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Understanding the global carbon (C) cycle is critical to accurately model feedbacks between climate and soil. Thus, many climate change studies focused on soil organic carbon (SOC) stock changes. Pyrogenic carbon (PyC) is one of the most stable fractions of soil organic matter (SOM). Accurate maps based on measured PyC contents are required to facilitate future soil management decisions and soil-climate feedback modelling. However, consistent measurements that cover large areas are rare. Therefore, this study aimed to map the PyC content and stock of the Iberian Peninsula, which covers contrasting climatic zones and has long-term data on wildfire occurrence. A partial least square (PLS) regression using the mid-infrared spectra (1800–400 cm−1) was applied to a dataset composed of 2961 soil samples from the Iberian component of the LUCAS 2009 database. The values of PyC for LUCAS points were modelled to obtain a map of topsoil PyC by a random forest (RF) approach using 36 auxiliary variables. The results were validated through comparison with documented historical wildfire activity and anthropogenic energy production. A strong relationship was found between these sources and the distribution of PyC. Our study estimates that the accumulated PyC in Iberian Peninsula soils comprises between 3.09 and 20.39% of total organic carbon (TOC) in the topsoil. Forests have higher PyC contents than grasslands, followed by agricultural soils. The incidence of recurrent wildfires also has a notable influence on PyC contents. This study shows the potential of estimating PyC with a single, rapid, low cost, chemometric method using new or archived soil spectra, and has the ability to improve soil-climate feedback modelling. It also offers a possible tool for measuring, reporting and verifying soil C stocks, which is likely to be important moving forward if soils are used as sinks for C sequestration

Rapid and unplanned urbanisation can lead to altered local climate by increasing land surface temperature (LST), particularly in summer months. This study investigates the Urban Heat Island (UHI) in Greater Cairo, Egypt, using remote sensing techniques to estimate LST of summer months over 45 years (1986, 2000, 2017, and predicted year 2030). The research objectives and steps were, 1- mapped land use/ land cover (LULC), 2- conducted spatiotemporal analysis of LST, with a comparison of change in LST across different land cover types, 3- predicted future LST for 2030, and 4- examined this temporal change for a hot-spot area (ring road) and a cool-spot area (the River Nile). The results showed that urban areas have increased over the last 30 years by 179.9 km2 (13 %), while agriculture areas decreased by 148 km2 (12 %) and water bodies decreased by 6 km2 (0.5 %). The mean LST over Greater Cairo increased over time, from 31.3 °C (1986) to 36.0 °C (2017) and is predicted to reach 37.9 °C in 2030. While a notable rise of mean LST in the Cairo ring road buffer zone (88 km2), where it was 31.1 °C (1986), and 37 °C (2017) due to the triple increase of urban areas on account of agriculture areas, and the LST it may reach 38.9 °C by 2030. The mean LST increased slightly more in urban hot-spot areas than in cooler cultivated areas. UHI may induce a modification in the local climate that can negatively affect agricultural land, and human thermal comfort and unfortunately lead to a less sustainable environment.

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 CO2 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) sho

Biocrust cyanobacteria are ubiquitous organisms in dryland environments that can enhance soil stability and improve nutrient conditions in reconstructed or disturbed soils. Despite the demonstrated benefits of cyanobacterial inoculation for promoting soil fertility, there is limited knowledge about the impacts of introducing cultured cyanobacteria on the indigenous microbial communities. Here, we conducted a microcosm experiment under controlled conditions using indigenous biocrust cyanobacteria and locally sourced topsoil substrate commonly used in dryland restoration in the Australian arid zone. We inoculated the topsoil with a consortium composed of Leptolyngbya sp. and Scytonema sp. cyanobacteria, to assess the effects of the inoculated cyanobacteria on (i) the soil chemical properties, i.e., pH, electrical conductivity (EC), total organic carbon (TOC), and nitrogen (TN) of the inoculated topsoil, (ii) the early formation of an artificial soil biocrust and iii) the composition and diversity of the resident bacterial community. Our results showed that the inoculated cyanobacterial consortia decreased the EC, but changes in the TOC and TN were not significant after 80 days. A higher content of chlorophyll a in the inoculated samples compared to the control, confirmed the survival of the cyanobacterial inoculants after 80 days. The inoculated cyanobacteria promoted gypsum formation on the soil surface indicating that they are actively modifying the upper layers of the soil profile and improving habitability. Although immediately after inoculation cyanobacteria dominated the abundance of bacterial phylotypes, these were replaced by other phyla such as Actinobacteria, after 80 days. The Shannon diversity and Simpson diversity indexes between control and inoculated soils differed at the time of inoculation but were similar at the end of the experiment. These results suggest that, in the short term, the introduced cyanobacteria do not significantly affect the native re

[EN]: Increasing food production while avoiding the progressive degradation of agricultural soils has become one of the major challenges at a global level. In consequence, the development of sustainable tillage methodologies or cultivation strategies is an important subject of current research. In fact, it has been observed that the implementation of reduced tillage (RT) vs. traditional tillage (TT) in the long term not only improves soil physicochemical properties but also global soil quality in terms of soil health. In particular, the increase of the soil organic carbon (SOC) content under RT conditions is one of the most important factors, but there is little information about the chemical composition and humification level of this carbon, and thus about its persistence at long-term. This is of particular importance considering the policies of carbon sequestration and climate change mitigation, such as the “4 per 1000” initiative. In this study, molecular-level characterization of the humic acid (HA) and fulvic acid (FA) fractions isolated from a soil after 19 years under RT and TT practices was carried out. This study would provide objective descriptors of the impact of these two tillage practices in the chemical composition of the resulting SOC. With this purpose, the potential of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) for assessing changes in the molecular assemblages released from the humic fractions (HA and FA) was examined. The results showed enhanced diversity and chemical richness (expressed as number of molecular formulas) in the composition of SOC under RT. Different tillage-specific compound classes were associated with both tillage practices. As a whole, the humic fraction showed a higher proportion of molecular formulas for lipid and hydroaromatic families in the case of RT compared to TT, while the same fraction under TT showed a greater richness of oxidized protein-derived formulas than RT. In the case of FAs, a simi, [ES]: El incremento en la producción de alimentos evitando los procesos de degradación de los suelos agrícolas se ha convertido en uno de los mayores retos a nivel global. Por esta razón, el desarrollo de nuevas técnicas de laboreo o estrategias de cultivo sostenible es uno de los principales campos de investigación en agricultura. La implementación de estrategias como el laboreo reducido (RT) frente al laboreo tradicional (TT) mejoran ciertas propiedades físico-químicas del suelo. Una de estas propiedades de especial relevancia es el contenido de carbono orgánico del suelo (SOC), que se ve favorecido en términos de cantidad bajo condiciones de RT. Sin embargo, se tiene poca información acerca de la composición química de este carbono y su persistencia a largo plazo. Esta información es particularmente importante considerando las políticas de secuestro de carbono y mitigación del cambio climático, como la iniciativa 4 por 1000. En este estudio, se realizó una caracterización química comparativa de las fracciones de ácidos húmicos y fúlvicos aisladas de la materia orgánica de un suelo proveniente de un experimento de campo en el que se han comparado dos tipos de laboreo (RT y TT) durante 19 años. Para la caracterización química se empleó la espectrometría de resonancia ciclotrónica por transformada de Fourier (FTICR-MS). Los resultados mostraron una mayor diversidad y riqueza química en la SOC en el tratamiento RT. La fracción húmica mostró una mayor proporción de compuestos de naturaleza lipídica e hidroaromática en el caso de RT, mientras que la misma fracción en suelos bajo TT mostró una mayor riqueza en compuestos más oxidados de naturaleza proteica. En el caso de los ácidos fúlvicos, se observó una tendencia similar a la de los húmicos; sin embargo, en este caso la proporción de lípidos insaturados fue mayor en los suelos bajo TT que en los suelos RT. Además, se observó un ligero aumento en el número de compuestos con carácter aromático y aromático condensado en

Land degradation, as a result of increased soil erosion and loss of fertility among other factors, is currently one of the most serious environmental problems. In recent years, the role of cyanobacteria from soil biocrusts in re-establishing soil function of degraded areas is gaining interest due to the potential of these organisms for soil stabilization and increase of soil fertility. In order to fully exploit the use of cyanobacteria in large-scale restoration of degraded lands, new approaches that facilitate their application must be explored in order to face with the harsh abiotic conditions of these environments. In this presentation, we showcase two different methods for the inoculation of cyanobacteria from soil biocrust in degraded soils of Australian dryland ecosystems: i) direct inoculation of cyanobacteria cultures and ii) incorporation of cyanobacteria within extruded pellets. Three soil native cyanobacterial strains from two representative N-fixing genera (Nostoc and Scytonema) and a non-heterocystous filamentous genus (Leptolyngbya) previously collected from the Pilbara region (north-west Western Australia), were used as inoculum. Then, in a multifactorial microcosm experiment under laboratory conditions, we evaluated the survival and establishment of the cyanobacteria for both methods. For the direct inoculation, cultures of isolated cyanobacteria and a mixture of them were applied as a liquid inoculum directly into a degraded soil from the Pilbara. In the case of application using extruded pellets, fresh cultures of each strain alone and an equal mixed of them were added into a substrate composed of commercial bentonite powder and sand (1:10 weight ratio). The composed solution was extruded through a jerky gun with an extruder nozzle into pellets (1 cm diameter x 2 cm length) and dried at 30oC for 24h. Pellets were then placed on the surface of three different degraded soils representative of Australian drylands

There is currently an active controversy about the variable influence of the factors involved in the total content and the quality of the soil organic matter (SOM), which translates into its resilience and stability against biodegradation, and importantly on the rates of release of CO into the atmosphere. The aim of this work is to study the molecular composition of SOM in contrasting environments in order to evaluate the extent to which such conditions may affect SOM characteristics in addition to the levels of soil organic C (SOC). Up to 33 soils from different environmental scenarios of Spain were analysed by pyrolysis combined with gas chromatography mass spectrometry (Py-GC/MS). The 193 major pyrolysis compounds released from the soils were included in a chemostatistical study based in discriminant analysis to assess the impact of classical soil forming factors (i.e., climate, vegetation and geological substrate) in SOM content and composition. Improved van Krevelen diagrams were used to facilitate the recognition of different patterns in SOM composition dependent on soil forming factors. The results showed that the molecular composition of SOM varies systematically according to environmental factors, with a decreasing influence in the order: climate > vegetation > geological substrate. In addition, the total levels of SOM were also different depending on the environmental scenarios on these soils, suggesting both qualitative and quantitative control of soil C sequestration.

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.

Soil organic matter (SOM) is a major component of the biogeochemical cycle contributing to soil general properties and conservation. The progressive depletion of the SOM in Mediterranean ecosystems results in an increased advance of desertification. The SOM not only plays a crucial role in soil resilience, but also represents a repository of environmental information on soil forming factors in particular climatic constraints. In this research, analytical pyrolysis (Py-GC/MS) is used to study SOM composition in 30 soils under contrasting bioclimatic scenarios as defined by the classical Emberger (Q) index. Partial least squares (PLS) regression using the major pyrolysis compounds as descriptors allowed to predict (P < 0.05) Q indexes, and provide molecular proxies of climatic variability effects on SOM composition. In addition, pyrolytic compound assemblages from soils developed under extreme climatic conditions were compared using a graphical approach based on surface density plots built from the major 193 pyrolysis compounds represented in the plane defined by their H/C and O/C atomic ratios. The differences between the proportions of the individual pyrolysis compounds in terms of the bioclimatic scenarios were also illustrated by a simulation of SOM molecular composition under extreme conditions of aridity or wetness. Although no cause-to-effect is inferred, the results show that SOM composition retains environmental information on the Q index. This is mainly related with the total abundances of methoxyphenols and alkylbenzene compounds, suggesting that the degree of transformation of SOM could be related with the variable accumulation of microbial and plant biomass controlled by climatic factors which is a potential field for future research. © 2020 Elsevier B.V.

The knowledge of biogeochemical mechanisms involved in soil organic carbon (SOC) storage is crucial to control its release to the atmosphere. In particular, the chemical composition of soil organic matter (SOM) plays an important role in the performance of the C storage and resilience in soils. The structural information provided by infrared spectroscopy (IR) of soil humic acid (HA) was used in the assessment of the C storage potential of 35 Spanish soils. Partial least squares (PLS) regression using the intensities of the points of the IR spectra of the HAs (4000–400 cm) as descriptors shows that a relationship exists between IR spectral pattern and the SOC content. This was also the case for E4 (humification index based on HA optical density at 465 nm). In addition, the chemical characteristics of the HAs correlated with the SOC levels were identified from digital data treatments of the IR spectra. Additional application of principal component analysis (PCA) and multidimensional scaling (MDS) suggested that bands assigned to carboxyl and amide structures were characteristic in HAs from soils with low C content, whereas HA spectra from soils with high C levels showed a conspicuous band pattern suggesting structural units of lignin from slightly transformed plant residues. The spectral profiles were analyzed in detail by an approach based on digital subtraction of IR spectra obtained by averaging those from HAs extracted from soils in the upper and lower quartiles of the SOC distribution. The results showed that significant relationships exist between the molecular composition of HAs and SOC levels and E4 values in a way in which aromatic, carboxyl and amide groups were predominant in HAs from soils with low SOC content, whereas lignin-derived structures were more characteristic of HAs from soils with high SOC content.

The progress of desertification, which is very pronounced in the Mediterranean area, and the emission of greenhouse gases into the atmosphere, especially CO2, are taking the attention of the researchers. The establishment of the factors involved in these processes as well as the development of emergent technologies to solve these issues is a main objective in different scientific fields. In this perspective, soil conservation plays an important role, due to the content of soil organic matter (SOM) and its stability are important factors. Biogeochemical processes involved in the stabilization of soil organic carbon (SOC) are being the subject of study in this field. Some studies focus on the organo-mineral interactions, while others are interested in the relationship between the molecular structure or the SOM and its resilience. This thesis deals with the molecular characterization of SOM accumulated in different types of soil to establish its relationship with the potential for carbon storage in the corresponding soils, as well as the factors with a bearing on SOM quality. For this purpose, 35 soils with high variability in their organic carbon content (17–157 g·kg-1) have been selected. A detailed characterization of the organic matter has been carried out using destructive and non-destructive techniques such as analytical pyrolysis (Py-GC/MS), 13C solid-state nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, visible spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). In particular, the attention has been focused on certain types of biomarker compounds that could act as environmental indicators of soil biogeochemical processes. The families of alkanes and methoxyphenols were analyzed in detail; its molecular composition was used to distinguish between microbial synthesis and transformation of plant biomass. This study examines the utility of the Shannon diversity index (H’), calculated from the abundance of alkanes a, From a quantitative point of view, PLS regression models based exclusively on the total abundance of the 12 major methoxyphenols were especially effective in predicting carbon storage in the soil. After studying the information provided by analytical pyrolysis of SOM from different scenarios of carbon storage activity, the study was focused to the fraction traditionally considered most representative of the SOM, humic acids (HA). This fraction corresponds to a colloidal product of advanced transformation of plant and microbial biomass. The HA characterization was carried out using visible, IR and NMR spectroscopies. A PLS study using the intensities of digital IR spectra points (4000–400 cm-1) as descriptors showed that there is a relationship between IR spectral patterns and SOC content. This was also the case with E4 index (i.e., indicative of progressive humification, and based on the optical density of HAs at 465 nm). The use of principal component analysis (PCA) and MDS suggested that the bands assigned to carbonyl and amide groups were characteristic in HA of soils with low C content, while the spectra of HAs from soils with high levels of C showed a typical pattern of lignin bands, which indicates accumulation of less transformed plant residues. The IR spectral patterns were analyzed in detail by digital treatments including weighted subtraction of spectra obtained by averaging those of HAs from soils classified in the upper and lower quartiles according the SOC distribution, respectively, and calculating the statistical significance level of the differences. The results showed significant differences between the molecular composition of the HAs, according the SOC and E4 values. Peaks corresponding to aromatic, carboxyl and amide groups showed comparatively high intensity in HAs from soils with low SOC content, while peaks corresponding to lignin-derived structures were more marked in the spectra of the HAs from soils with high SOC content. In the second leve

Environmental research about causes and effects of progressive soil desertification is attracting the attention of the scientific community. Factors that accelerate soil organic matter (SOM) mineralization may also induce structural changes in its molecular composition, but the specific impact of different climatic factors (annual precipitation, number of days with precipitations, temperature…) has not yet been the subject of systematic research. This study attempts to identify potential climatic molecular proxies in SOM, i.e., pyrolytic compounds correlated with specific climatic indices. A set of 16 Spanish soils under differing climatic conditions were selected. Molecular characterization of SOM was done by pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS) applied to whole soil samples. Up to 193 compounds were identified corresponding to alkanes, alkylbenzenes, N‐compounds, olefins, phenols and carbohydrate derivatives. Simple and multiple correlations between pyrolytic and climatic ariables showed that the concentration of several compounds was significantly correlated (P < 0.05) with particular climatic variables. In a second stage, using van Krevelen [1,2] diagrams, it was possible to display different patterns of pyrolytic compounds responsive to specific climatic characteristics. Finally, extrapolation functions were applied to simulate SOM molecular composition under future climatic scenarios, e.g., changes in temperature, evapotranspiration, rainfall, etc. This study demonstrates the potential of Py‐GC/MS in identifying specific molecules and compound families useful for predicting climate impact on SOM, as well as for monitoring soil quality in terms of the progress of desertification, References [1] D.W. van Krevelen, Fuel 29, 269–284 (1950). [2] G. Almendros, Z. Hernández, J. Sanz, S. Rodríguez‐Sánchez, M.A. Jiménez‐González, J.A. González‐Pérez, J. Chromatogr. A. 1533, 164–173 (2018).

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

There is a worldwide concern to predict and control the effects of climatic change on the soils. In fact, soil organic matter (SOM) is the largest reservoir of C in Earth’s surface and one of the most dynamic pool. Apart from this, the structure and composition of the SOM are largely responsive to environmental changes. For this reason, the most current research focuses on the study of the mechanism of C stabilization in soils that generally depends on both natural and anthropogenic factors, including soil use and management practices. In particular, a controversial aspect about SOM formation processes is to understand how the C from the biomass is stabilized in the soil by either physical protection based on organo-mineral interactions or after a structural rearrangement the SOM macromolecular structure. Organic N-compounds in soil are of particular interest, to the extent that its chemical structure and speciation status in the SOM can play an important role in its N bioavailability. In fact, the accumulation of recalcitrant N-compounds can be an indicator of soil quality and could be related with the potential of C sequestration in certain soils. In this study, 35 soils from different areas of Spain with a wide range of SOM content were selected. Analytical pyrolysis (Py-GC/MS) was used to identify N moieties and pattern in the SOM of whole soil samples. A general characterization of SOM quality was performed using solid state 13C NMR and UVvis spectroscopy. The N-compounds identified by Py-GC/MS corresponded to seven principal chemical structures: indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolones. These compounds are considered as pyrolytic markers of well-defined classes of biomass constituents, viz., protein derivatives, amino sugars and chlorophylls. Partial least squares (PLS) regression was used to explore the possible prediction of SOM quality and content using the percentages of pyrolytic N-compounds as descriptors. This ap

Wildfires are a global phenomenon occurring in tropical, temperate and boreal regions that affect 300-460 Mha every year (Randerson el al. 2012; Giglio el al. 2013) (Chapter 1). Temperate regions have been affected historically by wildfires (Pausas el al. 2008; Lasheras-Álvarez el al. 2013) and during the 20th century a drastic increase in the extent and severity of burnt areas has occurred. This has made wildfires one of the main threats for ecosystems, particularly those in Mediterranean areas. The obvious consequence of a wildfire is the partial or complete loss of the vegetation. However, wildfires may also cause changes in the physical, chemical and biological properties of soil (González-Pérez el al. 2004; Certini, 2005). The alteration of geomorphological and hydrological processes of the topsoil following a wildfire (de la Rosa el al. 2013a; Faria el al. 2015a) can boost erosion processes that, in turn, are often associated with quantitative and qualitative changes in soil organic matter (SOM) (Chapters 3, 4, 5, 6, 7, 8, 9, 10 and 11). In fact, desertification risk in connection with increased wildfire recurrence is very high in Mediterranean ecosystems (Shakesby, 2011) and the design and application of proper post-fire restoration strategies is paramount for reducing such risk. Restoration of both quality and quantity of SOM is important for soil rehabilitation after a wildfire. Nevertheless, many studies have focused only on the effects of fire in the total amount of SOM, with only scarce the attention paid to the SOM chemical composition and molecular structure, despite its relevance (Almendros el al. 1984a,b; González Pérez el al. 2004, 2008; Certini el al. 2011, de la Rosa el al. 2013a; Faria el al. 2015a,b; Aznar el al. 2016; Jiménez-González el al. 2016; Jiménez-Morillo el al. 2016a, 2017).

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.

In present days there is a growing concern about the progress of desertification in different areas of the world. The unsuitable management of the soils and the change of land use may increase the desertification risk. On the other hand, desertification is typically associated with the decrease of soil organic matter (SOM) levels with the consequent loss of fertility in the soil. Presumably, all these aspects are reflected in the molecular composition of SOM. Previous studies have evidenced that a correlation exists between the carbon sequestration efficiency and the relative abundance of specific SOM constituents, e.g., alkane homologous series [1] or lignin-derived methoxyphenols [2]. This study aims to identify molecular descriptors of the SOM composition, which are responsive for the impact of climate, quantified with bioclimatic indices defining a continuous gradient between wet and dry areas. A total of 33 soil samples were collected from different areas of Spain. The studied soils presented a large variability in their chemical and physical properties, and were developed under different geological substrate and vegetation type. The sampling was carried out in the topsoil (0–10 cm) where the SOM content is higher. In order to assess desertification levels we used the De Martonne aridity index. This index was calculated from the annual average rainfall and annual average temperature for each soil sampling point. The SOM was analyzed by pyrolysis - gas chromatography mass spectrometry (Py-GC/MS) of whole soil samples. A total of 193 pyrolysis compounds were identified, and used as predictor variables in Partial Least Squares (PLS) regression models forecasting the De Martonne aridity index. The results showed that a significant prediction of this index (R = 0.869) exclusively using the information provided by Py-GC/MS analysis of the corresponding soils is possible. A graphical-statistical method based in the classical van Krevelen diagram was used for displayin, [1] M.A. Jiménez-González, A.M. Álvarez, Z. Hernández, G. Almendros, Biology and Fertility of Soils 54 (2018) 617–629. [2] M.A. Jiménez-González, A.M. Álvarez, P. Carral, F.J. González-Vila, G. Almendros J. Chromatography A 1508 (2017) 130–137. [3] D.W. van Krevelen, Fuel 29 (1950) 269–284.

In present days there is a growing concern about the progress of desertification in different areas of the world. The unsuitable management of the soils and the change of land use may increase the desertification risk. On the other hand, desertification is typically associated with the decrease of soil organic matter (SOM) levels with the consequent loss of fertility in the soil. Presumably, all these aspects are reflected in the molecular composition of SOM. Previous studies have evidenced that a correlation exists between the carbon sequestration efficiency and the relative abundance of specific SOM constituents, e.g., alkane homologous series [1] or lignin-derived methoxyphenols [2]. This study aims to identify molecular descriptors of the SOM composition, which are responsive for the impact of climate, quantified with bioclimatic indices defining a continuous gradient between wet and dry areas. A total of 33 soil samples were collected from different areas of Spain. The studied soils presented a large variability in their chemical and physical properties, and were developed under different geological substrate and vegetation type. The sampling was carried out in the topsoil (0–10 cm) where the SOM content is higher. In order to assess desertification levels we used the De Martonne aridity index. This index was calculated from the annual average rainfall and annual average temperature for each soil sampling point. The SOM was analyzed by pyrolysis - gas chromatography mass spectrometry (Py-GC/MS) of whole soil samples. A total of 193 pyrolysis compounds were identified, and used as predictor variables in Partial Least Squares (PLS) regression models forecasting the De Martonne aridity index. In order to assess desertification levels we used the De Martonne aridity index. This index was calculated from the annual average rainfall and annual average temperature for each soil sampling point. The SOM was analyzed by pyrolysis - gas chromatography mass spectrometry (P, A graphical-statistical method based in the classical van Krevelen diagram was used for displaying the pyrolysis results [3], representing difference values between the proportions of the 193 pyrolysis products, calculated between average pyrograms for soils in the uppermost quartile of the De Martonne index, and the average of those in the lower quartile. These values are shown as a scatterdiagram where the coordinates in the plane of the individual molecules correspond to their H/C and O/C atomic ratios, calculated from their empirical formulas. The difference values between relative abundances were represented as a density map where the green colour indicates compounds predominant in the SOM of soils formed in sites with high De Martonne index, and the red colour indicate compounds predominant in soils with low index (i.e., comparatively dry ecosystems). The Student’s t (p > 90%) was also used to evaluate the significant differences between the proportions of compounds, and was represented as a superimposed contour diagram in the Figure. The progressive desertification of the soils is associated to a molecular composition of the SOM defined by the selective accumulation of lignin-derived compounds at different stages of transformation in soil (phenols and methoxyphenols), whereas aromatic compounds and aliphatic hydrocarbons tend to be major SOM constituents in soils developed under comparatively humid climatic conditions. As a whole, the results suggests that progressive evolution towards arid climate leads to a decrease in SOM quality, as reflected by the accumulation of raw organic matter where structural units of plant macromolecules can still be easily recognized. Conversely, in soils formed in ecosystems with comparatively longer wet season, the SOM formation is carried out at expenses of the incorporation of aliphatic material of plant and microbial origin (mainly alkenes and alkanes) together with condensed aromatic structures, which are traditionally con, Referencias: [1] M.A. Jiménez-González, A.M. Álvarez, Z. Hernández, G. Almendros, Biology and Fertility of Soils 54 (2018) 617–629. [2] M.A. Jiménez-González, A.M. Álvarez, P. Carral, F.J. González-Vila, G. Almendros J. Chromatography A 1508 (2017) 130–137. [3] D.W. van Krevelen, Fuel 29 (1950) 269–284

Long-term stabilization of soil organic matter (SOM) as recalcitrant forms plays an important role in the Earth's carbon cycle. Hence, understanding biogeochemical mechanisms of soil C sequestration is crucial to control its emission to atmosphere. In this research the composition of humic acid (HA) is analyzed by Electrospray Ionization (ESI) Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS). Using Partial Least Squares (PLS) regression, a forecasting model for total soil C is obtained from the common compounds and its importance in the prediction is plotted in the space defined by their atomic ratios.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) of humic acids (HAs) from 30 agricultural soils from a volcanic island (Tenerife, Spain) was used to discern the molecular characteristics of soil organic matter (SOM) associated to resilience. For faster perceptual identification of the results, the yields of the pyrolysis products in the form of surface density plots were compared in an update of the Van Krevelen graphical statistical method. This approach, with respect to data reduction and visualization, was also used to collectively represent statistical indices that were obtained after simple and partial least squares (PLS) regression. The resulting plots illustrate different SOM structural domains (for example, carbohydrate- and lignin-derived and condensed lipid). The content of SOM and total mineralization coefficient (TMC) values can be well estimated from the relative abundance of 57 major pyrolysis compounds: SOM content and composition parallels the accumulation of lignin- and carbohydrate-derived structures (lignocellulosic material) and the depletion of condensed polyalkyl structures. In other words, in the volcanic ash soils that were studied, we found that the higher the amount of SOM, the lower its quality in terms of resilience. Although no cause-and-effect is inferred from this fact, it is evident that the resistance to biodegradation of the SOM is related to its molecular composition

The soil lipid fraction consists of a complex mixture of low-polarity compounds of microbial or plant origin [1]. Homologues series of alkanes are present in all soil types: long-chain homologues are frequently considered to derive from epicuticular waxes of vascular plants, whereas short-chain homologues are thought to derive mainly from microorganisms. In general the free lipids (i.e., directly extracted with organic solvents) amount to a little proportion of the soil organic matter (2¿ 50 g/kg), but may have a direct influence in soil processes viz, enhancing soil water repellency or promoting soil aggregation. On the other hand, alkanes as many other soil lipids, are also a valuable source of environmental information: In fact, homologous series of alkanes may vary in terms of the vegetation and microbial activity, but also reflect different soil abiotic and ecophysiological properties with a bearing on the selective accumulation of the individual alkane molecules. In this exploratory research we focus exclusively on the n-alkanes found in the free soil lipid fraction, which are studied by gas chromatography¿mass spectrometry (GC¿MS), using the characteristic ion trace at m/z 85 to identify and quantify the alkane series. For this study we collected 35 topsoil samples from Spanish soils (0¿10 cm depth) in contrasted environments. The lipids were Soxhletextracted for 12 h with petroleum ether (40¿60 ºC). Thehomologous series were considered in the range between C15 (pentadecane) to C35 (pentatriacontane). The proportions of the different homologues were used as descriptors to explore possible correlations with a large series of soil properties. Complementary chemometric methods were used to examine the information latent in the alkane patterns viz, partial least squares regression (PLS), multidimensional scaling (MDS) and principal component analysis (PCA). We found a large forecasting potential of the n-alkanes mainly as regards soil physical properties (i.e., a

Fire is considered one of the main disturbing factors of ecosystems at global scale. Causing physical and chemical changes on soil organic matter (SOM) (González-Pérez et al., 2004). One of the main consequences of burning is the formation of refractory material with a high degree of aromatization, black carbon (BC) (Hedges et al., 2000). The chemical composition of BC and the intensity of fire impacts on SOM depend strongly on fire conditions (De la Rosa et al., 2008). Therefore, fire intensity and duration may help to understand these processes. Nonetheless, in situ-determination is difficult. Burning experiments under laboratory conditions and traditional analytical techniques have provided contradictory results with respect to the impact of temperature and time of heating. Therefore the present research studies the relations between temperature and fire duration (are you sure that this is the right word?) with the composition of the resulting fire-affected organic matter. To achieve this, litter samples were collected below well-developed oak-canopy (Quercus suber) in the Doñana National Park (SW Spain). Litter was air dried and heated at 250, 300 and 350 oC in a muffle furnace during two different time periods (5 and 15 minutes). Unburnt air-dried litter samples were used as control. After heating, litter samples were analysed using solid-state CPMAS 13C-NMR spectroscopy and carbon isotope ratio mass spectrometry (C-IRMS).

The assessment of factors involved in soil carbon storage is crucial for developing the scientific bases of Earth¿s biogeochemical cycles. This is also the subject of current controversy about the extent to which the recalcitrance of soil organic matter depends on its molecular composition, or on external factors such as organo-mineral interactions. In any case, some studies have suggested significant correlations and possible cause-to-effect relationships between molecular-level descriptors and the recalcitrance of the soil organic matter. Assuming this background, we identify and quantify the N-compounds released by analytical pyrolysis from whole soil samples by using gas chromatography/mass spectrometry. The interest of focusing on N-compounds could be justified by: i) the fact that they amount to a reduced number of diagnostic molecules in pyrograms with more than a hundred of major compounds, ii) a progressively decreased C/N ratio is a classical index of stability and quality of soil organic matter, then changes in organic N-forms should parallel efficient soil C sequestration, iii) concentration of nonhydrolyzable ¿unknown¿ N-fractions is a general diagenetic trend in terrestrial soils, suggesting that organic matter maturity could be reflected by the pyrolytic assemblages of N-compounds. Consequently, this study explores the relationships between pyrolytic N-compounds and different soil properties with special emphasis on soil carbon storage. A total of 35 samples were collected from different Spanish soils with large variability in the concentration of organic carbon (from 17 to 157 g oxidizable C / kg soil). After identifying the major pyrolytic N-compounds (mainly indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolines) its origin is discussed as the different concentration in the soils of e.g., amino acids, amino sugars, nucleotides and chlorophylls. In a second step, a series of chemometric approaches were used to unravel poss