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Since the isolation of soil organic matter in 1786, tens of thousands of publications have searched for its structure. Nuclear magnetic resonance (NMR) spectroscopy has played a critical role in defining soil organic matter but traditional approaches remove key information such as the distribution of components at the soil-water interface and conformational information. Here a novel form of NMR with capabilities to study all physical phases termed Comprehensive Multiphase NMR, is applied to analyze soil in its natural swollen-state. The key structural components in soil organic matter are identified to be largely composed of macromolecular inputs from degrading biomass. Polar lipid heads and carbohydrates dominate the soil-water interface while lignin and microbes are arranged in a more hydrophobic interior. Lignin domains cannot be penetrated by aqueous solvents even at extreme pH indicating they are the most hydrophobic environment in soil and are ideal for sequestering hydrophobic contaminants. Here, for the first time, a complete range of physical states of a whole soil can be studied. This provides a more detailed understanding of soil organic matter at the molecular level itself key to develop the most efficient soil remediation and agricultural techniques, and better predict carbon sequestration and climate change.

International audienceThe fundamental mechanisms whereby organic inputs stabilize in soil are poorly resolved, which limits our current capacity to predict the dynamics of soil organic matter (OM) turnover and its influence on soil quality and functioning. Here we fractionated soil OM from long-term experimental field plots either unamended or amended with two organic materials of different quality (i.e., solid cattle manure and crop residues) for 44 years into five measurable and meaningful pools directly related to conceptual preservation mechanisms: dissolved OM, mineral-free particulate OM located outside aggregates (unprotected from decomposition), OM occluded within both macroaggregates and microaggregates (weakly and strongly protected by physical mechanisms, respectively), and OM intimately associated with soil mineral particles (protected by chemical mechanisms). Compared to the unamended soil, the application of cattle manure and crop residues increased total organic C content by 35 and 10%, respectively. Most of these increases (up to 60 and 72% for cattle manure and crop residues, respectively) were explained by the mineral-associated OM pool, followed by the intra-microaggregate OM fraction. In general, the distribution and dynamics of N content paralleled those of C content. As determined by a range of modern nuclear magnetic resonance (NMR) techniques, including 13C cross polarization magic angle spinning (MAS), 1H high resolution (HR)-MAS, and 1Hsingle bond13C heteronuclear single quantum coherence HR-MAS NMR, the mineral-associated OM fraction was found to be predominately of microbial origin, unlike free and intra-aggregate OM pools, which were dominated by plant structures at different stages of decomposition. As a whole, our results indicate that the main mechanism by which organic inputs are stabilized and OM accrues in soils is not the physical and chemical protection of undecayed or partially degraded organic structures, but the adsorption on mineral surfaces of microbial biomass and microbial by-products resulting from microbial growth, transformation, and degradation processes. It is possible that organic amendments increase more than previously thought the microbial populations of the soil, which live, thrive, and die in close association with the mineral surfaces. This mechanism appears to be enhanced with the addition of stable organic materials

The results of the ring conformational analysis of L-proline, N-acetyl-L-proline, and trans-4-hydroxy-L-proline by NMR combined with calculations using density functional theory (DFT) and molecular dynamics (MD) are reported. Accurate values of 1H-1H J-couplings in water and other solvents have been determined. Using a two-site equilibrium model, the Cgamma-endo conformer of L-proline in water has been identified as intermediate between gammaTdelta [twist(Cgamma-endo, Cdelta-exo)] and gammaE [envelope(Cgamma-endo)] and the Cgamma-exo conformer as betaTgamma. Both conformers were equally populated at room temperature. The N-acetyl [cis-rotamer gammaTbeta(80%)/gammaE(20%) and trans-rotamer gammaTbeta(61%)/gammaE(39%)] and 4-hydroxy (gammaEpsilon) derivatives showed significant changes in both the population and the geometries of the preferred ring conformers. The combination of NMR predicted populations with the DFT B3LYP/6-311+G(2d,p)/IEFPCM calculations proved successful, resulting in fairly accurate predictions of J-couplings. Simulations using MD were mostly in favor of the two-site equilibrium model between Cgamma-endo and Cgamma-exo conformers, similar to that used for the analysis of NMR J-couplings. Various force fields examined for MD simulations failed to reproduce the ring conformational geometries and populations of L-proline in water accurately, while significantly better agreement with NMR was found for trans-N-acetyl-L-proline using GROMOS and AMBER force fields.

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Nuclear magnetic resonance (NMR) and quantum mechanical (QM) studies have been carried out for proline (Pro) containing peptides: N-acetyl-l-proline (AcProOH) and N-acetyl-4-hydroxy-l-proline (AcHypOH). Preliminary results of variable temperature NMR measurements for Gly-Pro-Gly-Gly (GPGG), Val-Ala-Pro-Gly (VAPG), and Ala-Pro-Gly-Trp amide acetate salt (APGW) are also reported. The effect of solvent (D(2)O, DMSO-d(6) and CD(3)CN) on the pyrrolidine ring conformation and cis/trans-rotamerisation along the amide bond preceding Pro was investigated by temperature dependent NMR followed by detailed transition state (TS) searches for both conformational equilibria using QM methods. The results revealed the energetic characteristics of the TS, which were in satisfactory agreement with NMR, and the corresponding TS geometries, which are not available from experiment. The most remarkable feature of the cis/trans-rotamerisation is that the amide nitrogen in AcProOH and AcHypOH adopts a tetrahedral geometry in the TS. Various HF, DFT, and MP2 calculations together with implicit solvation modeling were employed in order to identify the most suitable QM protocols for reliable predictions of the geometry and the relative energies of the conformations of Pro and Hyp containing peptides in aqueous solution. Solution NMR results were used for the verification of the reliability of the QM predictions. The results indicate that the MP2 calculations combined with implicit solvation models are reasonably accurate in reproducing NMR measured populations of four different conformations of either AcProOH or AcHypOH in different solvents, whereas HF and DFT B3LYP calculations were significantly less accurate.

Environmental contextThe association of specific organic matter (OM) compounds with clay mineral surfaces is believed to protect these compounds from degradation and thus result in long-term protection in soil. The molecular-level composition of soil OM associated with soil fractions was measured and compared using solid-state 13C nuclear magnetic resonance (NMR) and solution-state 1H NMR methods. Combining these methods allowed more detailed characterisation of OM associated with different soil fractions and will improve the understanding of OM dynamics in soil. AbstractOrganic matter (OM) associated with fine soil fractions is hypothesised to be protected from complete biodegradation by soil microbes. It is therefore important to understand the structure and stage of decomposition of OM associated with various soil fractions. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has been used extensively to investigate the OM composition of soils and soil fractions. Solution-state 1H NMR spectroscopy has not been used as much but is an emerging tool for analysing soil OM because 1H NMR spectra are often better resolved and provide information that complements the structural information obtained from solid-state 13C NMR experiments. This study compares one-dimensional solution-state 1H NMR and solid-state 13C NMR methods for assessing the degradation and composition of OM in three different soils, and their light and clay-size fractions. The alkyl/O-alkyl degradation parameter was consistent across all NMR methods and showed that OM associated with clay-size fractions were at more advanced stages of degradation as compared to that in light density soil fractions. Solution-state 1H and diffusion edited (DE) 1H NMR results showed the presence of high concentrations of microbial-derived peptidoglycan and peptide side-chains in clay-sized fractions. Lignin was also identified in clay-sized fractions using solid-state 13C and solution-state 1H NMR techniques. The combination of solid-state 13C and solution-state 1H NMR methods provides a more detailed analysis of OM composition and thereby facilitates a better understanding of the fate and preservation of OM in soil.