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Biogeochemical factors responsible of the highly variable content of soil organic matter (SOM) in the different types of soils are poorly known. In particular, the role of organo-mineral interactions has frequently been considered, but less attention has been paid to the molecular composition of the SOM. The aim of this work was to contribute to a better qualitative and quantitative assessment of the soil organic C (SOC) accumulation, using chemometric approaches that do not require the absolute knowledge of the structure and functioning of the whole system under study. For this reason, we monitored the n-alkanes released by analytical pyrolysis from 35 widely different Mediterranean soils. The H′ Shannon diversity index was calculated to evaluate the origin and transformations of the alkane homologous series (C9-C31). A series of multivariate data treatments succeeded in showing significant relationship between the diversity of alkanes and the SOC concentration, and additional indicators of SOM quality were also used. All statistical analyses pointed out the significant correlation (P < 0.01) between the H′ diversity of the pyrolytic alkanes and the amount of SOC. In particular, a significant relationship between SOC levels and the percentage of long-chain alkanes was found, whereas the percentage of short-chain alkanes was correlated with specific descriptors of SOM quality. Finally, the partial least squares (PLS) predicted the SOC content from the alkane patterns. [ABSTRACT FROM AUTHOR]

The variable extent to which environmental factors are involved in soil carbon storage is currently a subject of controversy. In fact, justifying why some soils accumulate more organic matter than others is not trivial. Some abiotic factors such as organo-mineral associations have classically been invoked as the main drivers for soil C stabilization. However, in this research indirect evidences based on correlations between soil C storage and compositional descriptors of the soil organic matter are presented. It is assumed that the intrinsic structure of soil organic matter should have a bearing in the soil carbon storage. This is examined here by focusing on the methoxyphenols released by direct pyrolysis from a wide variety of topsoil samples from continental Mediterranean ecosystems from Spain with different properties and carbon content. Methoxyphenols are typical signature compounds presumptively informing on the occurrence and degree of alteration of lignin in soils. The methoxyphenol assemblages (12 major guaiacyl- and syringyl-type compounds) were analyzed by pyrolysis-gas chromatography-mass spectrometry. The Shannon-Wiener diversity index was chosen to describe the complexity of this phenolic signature. A series of exploratory statistical analyses (simple regression, partial least squares regression, multidimensional scaling) were applied to analyze the relationships existing between chemical and spectroscopic characteristics and the carbon content in the soils. These treatments coincided in pointing out that significant correlations exist between the progressive molecular diversity of the methoxyphenol assemblages and the concentration of organic carbon stored in the corresponding soils. This potential of the diversity in the phenolic signature as a surrogate index of the carbon storage in soils is tentatively interpreted as the accumulation of plant macromolecules altered into microbially reworked structures not readily recognized by soil enzymes. From a quantitative viewpoint, the partial least squares regression models exclusively based on total abundances of the 12 major methoxyphenols were especially successful in forecasting soil carbon storage. [ABSTRACT FROM AUTHOR]

• The patterns of pyrolytic N -compounds are related with biogeochemical C processes. • Unsubstituted pyridines and benzonitriles are indicators of organic matter quality. • Alkylpyridines, alkylbenzonitriles and indoles are indicators of soil carbon storage. The increasing land degradation is a problem that affects many soils in countries with a Mediterranean climate. In this aspect the soil organic matter (SOM) plays an important role, due to its progressive biodegradation parallels to desertification and the concomitant emissions of CO 2 to the atmosphere. These facts make basic research on the structure and composition of SOM important for soil conservation. Organic N -compounds in soil are of particular interest due to their chemical structure and speciation status in the SOM which can play an important role in soil N bioavailability and in the whole biogeochemical activity of the soil. For this reason, studying the possible relationships between the different N -compounds and soil properties, such as SOM content and its chemical characteristics, can provide new information on the stabilization and storage of organic C in soil. For this research, 30 soils from Spanish ecosystems with a wide range of SOM content were selected. The molecular composition of SOM in whole soil samples including N -compounds, was analyzed by analytical pyrolysis (Py-GC/MS). A parallel characterization of SOM quality was carried out using solid state 13C NMR and UV–vis spectroscopy. Based on their chemical structure, the N -compounds identified by Py-GC/MS were classified into seven main groups: indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolines. Multivariate statistical analyses were used to explore the relationship between the distribution of the above compounds and the SOM content. A significant predictive model was obtained for the SOM using partial least squares (PLS) regression, which was used to predict SOM content using the pyrolytic N -compounds as descriptors. This would show that there is a relationship between the patterns of N -compounds and the biogeochemical mechanisms involved in the different C storage levels the soils. Also, multidimensional scaling (MDS) and principal components analysis (PCA) showed to what extent the individual N -compounds are informative of status and quality of the humic acid fraction of SOM. As a whole, the results obtained by Py-GC/MS suggest that indoles, alkylindoles, alkylbenzimidazoles and alkylpyridines could be indicators of SOM accumulation while unsubstituted benzonitrile and pyridine are related to SOM quality. [ABSTRACT FROM AUTHOR]

• A relationship exists between SOC content and the composition of soil HAs. • Unsaturated lipid and lignin structures prevail in high SOC content scenarios. • Saturated lipid prevails in low SOC content scenarios. • Humic acid structure keeps information on soil properties as SOC content. • The SOC can be predicted from the molecular composition of HAs. Long-term stabilization of soil organic matter (SOM) plays an important role in the carbon cycle. Hence, understanding biogeochemical mechanisms of soil C sequestration is crucial to control its release to the atmosphere. This study aims at investigating the biogeochemical mechanisms of soil C sequestration. An exploratory assessment was carried out on the information about the soil C levels provided by the molecular composition of humic acids (HAs) analyzed by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Significant PLS forecasting model for total soil C was obtained using as descriptors the 131 compounds in common in all the HAs detected by FTICR-MS, and its variable importance for projection (VIP) was plotted in the space defined by their atomic ratios using van Krevelen diagrams. The results indicated that significant relationship exists between the HAs molecular composition and the soil organic C levels. The VIP values for the different groups of compounds illustrate how HA contains information about the amounts of C stored in the soil: the HAs in the soils with high levels of organic C have significantly (P < 0.1) higher proportions of unsaturated lipid and lignin-derived compounds; on the other side, low soil organic C levels are associated to HAs with comparatively high proportions of saturated lipid compounds. [ABSTRACT FROM AUTHOR]

A detailed and global quantitative assessment of the distribution of pyrogenic carbon (PyC) in soils remains unaccounted due to the current lack of unbiased methods for its routine quantification in environmental samples. Conventional oxidation with potassium dichromate has been reported as a useful approach for the determination of recalcitrant C in soils. However, its inaccuracy due to the presence of residual non-polar but still non-PyC requires additional analysis by 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, which is expensive and time consuming. The goal of this work is to examine the possibility of applying infrared (IR) spectroscopy as a potential alternative. Different soil type samples (paddy soil, Histic Humaquept, Leptosol and Cambisol) have been used. The soils were digested with potassium dichromate to determine the PyC content in environmental samples. Partial Least Squares (PLS) regression was used to build calibration models to predict PyC from IR spectra. A set of artificially produced samples rich in PyC was used as reference to observe in detail the IR bands derived from aromatic structures resistant to dichromate oxidation, representing black carbon. The results showed successful PLS forecasting of PyC in the different samples by using spectra in the 1800–400 cm−1 range. This lead to significant (P < 0.05) cross-validation coefficients for PyC, determined as the aryl C content of the oxidized residue. The Variable Importance for Projection (VIP) traces for the corresponding PLS regression models plotted in the whole IR range indicates the extent to which each IR band contributes to explain the aryl C and PyC contents. In fact, forecasting PyC in soils requires information from several IR regions. In addition to the expected IR bands corresponding to aryl C, other bands are informing about the patterns of oxygen-containing functional groups and the mineralogical composition characteristic of the soils with greater black carbon storage capacity. The VIP traces of the charred biomass samples confirm that aromatic bands (1620 and 1510 cm−1) are the most important in the prediction model for PyC-rich samples. These facts suggest that the mid-IR spectroscopy could be a potential tool to estimate the black carbon. • Effective prediction of black carbon in soils by IR spectroscopy and PLS regression. • The reliability of PyC prediction increases with the C content of the samples. • PLS-VIP inform on the IR spectral bands valid as surrogates for PyC. • Aromatic bands are the most important in the prediction model for PyC-rich samples. [ABSTRACT FROM AUTHOR]

Increasing soil organic carbon (SOC) content is crucial for soil fertility, conservation, and combating climate-related issues by sequestering CO 2. While existing studies explore the total content of SOC, few of them investigate the factors that favor its sequestration and the impact of land use type and management. This research aims to study the spatial variation of the total content and the quality or maturity (in terms of aromaticity) of the humic acid (HA) fraction, along with the factors that enhance its formation and conservation for a longer time in the soil. In addition, the study tries to evaluate the performance of the Regression Kriging (RK) method in producing interpolation maps that describe the natural variation of the SOC and its quality with the aim of defining and preventing soil degradation. Finally, the study aims to evaluate the impact of the land use type and the importance of dense vegetation in the sequestration of the organic carbon (OC) in the soil. The analysis of the SOC was performed in northeast Algeria's semi-arid climate, examining content, quality, and chemical composition. Using geostatistical methods (RK), SOC is correlated with most related factors, producing detailed interpolation maps. The results showed that the SOC and its HA fraction (both its total content and its degree of transformation or maturity (measured in terms of aromaticity and structural condensation) are highly correlated to the topography of the area (P < 0.05). Results reveal variations in HAs' composition across land covers. Notably, areas subjected to burning exhibited a 21% increase in HA aromaticity compared to forested regions and a 29% increase relative to cultivated areas. The study highlights that soil cover has a substantial influence on the performance of SOC sequestration, the forested areas have a positive impact on the storage of SOC in the form of HA with a more complex chemical composition that suggests increased aromaticity and resilience. As a whole, the results indicate the potential of geostatistical methods to provide valuable information about the factors that influence the current status and evolution of SOC in the study area. [Display omitted] • Humic acids are used as an indicator of soil organic carbon quality. • The chemical composition of soil organic carbon controls its sequestration. • Soil organic carbon sequestration is considerably affected by land cover. • Soil organic matter rich in aromatic structures is promoted by wildfires and erosion. • The interpolation of soil organic carbon stocks can be improved through regression kriging. [ABSTRACT FROM AUTHOR]