE (30%) and CUO (19%), but very low for the AHY fraction (5%) indicating that the fractions contain polar or high molecular weight compounds. Aliphatic lipids (62%), phenols and benzyls (17%) were the predominant classes, accompanied by minor abundances of mono- and disaccharides, LMW acids, terpenoids, steroids, amino acids, and amino sugars. The TSE and BHY fractions contained mainly aliphatic lipids derived from plant waxes, cutin, and suberin. Lignin-derived phenols are the major products in the CUO fraction, and amino compounds and carbohydrates of various sources were identified in the AHY products. The sequential degradation method is useful for the isolation and identification of apolar, LMW biomarkers in soil.     

Effects of soil pH and organic matter on distribution of thorium fractions in soil contaminated by rare-earth industries

The labilities of thorium fractions including mobility and bioavailability vary significantly with soil properties. The effects of soil pH and soil organic matter on the distribution and transfer of thorium fractions defined by a sequential extraction procedure were investigated. Decrease of soil pH could enhance the phytoavailability and the potential availability of thorium in soil. Increase of organic matter reduced the phytoavailability of thorium, but enhanced the potential availability of it. The reasons why soil pH and soil organic matter affect thorium fractions were discussed, and the behavior of the effects of soil properties on thorium fractions was elucidated. Fourier-transform infrared (FTIR) spectra were employed to reveal the positive relationship between the amounts adsorbed in humic material and/or amorphous oxides and the content of soil organic matter.     

Isomeric transformations of polyfluorobutenes caused by SbF5

Conclusions The 4-H- and 4-Cl-perfluoro-1-butenes when treated with SbF₅ rearrange to terminal polyfluorobutenes that contain either H or Cl at the multiple bond.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 681–683, March, 1979.     

Relationship between soil organic C degradability and the evolution of the delta13C signature in profiles under permanent grassland

The main objective of this research was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of 13C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the 13C natural abundance (delta13C value) of SOM were investigated in three soil profiles (0-40 cm depth) under permanent grassland of varying texture (a loamy sand, a loam and a clay loam soil). The delta13C value of the SOM showed a gradual increase with increasing depth and decreasing C content in the profiles, ranging from 1.9 per thousand (loamy sand soil), 2.9 per thousand (clay loam soil) and 4 per thousand (loam soil) in relation to the delta13C value of SOM at the surface. The relationship between the 13C enrichment and total organic C content at different depths in the profiles (down to 40 cm depth in the loam and clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors epsilon, associated with the Rayleigh approximation of the data, ranged from -1.57 per thousand (clay loam soil) to -1.64 per thousand (loamy sand soil) and -1.91 per thousand (loam soil). The potential C dynamics in four depth intervals from the profiles (0-10, 10-20, 20-30 and 30-40 cm depth) were determined by means of an incubation experiment. The C decomposition rate constants from the four sampling depths in the profiles showed a significant, positive correlation (y = 0.21x + 0.018, R(2) = 0.66, p < 0.005) with the corresponding Deltadelta13C values (change of the delta13C value per depth increment). A better correlation was obtained when only the data from the upper 20 cm in the profiles (y = 0.21x + 0.019, R(2) = 0.78, p < 0.05) were considered. These results suggest that the Deltadelta13C values in the surface layers of profiles under permanent grassland may serve as an indicator of the potential degradability or the stability of the SOM (in terms of C decomposition rate constants).     

Pyrolysis—gas chromatography—mass spectrometry of a low organic matter calcareous soil

A modification to the general method of pyrolysis-gas chromatography of soils which improves the reproducibility of the technique is described. It has been applied to the study of a calcareous soil, low in organic matter and varations have been studied in programs from soil at different depths as well as from the corresponding humic acids, fulvic acids and extraction residues.Most of the major peaks have identified by gas chromatography-mass spectrometry: they correspond to those cited in earlier works. Data are presented on the characteristics of the pyrolysis products of this soil in the relation to their probobic origin. The evolution of organic matter through various pyrolysis relations is also discussed.     

Repeat‐pulse 13CO2 labeling of canola and field pea: implications for soil organic matter studies

Both the quantity and quality of plant residues can impact soil properties and processes. Isotopic tracers can be used to trace plant residue decomposition if the tracer is homogeneously distributed throughout the plant. Continuous labeling will homogeneously label plants but is not widely accessible because elaborate equipment is needed. In order to determine if the more accessible repeat‐pulse labeling method could be used to trace plant residue decomposition, this labeling procedure was employed using ¹³CO₂ to enrich field pea and canola plants in a controlled environment. Plants were exposed weekly to pulses of 33 atom% ¹³CO₂ and grown to maturity. The distribution of the label throughout the plant parts (roots, stem, leaves, and pod) and biochemical fractions (ADF and ADL) was determined. The label was not homogeneously distributed throughout the plant; in particular, the pod fractions were less enriched than other fractions indicating the importance of continuing labeling well into plant maturity for pod‐producing plants. The ADL fraction was also less enriched than the ADF fraction. Because of the heterogeneity of the label throughout the plant, caution should be applied when using the repeat‐pulse method to trace the fate of ¹³C‐labeled residues in the soil. However, root contributions to below‐ground C were successfully determined from the repeat‐pulse labeled root material, as was ¹³C enrichment of soil within the top 15 cm. Canola contributed more above‐ and below‐ground residue C than field pea; however, canola was also higher in ADF and ADL fractions indicating a more recalcitrant residue. Copyright © 2010 John Wiley & Sons, Ltd.     

Erratum to: Sources of 137Cs fluvial export from a forest catchment evaluated by stable carbon and nitrogen isotopic characterization of organic matter

Journal of Radioanalytical and Nuclear Chemistry -