Organo-mineral Complexes Research Articles

Sorption of Phenanthrene by Dissolved Organic Matters and their Complex with Iron Oxide Nanoparticles

The environmental behavior of polycyclic aromatic hydrocarbons (PAHs) has attracted great research attention. However, their sorption mechanisms in organo-mineral complexes are still not specific enough. Batch sorption experiments with humic acid (HA) were carried out with iron oxide and their complexes adsorption phenanthrene (PHE) to determine the adsorption characteristics and mechanisms in this study. These results showed that the weakly polar and aromatic fractions of DOM preferentially adsorbed on the surface of iron oxide minerals. The organic matter (OM) binding on the surface of iron oxide played an important role for the adsorption of PHE, where van der Waals interactions and hydrophobic bonding, with ligand exchange at an electrified interface being the dominant reaction of HA adsorption to mineral. The sorption of PHE on organo-mineral complexes was controlled by π-π interactions and hydrophobic interactions.

Carbon sequestration in clay and silt fractions of Brazilian soils under conventional and no-tillage systems

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension (<53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.

Reconstitution of cutin monomers on smectite surfaces: adsorption and esterification

This paper focuses on the physico-chemical interactions of cutin monomers with clay smectites. Adsorption of cutin monomers was measured, and esterification and reconstitution were studied using Fourier transformed infrared spectrometry (FTIR), X-ray diffraction (XRD) and mass spectrometry (MS) analyses. Adsorption of a common cutin monomer, ω-dihydroxypalmitic acid (diHPA) to smectite was found to be affected by its interactions with exchangeable cations. Inner-sphere complexation was observed for Fe3+-montmorillonite, and nontronite. Outer-sphere complexes were observed for Ca2+- and Wyoming Na+-montmorillonite. XRD measurements showed that cutin monomers were intercalated between smectite layers, and MS results demonstrated their enhanced esterification on the smectite surfaces (as compared to monomer solution). This study suggests that cutin monomers can be reconstituted and esterified on the smectite surfaces to form stable organo-mineral complexes.

Disputable issues in interpreting the results of chemical extraction of iron compounds from soils

In Russia, iron is chemically fractionated according to a parallel scheme. Pyrophosphate-soluble iron (Fepyr) is considered to participate in organomineral complexes, oxalate-soluble iron (Feox) is believed to enter amorphous + poorly crystallized compounds, and dithionite-soluble iron (Fedit) is meant to represent the free (nonsilicate) compounds. However, the investigations prove that the commonly used subtraction operations (Feox − Fepyr) and (Fedit − Feox) are invalid because of the nonadditive action of the reagents in the parallel scheme of extraction. The low selectivity of reagents requires a new interpretation of chemically extracted iron compounds. In automorphic soils, the content of oxalate-soluble iron should be interpreted as the amount of Fe(III) capable of complexing with organic ligands; in hydromorphic soils with a stagnant moisture regime, it should be interpreted as the amount of iron (III) capable of being reduced in a short time. The content of dithionite-soluble compounds should be regarded as the amount of iron (III) within both (hydr)oxides and silicates potentially prone to reduction.

Assessing the contributions of sesquioxides and soil organic matter to aggregation in an Ultisol under long-term fertilization

In tropical and subtropical soils, sesquioxides and soil organic matter (SOM) are major binding agents for aggregates. However, the biotic and abiotic contributions to aggregation are often difficult to distinguish. In this study, we attempted to assess their contributions to aggregation separately, as indicated by aggregate size distribution and specific surface area (SSA). Our objectives were (i) to determine aggregate size distribution and SSA before and after removal of sesquioxides and SOM, and (ii) to assess the contributions of sesquioxides and SOM to soil aggregation. An oxide-rich Ultisol under long-term fertilization was extracted by water as a control, oxalate, dithionite–citrate–bicarbonate (DCB), or by H2O2 in the absence of any physical disturbance. The aggregate size distribution, Fe/Al oxides, soil organic C (SOC), and SSA of the soil before and after extraction were determined. Our results showed that the DCB and oxalate solutions broke down the sand-sized aggregates most intensively, whereas the H2O2 treatment disrupted 0.25–2.0mm aggregates intensively, indicating that SOM is the major binding agent for aggregates of this size. A slight change either in SOC stock after removal of Fe/Al oxides by DCB and oxalate or in Fe/Al oxides after removal of SOC by H2O2 indicated that organo-mineral complexes are a minor binding mechanism of aggregation in the soil studied. The SSA was reduced by 72–84% in the soil extracted by DCB, followed by 32.0–35.9% after the oxalate extraction, whereas the removal of SOM increased SSA by 3.8–12.6%. Our results showed that Fe/Al oxides played a major role in aggregation in the Ultisols studied. The difference in the major binding agent for different aggregate size classes is another reason to explain why the hierarchy aggregate concept is not applicable to oxide-rich soils. This study, however, could not assess their contributions to soil aggregation precisely, because of the difficulty in tracing aggregate dynamics. To better understand the mechanisms of soil aggregation we need more works in the future.

Changes in the agrochemical properties of dark gray soil in the Western Ukrainian forest-steppe under the effect of long-term agricultural use

The changes in the properties of dark gray forest soil in the Ukrainian Western Forest-Steppe under the effect of long-term agricultural use in a grain-row crop rotation without fertilization and with the application of mineral and organomineral complex fertilizers have been studied. The changes in the morphological properties of the soil, the reaction of the soil solution, the total and exchangeable acidities, the total exchangeable bases, the degree of base saturation, and the content and reserve of organic carbon over a 50-year-long period of plowing have been studied using different methods. It has been found that the acidification of the upper layer was intensified and the content of organic matter and the degree of base saturation decreased during the period studied (1961–2010). The effect of the management practice on the evolution and dynamics of the soil degradation has been studied. It has been shown that the cultivation of soils without fertilization primarily resulted in a decrease in the humus content; the application of mineral fertilizers increased the acidity of the soils.

Speciation of Cu and Zn in soil solution in a long-term fertilization experiment

Abstract The changes of the concentration of Cu and Zn in the soil solution and the percentage of particular forms of these elements in the soil solution were investigated in the long-term fertilization experiment. The soil solution was obtained following the vacuum displacement method. Speciation of copper and zinc ions was determined with MINTEQA2 for Windows software. The results of the investigation indicated that exclusive mineral fertilization (NPK) caused an increase of Cu and Zn concentration in the soil solution. Organic fertilization (FYM) resulted in a decrease of Cu and an increase of Zn concentration in the soil solution. Liming limited mobility of both analysed elements. The results of speciation analysis showed that regardless of the fertilization mode, the organo-mineral complexes are the main form of Cu occurring in soil solution. The percentage of Cu-DOC complexes ranges from 76.5 to 85.2% of the total concentration of Cu in the soil solution. The particular forms of copper can be sorted depending on the percentage in the soil solution as follows: Cu-DOC&gt;Cu2+&gt;Cu-CO3. The main form of Zn in the soil solution are active Zn2+ ions. The share of Zn2+ in total zinc concentration in the soil solution ranged from 76.9% to 86.4%. Forms of zinc in the soil solution can be arranged with regard to their percentage as follows: Zn2+&gt;Zn-DOC&gt;ZnCl+&gt;ZnHCO3+.

Pyrolysis Temperature-Dependent Changes in Dissolved Phosphorus Speciation of Plant and Manure Biochars

Pyrolysis of plant and animal wastes produces a complex mixture of phosphorus species in amorphous, semicrystalline, and crystalline inorganic phases, organic (char) components, and within organo-mineral complexes. To understand the solubility of different phosphorus species, plant (cottonseed hull) and manure (broiler litter) wastes were pyrolyzed at 350, 500, 650, and 800 °C and exposed to increasingly more rigorous extraction procedures: water (16 h), Mehlich 3 (1 mM EDTA at pH 2.5 for 5 min), oxalate (200 mM oxalate at pH 3.5 for 4 h), NaOH-EDTA (250 mM NaOH + 5 mM EDTA for 16 h), and total by microwave digestion (concentrated HNO3/HCl + 30% H2O2). Relative to the total (microwave digestible) P, the percentage of extractable P increased in the following order: M3 < oxalate ≈ water < NaOH-EDTA for plant biochars and water < M3 < NaOH-EDTA < oxalate for manure biochars. Solution phase (31)P NMR analysis of NaOH-EDTA extracts showed the conversion of phytate to inorganic P by pyrolysis of manure and plant wastes at 350 °C. Inorganic orthophosphate (PO4(3-)) became the sole species of ≥ 500 °C manure biochars, whereas pyrophosphate (P2O7(4-)) persisted in plant biochars up to 650 °C. These observations suggested the predominance of (i) amorphous (rather than crystalline) calcium phosphate in manure biochars, especially at ≥ 650 °C, and (ii) strongly complexed pyrophosphate in plant biochars (especially at 350-500 °C). Correlation (Pearson's) was observed (i) between electric conductivity and ash content of biochars with the amount of inorganic P species and (ii) between total organic carbon and volatile matter contents with the organic P species.

Pedogenic manganese oxide coatings (calcium buserite) on fracture surfaces in Tortonian (Upper Miocene) red mudstones, southern Turkey

Fracture surfaces of red mudstones (overbank deposits within the Kuzgun Formation of Late Miocene-Tortonian — age) are coated by thin black colored manganese oxide beneath a thin soil cover in the Mersin area of southern Turkey. These manganese oxide coatings have been investigated by a range of analytical techniques, including X-ray diffraction (XRD), infrared-spectral (IR), differential thermal analysis–thermal gravimetric (DTA–TG), scanning electron microscopy and energy-dispersive X-ray (SEM–EDX) analyses. The basal reflections at 10.00, 7.22, 5.00, 3.35, 2.50, 2.29 and 2.10Å, sharp infrared spectral lines at 469 and 519cm−1 and typical dehydration of interlayer water<200°C, together with results from the EDX analysis, indicate the presence of calcium buserite — a manganese oxide mineral. SEM images of buserite crystals show degraded platy pseudo-hexagonal forms suggesting a non-biological origin. Pedogenic processes taking place at the soil material overlying red mudstones, likely induced the reducing or acidic conditions under which Mn2+ was released by the dissolution of Mn-bearing minerals during the wet periods. Organic compounds were also most likely leached during these periods and subsequently, during the dry climatic intervals, manganese oxide minerals precipitated from the infiltrating soil solutions under oxidizing conditions. The nanometer-scale layering in manganese oxide coatings is here attributed to repeated changes in redox conditions. Some trace elements in the coatings, such as Ba, Ni, Co, and Ce have either been absorbed by the Mn oxide minerals or organo-mineral complexes.

Soil organic matter stability in organo-mineral complexes as a function of increasing C loading

Soil carbon (C) saturation behavior predicts that soil C storage efficiency observed under field conditions decreases as a soil approaches C saturation. This may be due to a decline in soil organic matter (SOM) stability as the result of changes in the type, strength or turnover time of organo-mineral interactions with increasing organic C input. The goal of this study was to test whether the stability of organic matter bound to soil minerals decreases as organo-mineral complexes approach C saturation with increasing C loading. A series of batch sorption experiments with natural dissolved organic matter (DOM) and soil mineral components was conducted to obtain organo-mineral complexes with a range of organic C loadings. The relative stability of C in these organo-mineral complexes was subsequently assessed using evolved CO2 gas analysis during thermal analyses and laboratory incubations. Results indicated that differences in dissolved organic C before and after sorption overestimated the amount of sorbed C when compared to differences in solid-phase C concentrations. Values of C/N, δ13C, and δ15N of the organo-mineral complexes were significantly smaller or more negative than initial soil samples or the stock DOM solution, consistent with the concept of molecular fractionation by sorption to minerals and suggesting that the composition of the organic matter in the organo-mineral complexes may have changed as the amount of sorbed organic matter increased. Observations that organic C loadings at maximum sorption did not substantially exceed 1 mg C m−2 and linear rather than asymptotic relations between sorbed C and initial DOM concentrations suggest that organo-mineral complexes may not have reached C saturation under the conditions in the batch sorption experiments of this study. The temperature at which half of the CO2 evolved during thermal analysis (i.e., CO2-T50) increased with increasing C loading, suggesting the sorbed C required greater energy input for combustion. Results of the laboratory incubations to determine relative biological stability of sorbed C were not consistent with the initial hypothesis. The size of potentially mineralizable C pool relative to the total sorbed C decreased with increasing C loading, even though this pool was being degraded more rapidly. Overall, the results did not support the hypothesis that SOM stability decreases with increasing C loading. In spite of generating a wide range of C loadings substantially greater than previous studies, the conditions of the DOM sorption experiments conducted in this study appeared unable to generate organo-mineral complexes exhibiting C saturation behavior. We speculate that measurable decreases in SOM stability may occur only once the threshold of C saturation is reached.

Adsorption of Phenanthrene and Bisphenol A on Organo-Nano Iron Oxide Complexes

Organo-mineral complexes are important in regulating the transport and retention of hydrophobic organic contaminants in soils/sediments. Humic acid will be fractionated during adsorption on mineral. This study examined the sorption of model compounds with different properties (tannin acid, gallic acid, fumaric acid sodium and sodium oleate) on nanoiron oxide minerals and determined the sorption of phenanthrene and Bisphenol A on these organooxide complexes. The results showed that the sorption of model compounds on nanoFe2O3 followed the order of O &gt; T &gt; G&gt; F. The higher adsorption of O and T was likely attributed to their higher molecule weight. The sorption of PHE and BPA on organomineral complexes was controlled by hydrophobic interactions. HOCs will adsorb preferentially on hydrophobic aliphatic compositions of organic matter.

Establishing DFT-Based Linear Free Energy Relationships for the Study of Organo-Mineral Complexes in Soil Carbon Pool - The Sorption Mechanisms of Organic Acids with Iron Oxides

Density functional theory and the hard and soft acid-base theory are widely used in quantum mechanics and computational chemistry. This article combined them to describe the iron oxide adsorption of humic acid free energy relationship, and predict the free energy relationship equation, which plays an important role for the study of the formation mechanism of organo-mineral complexes in soil carbon sequestration.

Establishing DFT-Based Linear Free Energy Relationships for the Study of Organo-Mineral Complexes in Soil Carbon Pool - The Sorption Mechanisms of Organic Acids with Iron Oxides

Density functional theory and the hard and soft acid-base theory are widely used in quantum mechanics and computational chemistry. This article combined them to describe the iron oxide adsorption of humic acid free energy relationship, and predict the free energy relationship equation, which plays an important role for the study of the formation mechanism of organo-mineral complexes in soil carbon sequestration.

Aspects of soil phenolic matter (SPM): An explorative investigation in agricultural, agroforestry, and wood ecosystems

This paper assesses the distribution of soil phenolic matter (SPM) in soil environments from a pedological perspective, with particular emphasis on soil horizonation and its involvement in pedogenic processes.The study was conducted on 15 soil profiles (Andosols, Calcisols, Cambisols, Lixisols, Phaeozems, Podzols, Umbrisols) covering five distinct ecosystems. Although the ecosystems were virtually homogeneous in and of themselves, they represented strongly contrasting climates, geolithology and morphology, land use and vegetation cover (intensive agriculture, chestnut grove, alpine pasture, woody reforestations, and natural woods). Three main SPM fractions were categorised: (i) “total” (SPMt), (ii) “soluble” and/or “labile” (SPMs), and (iii) the fraction with a “high” (SPMh) affinity for the soil body.The SPMh and SPMs contents varied significantly, averaging 2.84±0.52gkg−1 and 0.07±0.02gkg−1, respectively. Thus, these fractions appear to belong to different soil organic matter (SOM) pools. Indeed, the SPMs fraction was characterised by transitional, ephemeral low-weight molecules, which continuously reached the soil through both plant metabolism/decay and SOM degradation and were subsequently transformed into larger materials (humic matter, organo-mineral complexes) and/or lost by leaching and mineralisation. The SPMh fraction is likely involved in pedogenic processes that are primarily driven by the transport/translocation of colloidal phases under the following conditions: (i) in the early stages of horizon differentiation or, more specifically, in transport processes from the overlying A horizons and (ii) in cheluviation phenomena leading to the formation of spodic horizons. Moreover, SPMh also appears to be involved in early “in situ” alteration of cambic horizons. This involvement may result from the strong etching/complexing activity of soluble polyphenols from the overlying horizon, which promotes “in situ” weathering of the parent material.The total amount of SPM in a given environment greatly depended on the environmental conditions, which ranged from 0.02gkg−1 in an intensively cultivated Haplic Cambisol to 13.14gkg−1 in an Andic Cambisol under natural downy oak cover. Additionally, the SPMh/SPMs and SPMt/SOM% ratios decreased significantly from natural woods to intensive agriculture systems and may therefore indicate the extent of different anthropic impacts, such as soil cover/land use, in different ecosystems.

A study of cetylpyridinium cation sorption on Na-vermiculite

The sorption of cetylpyridinium on Na-vermiculite has been studied by the adsorption, calorimetry, and X-ray diffraction methods. The ion-exchange component of cetylpyridinium sorption on Na-vermiculite has been distinguished and analyzed. Variations in the integral heat of ion exchange of cetylpyridinium cations on Na-vermiculite have been determined as depending on the amount of absorbed cetylpyridinium by the calorimetry method. X-ray diffraction analysis of cetylpyridinium-Na-vermiculite organomineral complexes with different contents of exchange cations has been carried out in parallel. The formation of monolayer (interlayer spacing Δd = 20.25 A) and bilayer (Δd = 37.27 A) intercalation structures with a tilt angle of cetylpyridinium ions to the silicate basal surface of ≈57° has been revealed.

Sorptive fractionation of organic matter and formation of organo-hydroxy-aluminum complexes during litter biodegradation in the presence of gibbsite

Solid and aqueous phase Al species are recognized to affect organic matter (OM) stabilization in forest soils. However, little is known about the dynamics of formation, composition and dissolution of organo-Al hydroxide complexes in microbially-active soil systems, where plant litter is subject to microbial decomposition in close proximity to mineral weathering reactions. We incubated gibbsite–quartz mineral mixtures in the presence of forest floor material inoculated with a native microbial consortium for periods of 5, 60 and 154days. At each time step, samples were density separated into light (<1.6gcm−3), intermediate (1.6–2.0gcm−3), and heavy (>2.0gcm−3) fractions. The light fraction was mainly comprised of particulate organic matter, while the intermediate and heavy density fractions contained moderate and large amounts of Al-minerals, respectively. Multi-method interrogation of the fractions indicated the intermediate and heavy fractions differed both in mineral structure and organic compound composition. X-ray diffraction analysis and SEM/EDS of the mineral component of the intermediate fractions indicated some alteration of the original gibbsite structure into less crystalline Al hydroxide and possibly proto-imogolite species, whereas alteration of the gibbsite structure was not evident in the heavy fraction. DRIFT, Py–GC/MS and STXM/NEXAFS results all showed that intermediate fractions were composed mostly of lignin-derived compounds, phenolics, and polysaccharides. Heavy fraction organics were dominated by polysaccharides, and were enriched in proteins, N-bearing compounds, and lipids. The source of organics appeared to differ between the intermediate and heavy fractions. Heavy fractions were enriched in 13C with lower C/N ratios relative to intermediate fractions, suggesting a microbial origin. The observed differential fractionation of organics among hydroxy-Al mineral types suggests that microbial activity superimposed with abiotic mineral-surface-mediated fractionation leads to strong density differentiation of organo-mineral complex composition even over the short time scales probed in these incubation experiments. The data highlight the strong interdependency of mineral transformation, microbial community activity, and organic matter stabilization during biodegradation.

Variation of heavy metal speciation during the pyrolysis of sediment collected from the Dianchi Lake, China

Sediment samples with high organic carbon were collected from the Dianchi Lake in China and thermally treated using a method analogous to biochar production. The speciation of the heavy metals Cu, Cd, Pb, and Zn in sediment and thermally treated sediments (TTSs) were analyzed by European Community Bureau of Reference (BCR) sequential extraction methods. Heavy metal bioavailability and eco-toxicity were assessed by risk assessment code. This study demonstrates that BCR sequential extraction methods and risk assessment code can be used as valuable tools to assess heavy metal mobility, bioavailability and eco-toxicity. Compared to biochar derived from biomass, TTSs had different characteristics, which may contribute to the formation of organo-mineral complexes. The heavy metals Cu, Cd, Pb, and Zn speciated in TTSs show different patterns from that of the sediment and pyrolysis temperature has a great influence on the fractional distribution of heavy metals. Those different distributions may attribute to the geochemistry of the sediment and the different physicochemical characteristics of heavy metals. In order for the safe application of thermally –treatment sediments (TTSs) as a soil amendment, further studies such as field experiments may be required.

Influence of Humic Acid on Interaction of Ammonium and Potassium Ions on Clay Minerals

Interaction of ammonium (NH+4) and potassium (K+) is typical in field soils. However, the effects of organic matter on interaction of NH+ and K+ have not been thoroughly investigated. In this study, we examined the changes in major physicochemical properties of three clay minerals (kaolinite, illite, and montmorillonite) after humic acid (HA) coating and evaluated the influences of these changes on the interaction of NH+4 and K+ on clay minerals using batch experiments. After HA coating, the cation exchange capacity (CEC) and specific surface area (SSA) of montmorillonite decreased significantly, while little decrease in CEC and SSA occurred in illite and only a slight increase in CEC was found in kaolinite. Humic acid coating significantly increased cation adsorption and preference for NH+4, and this effect was more obvious on clay minerals with a lower CEC. Results of Fourier transform infrared spectrometry analysis showed that HA coating promoted the formation of H-bonds between the adsorbed NH+ and the organo-mineral complexes. HA coating increased cation fixation capacity on montmorillonite and kaolinite, but the opposite occurred on illite. In addition, HA coating increased the competitiveness of NH+ on fixation sites. These results showed that HA coating affected both the nature of clay mineral surfaces and the reactions of NH+4 and K+ with clay minerals, which might influence the availability of nutrient cations to plants in field soils amended with organic matter.

Characterization of the Atrazine Sorption Process on Andisol and Ultisol Volcanic Ash-Derived Soils: Kinetic Parameters and the Contribution of Humic Fractions

Atrazine sorption was studied in six Andisol and Ultisol soils. Humic and fulvic acids and humin contributions were established. Sorption on soils was well described by the Freundlich model. Kf values ranged from 2.2-15.6 μg(1-1/n)mL(1/n)g⁻¹. The relevance of humic acid and humin was deduced from isotherm and kinetics experiments. KOC values varied between 221 and 679 mLg⁻¹ for these fractions. Fulvic acid presented low binding capacity. Sorption was controlled by instantaneous equilibrium followed by a time-dependent phase. The Elovich equation, intraparticle diffusion model, and a two-site nonequilibrium model allowed us to conclude that (i) there are two rate-limited phases in Andisols related to intrasorbent diffusion in organic matter and retarded intraparticle diffusion in the organo-mineral complex and that (ii) there is one rate-limited phase in Ultisols attributed to the mineral composition. The lower organic matter content of Ultisols and the slower sorption rate and mechanisms involved must be considered to assess the leaching behavior of atrazine.

Buried black soils on the slopes of Mt. Kilimanjaro as a regional carbon storage hotspot

Abstract Mt. Kilimanjaro attracts much scientific and public attention due to its dramatically shrinking ice caps, still known as “the white top” of Africa. In this mountain system forming a type of island within the surrounding savannah, a new phenomenon has recently been discovered. On the slopes of Mt. Kilimanjaro, Late Quaternary paleosol sequences, composed of dark or black buried soils, are widespread in the montane rainforest zone (1800–3000 m a.s.l.). In this study we investigated in detail the soil organic carbon (SOC) content and SOC stocks in soil profiles (mostly Andosols) along two altitudinal transects, situated on both the humid southern slopes and on the drier northern slopes of the mountain. In the montane forest zone, up to 3 m thick paleosol sequences are frequently found. SOC content is remarkably high, reaching values of up to more than 10%. This testifies to good preservation of soil organic matter (SOM) which may be due to such factors as rapid burial by dust, low temperatures alongside more resistant litter during glacial periods, formation of stable organo-mineral complexes and high black carbon (BC) content. The buried black soils are estimated to contain ~ 82 kg m − 2 mean SOC stocks in the montane rainforest. As compared to the SOC storage in the surrounding savannah soils of the Maasai Steppe, the buried black soils constitute a distinctive regional carbon storage hotspot.