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Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (δ13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ13C values.

International audience; Soil organic matter (SOM) is a complex heterogeneous mixture formed through decomposition and organo-mineral interactions, and characterization of its composition and biogeochemical stability is challenging. From this perspective, Rock-Eval® is a rapid and efficient thermal analytical method that combines the quantitative and qualitative information of SOM, including several parameters related to thermal stability. This approach has already been used to monitor changes in organic matter (OM) properties at the landscape, cropland, and soil profile scales. This study was aimed to assess the stability of SOM pools by characterizing the grain size fractions from forest litters and topsoils using Rock-Eval® thermal analysis. Litter (organic) and topsoil samples were collected from a beech forest in Normandy (France), whose management in the last 200 years has been documented. Fractionation by wet sieving was used to separate large debris (> 2 000 μm) and coarse (200–2 000 μm) and fine particulate OM (POM) (50–200 μm) in the organic samples as well as coarse (200–2 000 μm), medium (50–200 μm), and fine (< 50 μm) fractions of the topsoil samples. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In the organic layers, quantitative and qualitative changes were explained by the progressive decomposition of labile organic compounds from plant debris to the finest organic particles. Meanwhile, the grain size fractions of topsoils presented different characteristics. The coarse organo-mineral fractions showed higher C contents, albeit with a different composition, higher thermal stability, and greater decomposition degree than the plant debris forming the organic layer. These results are consistent with those of previous studies that microbial activity is more effective in this fraction. The finest fractions of topsoils showed low C contents, the highest thermal stability, and low decomposition degree, which can be explained by the stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM in the different size fractions be interpreted in the light of a plant-microbe-soil continuum. Finally, three distinct thermostable C pools were highlighted through the grain size heterogeneity of SOM: free coarse OM (large debris and coarse and fine particles), weakly protected OM in (bio)aggregates (coarse fraction of topsoil), and stabilized OM in the fine fractions of topsoil, which resulted from the interactions within organo-mineral complexes. Therefore, Rock-Eval® thermal parameters can be used to empirically illustrate the conceptual models emphasizing the roles of drivers played by the gradual decomposition and protection of the most thermally labile organic constituent

Loess-paleosol sequences preserve information that can be used to reconstruct paleoenvironments. A dense network of reliably analyzed sequences from different geographic locations is crucial to address ecological and climatic trends, which occurred during the Pleistocene. This paper focuses on a loess-paleosol sequence in Bůhzdař, situated 9 km NW of Prague, Czech Republic. Geochemical approaches (total organic carbon, XRF elemental analyses, XRD mineralogy, 13C and 18O stable isotopes) are combined with grain-size distributions and optically stimulated luminescence (OSL) to assess the climatic conditions at the time of formation of the strata. The oldest loess at this location was formed circa 200 ka ago (MIS 7), making it the oldest loess in Central Bohemia for now. The Bůhzdař loess-paleosol sequence was highly affected by several erosion events, Eemian paleosols (MIS 5e) being completely missing. The partly relocated paleosols situated at the lower part of the Bůhzdař sequence represent a period of increased humidity during late MIS 7 and MIS 6. Despite the fact that the Bůhzdař profile is not particularly thick (5 m), significant changes in paleoclimate reconstructions have been detected.

The northern part of the Chobe Enclave (an administrative district of northern Botswana) is an agricultural area situated between relatively pristine national parks situated in the Middle Kalahari Basin. It belongs to the Linyanti-Chobe structural basin and constitutes a syntectonic depocenter formed within a large structural depression, known as the Okavango Graben, a tectonic structure of a likely trans-tensional nature. The landscape includes fossil landforms, such as sand dunes, pans, sand ridges, and carbonate islands resulting from palaeo-environmental and palaeo-drainage changes through the Quaternary and associated to (neo)tectonic processes. In addition to river- and wind-reworked Kalahari sands, the sediments include diatomites and carbonate deposits, forming inverted reliefs and originating from palustrine palaeo-environments. The Linyanti-Chobe basin is at the convergence of several ecoregions from tropical and subtropical grasslands to savannas and shrubland biomes. The hydrological cycle in the northern Chobe Enclave is governed by a complex interplay between the Okavango, Kwando, and Upper Zambezi drainage basins, which originate from tropical watersheds of the Angolan highlands. Finally, the widespread development of termite mounds impacts the diversity of soils and sediments of the northern Chobe Enclave, which is also reflected in the vegetation.

This review covers more than twenty-five years of research listing and discussing the biogeochemical, mineralogical, and physical impacts of fungus-growing termites (or FGT, Macrotermitinae sub-family) on savanna sediments and landscapes. The main mechanisms by which FGT transform the surficial geological formations in tropical and sub-tropical environments is investigated from a geological perspective and the potential FGT legacies in the sedimentary facies are identified. In order to sustain a twenty million-year symbiosis with the fungus, in which fungi provide digestible food to termites, FGT must optimize the living conditions of the fungus for it to thrive. To do so, they build a biogenic structure maintaining a constant humidity of 80% and a temperature of 30 °C in any kind of environment and all year long. Indeed, FGT adapt to their environment by (i) modifying the grain-size distributions of sediments and soils where they develop, (ii) forming clay horizons below their mounds enabling water to be stored for long period of time in dry environments, (iii) increasing the alkalinity by an order of magnitude of two to three, (iv) mineralizing around 20% of all organic carbon in dry savannas, thus making them the predominant decomposing organisms and crucial actors in the carbon cycle, and (v) concentrating vital nutrients for plants and animals, creating patches of fertile land in sandy semi-deserts. Through their mound-building activities, termites substantially increase the clay fraction compared to the adjacent soil and alter 2:1 clay properties, particularly after the removal of potassium, leading to the formation of smectite layers, demonstrating their biogeochemical effects on silicate mineralogy. Through the binding of aggregates, FGT increase the strength of the mound by a factor of ten and provide exceptional weathering resistance to their mounds. Therefore, termites impact their environment from sub-millimetric transformations to solid voluminous landmarks. The water-holding capacity of a FGT mound leads to an array of positive feedbacks to the savanna landscape by enhancing protection from fires, delaying desertification, supporting rebounds by seedlings and reinforcing dryland resistance and recovery from drought. Termite bioturbation allows sediments to accumulate at a rate averaging 1 mm.y−1.ha−1, and tends to mitigate physical and chemical processes of soil degradation, boosting the heterogeneity at the landscape scale, providing it more resilience. Some of the modifications brought by FGT will remain in the landscape for long periods, testifying to past environmental conditions, and making these mounds potential proxies for paleoenvironmental reconstructions. To conclude, FGT are not only biological actors of the savanna ecosystem, but they act as a geological force by their impact on landscapes as well as by their major role in biogeochemical cycles. Finally, further research is recommended regarding the role of termite's saliva as a binding agent, as well as the age and the evolution of mounds over time.

The largest terrestrial carbon sink on earth is soil carbon stocks. As the climate changes, the rate at which the Earth’s climate warms depends in part on the persistence of soil organic carbon. Microbial turnover forms the backbone of soil organic matter (SOM) formation and it has been recently proposed that SOM molecular complexity is a key driver of stability. Despite this, the links between microbial diversity, chemical complexity and biogeochemical nature of SOM remain missing. Here we tested the hypotheses that distinct microbial communities shape the composition of SOM, and microbial-derived SOM has distinct decomposition potential depending on its community of origin. We inoculated microbial communities of varying diversities into a model soil matrix amended with simple carbon (cellobiose) and measured the thermal stability of the resultant SOM. Using a Rock-Eval® ramped thermal analysis, we found that microbial community composition drives the chemical fingerprint of soil carbon. While diversity was not a driver of SOM composition, bacteria-only communities lead to more thermally labile soil C pools than communities with bacteria and fungi. Our results provide direct evidence for a link between microbial community structure, SOM composition, and thermal stability. This evidence demonstrates the relevance of soil microorganisms in building persistent SOM stocks.

Increasing cropland topsoil organic carbon (SOC) content is a key goal for soil improving quality and adaptating soils to climate change. Moreover, the short term potential of climate mitigation by carbon sequestration is mostly attributed to increasing topsoil SOC content ( Balesdent and Arrouays, 1999 , Chambers et al., 2016 ; Minasny et al., 2017; Balesdent et al., 2018). However, the possibility to increase SOC content is highly disputed in current literature which is mostly based on field experiments. We quantified the on-farm SOC content deficit and SOC content change rate of cropland topsoil (0–20 cm) from western Switzerland using the data bases of Geneva and Vaud cantons containing more than 30,000 topsoil analyses, performed every ten years on every cultivated field of the region since 1993. SOC deficit was estimated as the amount of SOC necessary to reach the 0.1 SOC:clay ratio considered as the minimum required SOC amount for acceptable soil quality. Cropland topsoils of the Vaud and Geneva cantons displayed a 20% and 70% SOC content deficit, respectively. In both cantons, the range of observed rates of change in SOC content from 1993 to present was very large, from −30 to +30‰ per year, with a median value of 0. However, the time trends showed a highly significant linear increase of rates from −5‰ to +6‰ per year on average, in 1995 and 2015, respectively, with no change in SOC content reached by 2005–2007. These trends were attributed to the Swiss agri-environmental schemes applied at the end of 20th century, namely mandatory cover crops and minimum rotations of 4 crops. Further, SOC content increase was accordant with the continuing adoption of minimum tillage, conservation agriculture and multi-species intense cover crops. These findings oppose to those obtained in Swiss long-term experiments, which emphasizes the need to use on-farm information when adressing agriculture policy, climate mitigation or soil quality management issues.

Evaluating the impact and implications of eolian repositories that mark large-scale climatic transitions requires knowledge about the timing of their emplacement and the mechanisms responsible for their production, which remain highly uncertain. Here we apply numerical modeling of cosmogenic nuclide data, measured in the largest continuous terrestrial body of sand on Earth, to determine settings under which the sand was generated, by constraining the timing of sand introduction into the interior of southern Africa. Our findings reveal that major events of sand formation and accumulation in the Kalahari Basin occurred between ~2.2 and 1 Myr ago. The establishment of the Kalahari sand field corresponds to regional, continental, and global scale morphotectonic and climatic changes that contributed to the mass production and widespread dispersion of sand. These changes substantially altered existing habitats, thus constituting a crucial milestone for flora, fauna, and hominins in southern Africa during the Pleistocene.

Aim: To investigate the potential of a large range of soil variables to improve topo-climatic models of plant species distributions in a temperate mountain region encompassing complex relief. Location: The western Swiss Alps. Methods: Fitting topo-climatic models for >60 plant species across >250 sites with and without added soil predictor variables (>30). Testing included: (i) which soil variables improve plant species distribution models; (ii) whether an optimal subset of soil variables can improve models for the majority of species and habitat types; and (iii) how much variation in plant species distributions soil variables alone explain. Results: Geochemical variables (i.e., CaO, pH and inorganic carbon) and a drainage indicator (i.e., bulk soil water content) improved the predictive abilities of the models across the large majority of alpine plant species. The improvement of the models after the addition of soil information varied strongly between plant species and habitat types, but a trade-off was found between the number of soil variables and the associated gain in model performance. Finally, across all species, one specific combination of soil variables–bulk soil water content + total phosphorus + δ13C–outperformed the commonly used topo-climatic variables. Main conclusions: Several soil variables significantly increased the predictive power of plant species distribution models in the temperate mountain region. Geochemical and drainage variables proved most important.

International audience; Provenance determination of archaeological remains is a valuable tool for reconstruction of past exchange networks. Among these materials, oyster shells are ubiquitous in sites from all prehistorical and historical periods. Thus, they seem to be promising candidates for provenance identification as they include chemical elements from the environment in their shells, which implies that an elemental fingerprint of the region of origin can be recorded in the shell composition. In this study, we present elemental measurements from 15 groups of modern and archaeological shells from 13 continental localities in mainland France and the island of Corsica (western Mediterranean Sea). Two of these localities had two oyster species (Crassostrea gigas and Ostrea edulis). Results indicate that (i) a species-specific elemental fingerprint exists and (ii) the Atlantic Ocean and Mediterranean Sea provenances can be identified for O. edulis shells. Moreover, if the shell originated from a locality only partially connected to the ocean (e.g. an estuary or lagoon), a fingerprint specific to the watershed can also be observed, even between groups originating from the same bay. Using these measurements as reference fingerprints, we characterize the Mediterranean origin of two groups of shells unearthed at Lyons (central France, 200 km away from the nearest shoreline), dated from the 1st c. CE.

Monique Fort (Pr. Emerite, Universite de Paris, UMR 8586 PRODIG CNRS) Yvette Dewolf nous a quittes le 5 avril 2021. Nee en 1928, elle debuta sa carriere a l’Universite de Caen dans l’equipe d’Andre Journaux, et elle s’investit tres vite au Centre de Geomorphologie ou elle participa, entre autres avec Jean-Pierre Lautridou et Jean-Pierre Coutard, aux experiences sur les milieux periglaciaires et la cryoclastie. Nommee a Paris en 1965, elle co-dirigea, avec F. Durand-Dastes et sous la Direction...

International audience; Complex sequences of paleosols are often formed by the interaction between pedogenesis and geomorphological evolution. Their study, particularly in mountain areas, is useful to reconstruct past environmental conditions as well as climate shifts, and to gather information on the morphodynamical processes affecting the landscape through time.Since the combined role that all different factors can play in the soil formation and evolution through time and space influences the formation and evolution of those complex paleosol sequences, a multidisciplinary study was conducted at the NW slope of Mt. Cusna (Northern Apennines, Italy). This work aims to reconstruct and to evaluate how the interactions between the geomorphological context, the Holocene climate variations, and the modification of the vegetation cover and composition influence the soil development of this area.A combination of routine soil analyses (i.e., grain-size distributions, total organic carbon, total nitrogen, pH, and Fe/Al extractions), soil micromorphology and the Rock-Eval® pyrolysis allowed to characterize and to correlate the different soil units constituting a toposequence of six soil profiles.The presence of different pedological units that can be correlated along the slope underlines the occurrence of separate events of pedogenesis, spatio-temporally linked to recognizable stability phases at slope scale. These phases of biostasy, characterized by vegetation cover and soil development, alternate to phases of rhexistasy, characterized mainly by slope instability (i.e., aggradation/degradation).In detail, in the Mt. Cusna toposequence three different soil units, linked to three different stability phases, have been identified: the earliest stability phase, characterized by the presence of well-developed Luvisols, the subsequent stability phase typified by less expressed Luvisols, and the ongoing stability phase with Leptosols. This latter pedogenetic phase, in some cases, is superimposed to the previous one, so affecting the exhumed paleosols.In this light, the Mt. Cusna toposequence characterization allowed to enlighten the complexity of soil polygenesis in higher detail than the previous studies, not only reconstructing the past environmental conditions but also inferring the succession of phases of slope stability and phases characterized by erosion and deposition processes.

As stipulated by G. Stoops, “the aim of micropedology is to contribute to solving problems related to the genesis, classification and management of soils, including soil characterization in palaeopedology and archaeology. The interpretation of features observed in thin sections is the most important part of this type of research, based on an objective detailed analysis and description” (Stoops et al. 2018). To answer such questions, two major books contributed to the comparative knowledge necessary to tackle this objective: the first one was published in 1985 and used micromorphology to distinguish between different classes of soils (Douglas and Thompson 1985); the second one is an extensive guide of more than 1000 pages to the interpretation of micromorphological features encountered in thin sections of soil (Stoops et al. 2018). The aim of this Atlas is neither to be a substitution for these books nor a way to enter directly into the interpretation of soil genesis and classification. Nonetheless, this chapter presents the imprints of major soil processes that can be easily deduced from specific features observed in thin sections. These processes involve the dynamics of (a) clay, both translocation and swelling, (b) water, such as waterlogging, evaporation, and its role as ice and frost, (c) carbonate, gypsum, and iron oxyhydroxides, and finally (d) biogeochemical reactions within the solum.

Mineral and organic constituents belong to the basic components observed in soil thin sections. They can appear, for instance, as large rock fragments, or single minerals as sand grains; they can constitute large areas of micromass formed by clay minerals or display parts of plant roots or leaf fragments, i.e. organic material. These constituents comprise the body of the soil itself, and in soil micromorphology, they belong to the groundmass, as well as the material constituting the pedofeatures (see “10.1007/978-3-030-67806-7_1#Sec9”). Two types of basic components are recognized by Stoops (2003, 2021), those recognizable at the magnifications of the optical microscope and those which are not. Stoops (2003, 2021) pointed out the problem of the optical microscope resolution and the thickness of conventional thin sections. Indeed, it is preferable not to have a standard size limit between coarse and fine materials.

From a historical point of view, soil micromorphology was first used in order to decipher the expressions of pedogenic processes at the microscale (Kubiëna 1938). In the preceding chapters, the Atlas listed a series of descriptive tools to help with the identification of objects. This chapter deals with specific pedofeatures encountered in a large diversity of soils and directly related to pedogenic processes. Pedological features (Brewer 1964) or pedofeatures (Bullock et al. 1985) are “discrete fabric units present in soil materials that are recognizable from an adjacent material by a difference in concentration in one or more components or by a difference in internal fabric” (Stoops 2003, 2021). In Stoops (2003, 2021), pedofeatures are subdivided into two categories: matrix pedofeatures and intrusive pedofeatures. Matrix pedofeatures can be subdivided according to their relationship with the groundmass (depletion, impregnative, and fabric pedofeatures) and to their morphology (hypocoatings, quasicoatings, matrix infilling, intercalation, and matrix nodules). Regarding the intrusive pedofeatures, they include coatings, infillings, crystals and crystal intergrowth, intercalations, and finally nodules. The proposed nomenclature of this chapter is based on the nature and morphology of the pedofeatures, simplified from Bullock et al. (1985).

Summary This study presents the evolution of organic matter content using Rock-Eval® pyrolysis in Beech forest soils after their particle Size-fractionation. The amount of C in soil depends in microbial and physicochemical interactions between various biomass affecting its accumulation and its stability. As far as thermal stability of soil OM is supposed to mimic its biogeochemical stability, Rock-Eval® pyrolysis method is an efficient tool to quantify the organic and mineral carbon. To this end, litter and topsoil samples from a beech forest located in Normandy (France) were selected and fractionated by sieving from large debris to A-fine fractions. The TOC decreases from large debris to A-medium and drops in A-fine. Thermal stability inputs are highlighted through RE parameters when comparing litters and particle size fractions separated from topsoil samples (increase of TpS2 and OI, decrease of HI with decreasing fraction size in A horizons). Three distinct thermal stability C pools are identified

This open access atlas is an up-to-date visual resource on the features and structures observed in soil thin sections, i.e. soil micromorphology. The book addresses the growing interest in soil micromorphology in the fields of soil science, earth science, archaeology and forensic science, and serves as a reference tool for researchers and students for fast learning and intuitive feature and structure recognition. The book is divided into six parts and contains hundreds of images and photomicrographs. Part one is devoted to the way to sample properly soils, the method of preparation of thin sections, the main tool of soil micromorphology (the microscope), and the approach of soil micromorphology as a scientific method. Part two focuses on the organisation of soil fragments and presents the concept of fabric. Part three addresses the basic components, e.g. rocks, minerals, organic compounds and anthropogenic features. Part four lists all the various types of pedogenic features observed in a soil, i.e. the imprint of pedogenesis. Part five gives interpretations of features associated with the main processes at work in soils and paleosols. Part six presents a view of what the future of soil micromorphology could be. Finally, the last part consists of the index and annexes, including the list of mineral formulas. This atlas will be of interest to researchers, academics, and students, who will find it a convenient tool for the self-teaching of soil micromorphology by using comparative photographs.

The advancement of technology opens up new opportunities to soil micromorphology. Although a description using an optical microscope of the fabric and the various constituents of soils will be always necessary to investigate soil evolution, the uncovered thin section leaves soil material on which analyses can be performed. Since the 1970s, it was possible to observe thin sections at high resolution with the scanning electron microscope in its backscattered electron mode (see “10.1007/978-3-030-67806-7_1#Sec7”). It was also possible to generate chemical images with electron microprobes. But these conventional techniques, as well as new ones, greatly improve the study of matter interactions in soils, not only by enhancing the spatial resolution with incredible precision but also by providing chemical and mineralogical images, which substantially increased the accuracy of micromorphological diagnostics. By coupling morphological and chemical approaches, including stable isotope imaging in soil material, the future of soil micromorphology will undoubtedly offer new opportunities to solve specific problems, especially in the field of organomineral interactions in soils. It is wise to say that soil micromorphology, with its analytical and holistic approaches, will make it possible to build the necessary solid foundations needed for investigations that are increasingly oriented towards nanoscale objects: it will remind us that the trees should not hide the forest.

As emphasized by W. Kubiëna, “… there exists no other method capable of revealing the nature and complexity of soil polygenesis in so much detail as thin-section micromorphology and at the same time enabling one to follow and explain its formation...”. This sentence, cited by Fedoroff (1971), highlights the aim of soil micromorphology: looking at a soil from the inside and at various scales, from the optical microscope to synchrotron imaging. Soils constitute multiscalar objects by definition, from their soilscape (at the landscape scale), to their profile and its horizons to the atomic interactions between the smallest minerals and organic molecules. Micromorphology enters the soil investigations at the multi-centimetre scale (see “File 3”) at which the thin section is made.

Kubiëna (1938) was the first to introduce the concept of fabric in soil micromorphology, so this term has been used in soil micromorphology for a long time. The term “fabric” was initially applied to rocks by geologists and petrologists. This type of fabric is defined as the “factor of the texture of a crystalline rock which depends on the relative sizes, the shapes, and the arrangement of the component crystals” (Matthews and Boyer 1976). This definition has been adapted for soil micromorphology and its latest definition has been given by Bullock et al. (1985) as: “soil fabric deals with the total organization of a soil, expressed by the spatial arrangement of the soil constituents (solid, liquid, and gaseous), their shape, size, and frequency, considered from a configurational, functional and genetic view-point”. In conclusion, the soil micromorphologist should consider the fabric as an arrangement and∕or organization of soil constituents.

Carbonate platform islands are important landscape features in the northeastern part of the Okavango Delta region of Botswana, the Chobe Enclave. However, the formation processes and the timing of these “Chobe Islands” remain unclear. They are assumed to be the result of late Quaternary hydrological changes. Records of such changes are poorly preserved, though the occurrence of beach ridges in the Middle Kalahari Basin attests to the existence of large paleo-lakes in the past. Carbonate rocks from the Chobe Island appear to be relics of palustrine environments, but their relationship with the other hydrological archives is still unclear. Here, we report optically stimulated luminescence dating of key sedimentary beds in and around a single Chobe Island. It was necessary to model dose rate evolution for each sample individually, taking into account post-depositional changes in the sediment chemistry and its burial depth. The resulting ages suggest that the dated units were deposited between MIS6 to MIS1. The carbonate platform itself appears to have been deposited in two phases, separated by either an unexpectedly long (∼40 ka) depositional hiatus or an episode of erosion. This study demonstrates the potential of using luminescence dating in such settings, and offers the possibility of linking sedimentary processes within the Chobe Enclave to regional paleo-hydrological records.

Soil research in temperate to cool and humid regions has typically focused on acidic soils; there has been relatively little investigation of the effects of calcium carbonate (CaCO3) on unamended soil properties or function in these environments. The object of this study was to characterise the effects of small amounts of CaCO3 on pedogenic trajectories and soil biogeochemistry in a humid subalpine valley of Switzerland. To isolate the influence of CaCO3, six profiles were selected that had developed under almost identical conditions for soil formation, i.e. climate, topography, vegetation structure, time since deglaciation, silicate mineralogy and texture. The main difference between the profiles was that three contained a small quantity of CaCO3 (

Fungus-growing termites have long been considered as ecosystem engineers for the modifications they make to the soil, through their ability to concentrate nutrients and create patches of fertile land. However, few studies have highlighted the degree to which, in building their mounds, they are able to modify the grain-size distributions of the sediments and soils in their surroundings. Therefore, the aim of this study is to document the potential impact of fungus-growing termites on sediment sorting of their environment. The assessment is based on comparisons between two different mounds and their associated control soils developed in contrasting grain-size settings, one on sand and the other on fine material (diatomite). The sedimentary modifications carried out by termites between these two parent materials and associated constructed mounds are addressed using techniques mostly based on grain-size distributions (performed with End-Member Mixing Analyses) and soil micromorphology. In order to conduct this investigation, two fungus-growing termites’ mounds were selected in the Chobe Enclave District, northern Botswana. The key questions of this study center on the capacity of fungus-growing termites to (i) adapt to any kind of parent material to build their mounds, and (ii) enrich or deplete this parent soil to meet their texture requirements in terms of mound stability and other mechanical properties to insure the success of the colony. This study demonstrated that fungus-growing termites assemble the mandatory texture required for the functions and properties of their mounds, whatever a given parent material, by selecting, transporting, and mixing the various grain sizes. To conclude, the sorting by fungus-growing termites must be considered when reconstructing paleoenvironment based on particle-size distributions.

Explanatory studies suggest that using very high resolution (VHR, 1–5 m resolution) topo-climatic predictors may improve the predictive power of plant species distribution models (SDMs). However, the use of VHR topo-climatic data alone was recently shown not to significantly improve SDM predictions. This suggests that new ecologically-meaningful VHR variables based on more direct field measurements are needed, especially since non topo-climatic variables, such as soil parameters, are important for plants. In this study, we investigated the effects of adding mapped VHR predictors at a 5 m resolution, including field measurements of temperature, carbon isotope composition of soil organic matter (δ13CSOM values) and soil pH, to topo-climatic predictors in SDMs for the Swiss Alps. We used data from field temperature loggers to construct temperature maps, and we modelled the geographic variation in δ13CSOM and soil pH values. We then tested the effect of adding these VHR mapped variables as predictors into 154 plant SDMs and assessed the improvement in spatial predictions across the study area. Our results demonstrate that the use of VHR predictors based on more proximal field measurements, particularly soil parameters, can significantly increase the predictive power of models. Predicted soil pH was the second most important predictor after temperature, and predicted δ13CSOM was fourth. The greatest increase in model performance was for species found at high elevation (i.e. 1500–2000 m a.s.l.). Addition of predicted soil factors thus allowed better capturing of plant requirements in our models, showing that these can explain species distributions in ways complementary to topo-climatic variables. Modelling techniques to generalize edaphic information in space and then predict plant species distributions revealed a great potential in complex landscapes such as the mountain region considered in this study.

This special issue follows the 20th INQUA congress held in Dublin in July 2019, where we organized with the help of Julie Dabkowski a session on “Multiproxy studies on continental carbonates : palaeoclimates and palaeoenvironments”. Continental carbonates have been extensively studied during the last few decades as supports for multidisciplinary palaeoclimatic and palaeoenvironmental researches. Most efforts have been concentrated on speleothems, but investigations on tufa, travertine, lacust...

Rare earth elements (REEs) and yttrium in seawater originate from atmospheric fallout, continental weathering, and transport from rivers, as well as hydrothermal activity. Previous studies have reported the use of REE and Y measurements in biogenic carbonates as a means to reconstruct these surface processes in ancient times. As coastal seawater REE and Y concentrations partially reflect those of nearby rivers, it may be possible to obtain a regional fingerprint of these concentrations from bivalve shells for seafood traceability and environmental monitoring studies. Here, we present a dataset of 297 measurements of REE and Y abundances by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from two species (Crassostrea gigas and Ostrea edulis). We measured a total of 49 oyster specimens from six locations in France (Atlantic Ocean and Mediterranean Sea). Our study reports that there is no significant difference in concentrations from shell parts corresponding to winter and summer periods for both species. Moreover, interspecific vital effects are reported from specimens from both species and from the same locality. REE and Y profiles as well as t-distributed stochastic neighbour embedding processing (t-SNE; a discriminant statistical method) indicate that REE and Y measurements from C. gigas shells can be discriminated from one locality to another, but this is not the case for O. edulis, which presents very similar concentrations in all studied localities. Therefore, provenance studies using bivalve shells based on REEs and Y have to first be tested for the species. Other methods have to be investigated to be able to find the provenance of some species, such as O. edulis.

The study area of the Chobe Enclave (northern Botswana) is defined as mostly covered by Arenosols in available maps. However, recent explorations of the area showed that soils are more diverse than expected. This is because of complex interactions between current alluvial deposition processes, paleo-environmental effects (ancient alluvial deposition, ancient wind-blown sand deposits) and ongoing hydrological effects and colluvial effects on topographic gradients. An in-depth exploration of both soils and vegetation in the area was conducted with the aim (i) to survey the soil diversity at the Chobe Enclave, (ii) to study soil dynamics and identify the key factors of this diversity, and (iii) to create a soil map based on the analysis of the soil-vegetation relationship. For this purpose, thirty-six soil profiles were extensively described according to the World Reference Base for soil resources. In order to better classify these soils, physicochemical parameters, such as pHH2O, exchangeable cations, and particle size distributions, were measured for a specific set of soils (n = 16), representative of their diversity. To assess Soil Organic Matter (SOM) dynamics, samples were studied using Rock Eval pyrolysis. Results show a high soil diversity and heterogeneity with the presence of (i) Arenosols, as expected, but also of (ii) organic-rich soils, such as Chernozems, Phaeozems, and Kastanozems, (iii) salty/sodic soils, such as Solonchaks and Solonetz, and finally (iv) calcium-rich soils, such as Calcisols. Analyses of the different actors driving the soil diversity emphasized the importance of the surficial geology, composed of different sand deposits (red sands/white sands), carbonate and diatomite beds, as well as ancient salt deposits, in which high proportions of exchangeable Na+ were found, associated with high pHH2O (up to 11.3). In addition, as a parameter, the topography creates a complex hydrological system in the Chobe Enclave and therefore, induces a notable soil moisture gradient. Moreover, this study stressed the key role of termites: not only do they modify physicochemical patterns of soils, but they also decay and incorporate large quantities of fresh plant materials into soils. Finally, the analysis of Organic Matter (OM) showed that the Soil Organic Carbon (SOC) is composed essentially by recalcitrant Organic Carbon (OC) substances, such as charcoal, a common carbon type of tropical soils.

Aims The aims of this study were: 1) to test a new methodology to overcome the issue of the predictive capacity of soil water availability in geographic space due to measurement scarcity, 2) to model and generalize soil water availability spatially to a whole region, and 3) to test its predictive capacity in plant SDMs. Methods First, we modelled the measured Soil Water Holding Capacity (SWHC at different pFs) of 24 soils in a focal research area, using a weighted ensemble of small bivariate models (ESM). We then used these models to predict 256 locations of a larger region and used the differences in these pF predictions to calculate three different indices of soil water availability for plants (SWAP. These SWAP variables were added one by one to a set of conventional topo-climatic predictors to model 104 plant species distributions. Results We showed that adding SWAP to the SDMs could improve our ability to predict plant species distributions, and more specifically, pF1.8–pF4.2 became the third most important predictor across all plant models. Conclusions Soil water availability can contribute a significant increase in the predictive power of plant distribution models, by identifying important additional abiotic information to describe plant ecological niches.

The oxalate-carbonate pathway (OCP) is a biogeochemical process linking oxalate oxidation and carbonate precipitation. Currently, this pathway is described as a tripartite association involving oxalogenic plants, oxalogenic fungi, and oxalotrophic bacteria. While the OCP has recently received increasing interest given its potential for capturing carbon in soils, there are still many unknowns, especially regarding the taxonomic and functional diversity of the fungi involved in this pathway. To fill this gap, we described an active OCP site in Madagascar, under the influence of the oxalogenic tree Tamarindus indica, and isolated, identified, and characterized 50 fungal strains from the leaf litter. The fungal diversity encompassed three phyla, namely Mucoromycota, Ascomycota, and Basidiomycota, and 23 genera. Using various media, we further investigated their functional potential. Most of the fungal strains produced siderophores and presented proteolytic activities. The majority were also able to decompose cellulose and xylan, but only a few were able to solubilize inorganic phosphate. Regarding oxalate metabolism, several strains were able to produce calcium oxalate crystals while others decomposed calcium oxalate. These results challenge the current view of the OCP by indicating that fungi are both oxalate producers and degraders. Moreover, they strengthen the importance of the role of fungi in C, N, Ca, and Fe cycles.

Nāri-calcrete is a calcareous formation common in the semiarid Mediterranean region of the Levant. The profile studied in Nazareth (Galilee) is 2.75 m thick and divided into the four conventional layers, i.e. a chalky bedrock, a powdery lower nāri, an indurated and platy upper nāri, and a laminar crust capping the profile. In this study, mercury intrusion porosimetry (MIP) is used to assess the genetic processes at work in the formation of nāri-calcrete. Based on the former work by Yaalon and Singer (1974) and MIP results, various parameters are proposed to describe the different layers: total porosity (N P ), trapped porosity (N T ), permeability (K), and strength (σ c ). These parameters are compared to each other for each layer, as well as with their respective calcium carbonate contents. An unexpected relationship shows that the more CaCO 3 is present, the more porous the layer of the nāri. Consequently, the lower nāri is interpreted as a weathering layer of the parent chalk, whereas the upper nāri, which is the most porous and the richest in CaCO 3 , results from in situ recrystallization. The upper nāri permeability precludes it from playing the role of a “plugged horizon” to explain the formation of the laminar crust. In conclusion, the nāri-calcrete in Nazareth questions the conventional model of calcrete genesis as its cementation is due to in situ redistribution of CaCO 3 rather than its translocation. These results strengthen Yaalon and Singer (1974)'s conclusions.

Non-anthropogenic earth mounds, defined as mima-like mounds in this study, have recently been observed in non-carbonate watersheds along the Sudano-Sahelian belt in the Chad Basin. In the Diamare piedmont (northern Cameroon) they are particularly well developed within stream networks. In less eroded areas, they occur as whaleback, flattened morphologies, or even as buried features. All these shapes are composed of clay-rich sediment associated with high proportions of secondary carbonate nodules and Fe Mn micro-nodules. Their soil structure is prismatic to massive and vertical cracks are observed locally. Grain-size distributions emphasize the clay-rich nature of the sediment, with average clay contents of 32% ± 12.8% (n = 186), which is significantly higher than the clay content in the adjacent sediments in the landscape (mean = 10% ± 4%, n = 21). Moreover, high proportions of smectite characterize the soil, with average contents of 34 ± 7% (n = 25). At the micro-scale, the groundmass has a cross-striated b-fabric, with embedded smooth subangular quartz and feldspar grains of the silt-size fraction. All the characteristics point to altered vertic properties in the clay-rich sediment composing the mima-like mounds. Mima-like mounds are thus interpreted as degraded Vertisols. Compared to present-day Vertisols occurring in the piedmont, mima-like mounds are located upstream. It is thus proposed that the Vertisol areas were more extensive during a former and wetter period than the present-day. Subsequent changing climatic conditions increased erosion, revealing the gilgai micro-relief by preferential erosion in micro-lows rather than in micro-highs. Mima-like mounds of the Chad Basin might thus result from pedogenesis combined with later erosion. These local processes can be inherited from regional climatic variations during the Late Pleistocene-Holocene and likely be related to the African Humid Period.

Soil is one of the most complex systems on Earth, functioning at the interface between the lithosphere, biosphere, hydrosphere, and atmosphere and generating a multitude of functions. Moreover, soil constitutes the belowground environment from which plants capture water and nutrients. Despite their great importance, soil properties are often not sufficiently considered in other disciplines, especially in spatial studies of plant distributions. Most soil properties are available as point data and, to be used in spatial analyses, need to be generalised over entire regions (i.e. digital soil mapping). Three categories of statistical approaches can be used for such purpose: geostatistical approaches (GSA), predictive-statistical approaches (PSA), and hybrid approaches (HA) that combine the two previous ones. How then to choose the best approach in a given soil study context? Does it depend on the soil properties to be spatialized, the study area's characteristics, and/or the availability of soil data? The main aims of this study was to review the use of these three approaches to derive maps of soil properties in relation to the soil parameters, the study area characteristics, and the number of soil samples. We evidenced that the approaches that tend to show the best performance for spatializing soil properties were not necessarily the ones most used in practice. Although PSA was the most widely used, it tended to be outperformed by HA in many cases, but the latter was far less used. However, as the study settings were not always properly described and not all situations were represented in the set of papers analysed, more comparative studies would be needed across a wider range of regions, soil properties, and spatial scales to provide robust conclusions on the best spatialization methods in a specific context.

(IF 3.12; Q2); International audience; Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition.

Climate and environmental changes since the Last Glacial Maximum in the tropical zone of West Africa are usually inferred from marine and continental records. In this study, the potential of carbonate pedo-sedimentary geosystems, i.e. Vertisol relics, to record paleoenvironmental changes in the southwestern part of Chad Basin are investigated. A multi-dating approach was applied on different pedogenic organo-mineral constituents. Optically stimulated luminescence (OSL) dating was performed on the soil K-rich feldspars and was combined with radiocarbon dating on both the inorganic (C-14(inorg)) and organic carbon (C-14(org)) soil fractions. Three main pedo-sedimentary processes were assessed over the last 20 ka BP: 1) the soil parent material deposition, from 18 ka to 12 ka BP (OSL), 2) the soil organic matter integration, from 11 cal ka to 8 cal ka BP (C-14(org)), and 3) the pedogenic carbonate nodule precipitation, from 7 cal ka to 5 cal ka BP (C-14(inorg)). These processes correlate well with the Chad Basin stratigraphy and West African records and are shown to be related to significant changes in the soil water balance responding to the evolution of continental hydrology during the Late Quaternary. The last phase affecting the Vertisol relics is the increase of erosion, which is hypothesized to be due to a decrease of the vegetation cover triggered by (i) the onset of drier conditions, possibly strengthened by (ii) anthropogenic pressure. Archaeological data from Far North Cameroon and northern Nigeria, as well as sedimentation times in Lake Tilla (northeastern Nigeria), were used to test these relationships. The increase of erosion is suggested to possibly occur between c. 3 cal ka and 1 cal ka BP. Finally, satellite images revealed similar geosystems all along the Sudano-Sahelian belt, and initial C-14(inorg) ages of the samples collected in four sites gave similar ages to those reported in this study. Consequently, the carbonate pedo-sedimentary geosystems are valuable continental paleoenvironmental archives and soil water balance proxies of the semiarid tropics of West Africa. (C) 2018 Elsevier Ltd. All rights reserved.

In this study we investigated how the source of organic carbon (Corg) and nitrogen (Norg) affects the interactions between fungi of the genus Morchella and bacteria dispersing along their hyphae (fungal highways; FH). We demonstrated that bacteria using FH increase the hydrolysis of an organic nitrogen source that only the fungus can degrade. Using purified fungal exudates, we found that this increased hydrolysis was due to bacteria enhancing the activity of proteolytic enzymes produced by the fungus. The same effect was shown for various fungal and bacterial strains. The effect of this enhanced proteolytic activity on bacterial and fungal biomass production varied accordingly to the source of Corg and Norg provided. An increase in biomass for both partners 5 days post-inoculation was only attained with a Norg source that the bacterium could not degrade and when additional Corg was present in the medium. In contrast, all other combinations yielded a decrease on biomass production in the co-cultures compared to individual growth. The coupled cycling of Corg and Norg is rarely considered when investigating the role of microbial activity on soil functioning. Our results show that cycling of these two elements can be related through cross-chemical reactions in independent, albeit interacting microbes. In this way, the composition of organic material could greatly alter nutrient turnover due to its effect on the outcome of interactions between fungi and bacteria that disperse on their mycelia.

In the field of microbial biomineralization, much of the scientific attention is focused on processes carried out by prokaryotes, in particular bacteria, even though fungi are also known to be involved in biogeochemical cycles in numerous ways. They are traditionally recognized as key players in organic matter recycling, as nutrient suppliers via mineral weathering, as well as large producers of organic acids such as oxalic acid for instance, an activity leading to the genesis of various metal complexes such as metal-oxalate. Their implications in the transformation of various mineral and metallic compounds has been widely acknowledged during the last decade, however, currently, their contribution to the genesis of a common biomineral, calcite, needs to be more thoroughly documented. Calcite is observed in many ecosystems and plays an essential role in the biogeochemical cycles of both carbon (C) and calcium (Ca). It may be physicochemical or biogenic in origin and numerous organisms have been recognized to control or induce its biomineralization. While fungi have often been suspected of being involved in this process in terrestrial environments, only scarce information supports this hypothesis in natural settings. As a result, calcite biomineralization by microbes is still largely attributed to bacteria at present. However, in some terrestrial environments there are particular calcitic habits that have been described as being fungal in origin. In addition to this, several studies dealing with axenic cultures of fungi have demonstrated the ability of fungi to produce calcite. Examples of fungal biomineralization range from induced to organomineralization processes. More examples of calcite biomineralization related to direct fungal activity, or at least to their presence, have been described within the last decade. However, the peculiar mechanisms leading to calcite biomineralization by fungi remain incompletely understood and more research is necessary, posing new exciting questions linked to microbial biomineralization processes.

Pedogenic carbonate nodules have been observed in the Far North region of Cameroon. They have been interpreted as Vertisol relic components inherited from the African Humid Period (AHP), however their processes of formation remain poorly constrained. Here we explore the possibility of targeting the minerals trapped within such nodules, which represent a residual soil fraction, to date the deposition time of the soil parent material using optically stimulated luminescence (OSL). As the trapped minerals have a complex history, related to pedogenesis and sedimentary processes since their deposition, it was necessary to identify a suitable mineral population. To achieve this, a nodule collected at the surface was investigated in order to compare the OSL signals from feldspar minerals from within its external and internal parts. The resultant luminescence data were then compared with minerals trapped in a buried nodule, with the scatter in OSL signals between the different populations indicating potential partial bleaching of grains and/or pedoturbation processes. Our results show that the minerals from the internal part of the nodule are appropriate for OSL dating. Furthermore, the environmental dose rate has changed through time following carbonate precipitation and the radionuclide decay series may be in disequilibrium. Because of its solubility, the 238U-series is particularly affected, and when a complete absence of 234U daughters is assumed, our calculations show a maximum and significant decrease of 20% in the environmental dose rate compared to secular equilibrium. Finally, changing the time of carbonate nodule precipitation results in a maximum OSL age range of 12 ka (from 15 to 27 ka) for the deposition of the soil parent material. Despite this relatively high uncertainty, these ages suggest that the deposition occurred before the AHP. Consequently, if their formation time is well constrained from independent methods, the pedogenic carbonate nodules can be exploited as valuable time archives, providing temporal information on soil parent material deposition and thus a maximum age for the onset of pedogenesis.