Amazonian Soils Research Articles

Transformations in artisanal and small-scale gold mining work and production structures in the tapajós region of Brazil's amazon

Currently, artisanal and small-scale gold mining (ASGM), known in the Brazilian Amazon as “garimpagem”, generates direct income for approximately 35,000 gold miners in the Tapajos region. Most of the gold mining sites are located in the Pará cities of Itaituba, Jacareacanga and Novo Progresso, operating various technical and organizational structures that require distinct levels of capital and technological investments, constituting a complex commercial supply chain of services, inputs and equipment on a regional national and international scale based on the commercialization of gold, which in most cases is negotiated under informal and illegal conditions. The main objective of this ethnographic research is to analyze the predominant model of mining production used in the region in consolidation with the use of backhoe technology in the production process, focusing on the relations and working conditions of small miners. The results of this research are to understand and describe how the productive structures that currently exploit gold in the Tapajos region are organized under what conditions the small miners extract the ore from the Amazonian soil.

Characterization of an Amazon Soil Profile by Laser-Induced Breakdown, Raman, and Fluorescence Spectroscopies

This work aimed to investigate, in detail, the elemental and molecular composition of soil samples collected from the various horizons of an Amazon spodosol profile by combining the atomic technique laser-induced breakdown spectroscopy (LIBS) with two molecular techniques, i.e., Raman and fluorescence spectroscopies. The emission lines of the elements Fe, C, Si, Mg, Al, Ti, Ca, and K with various relative intensities were detected by using LIBS. In particular, C appeared to accumulate in the transition horizon and was proven to be mostly humified by fluorescence analysis. The Raman peaks detected at 465 cm−1 and 800 cm−1 corresponded with the symmetric stretching and bending modes of O-Si-O and Si-OH, respectively. Small shifts toward higher frequencies and slight increases in the width and full width at half maximum (FWHM) of the quartz band at 465 cm−1 appeared in the Tr to K2 horizons, which could be ascribed to a local distortion caused by the high contents of Al of kaolinite mineral phases, as also shown by the LIBS data. Thus, a small amount of kaolinite mineral phase and K measured by LIBS would be present also in the A1 to E2 horizons. The lifetime fluorescence was almost constant in the surface and middle horizons, whereas it increased sharply in the transition horizon and then decreased slightly in the kaolin horizons, which confirmed the presence of more humified recalcitrant organic matter in deeper soil horizons. In conclusion, the combined use of the three spectroscopic techniques appeared to be a very promising approach for studying Amazon soils.

Shifts in functional traits and interactions patterns of soil methane-cycling communities following forest-to-pasture conversion in the Amazon Basin.

Deforestation threatens the integrity of the Amazon biome and the ecosystem services it provides, including greenhouse gas mitigation. Forest-to-pasture conversion has been shown to alter the flux of methane gas (CH4 ) in Amazonian soils, driving a switch from acting as a sink to a source of atmospheric CH4 . This study aimed to better understand this phenomenon by investigating soil microbial metagenomes, focusing on the taxonomic and functional structure of methane-cycling communities. Metagenomic data from forest and pasture soils were combined with in situ CH4 fluxes and soil edaphic factors measurements and analysed using multivariate statistical approaches. We found a significantly higher abundance and diversity of methanogens in pasture soils. As inferred by co-occurrence networks, these microorganisms seem to be less interconnected within the soil microbiota in pasture soils. Metabolic traits were also different between land uses, with increased hydrogenotrophic and methylotrophic pathways of methanogenesis in pasture soils. Land-use change also induced shifts in taxonomic and functional traits of methanotrophs, with bacteria harboring genes encoding the soluble form of methane monooxygenase enzyme (sMMO) depleted in pasture soils. Redundancy analysis and multimodel inference revealed that the shift in methane-cycling communities was associated with high pH, organic matter, soil porosity, and micronutrients in pasture soils. These results comprehensively characterize the effect of forest-to-pasture conversion on the microbial communities driving the methane-cycling microorganisms in the Amazon rainforest, which will contribute to the efforts to preserve this important biome.

Investigation of mineral commodity residues based on alkalinity, solubility and other physicochemical aspects aiming the management of amazonian acidic soils

The large amounts of mineral residues generated by the bauxite-alumina industries in the Amazon — with a production scale in the same order of magnitude as their source commodities — have been seen as new sources of raw materials (secondary mines) and/or an inseparable part of a sustainable production system, i.e., a source for the generation of co-products within a circular economy system. In the present study, two alkaline residues from this mining-metallurgical industry were assessed for their potential to amend productive acidic Amazonian soils, namely, (1) insoluble solid residue from the Bayer process (bauxite residue, BR) and (2) ash from energy production from coal (coal combustion residues, CCRs: fly ash, FA, and bottom ash, BA). A physicochemical investigation was performed to evaluate the possible benefits that these residues can offer to the soil‒plant system. The alkalinity of the residues was adjusted “to a value of pH 8–10” by leaching with H3PO4 using a central composite experimental design. The chemical analyses indicated high levels (total and soluble) of essential elements such as Ca and S in the CCRs. All residues showed a high cation exchange capacity (CEC). Regarding the water holding capacity (WHC), FA showed a higher value than the other residues (68.6%). After pH adjustment, available P increased significantly for all residues, and the Ca and S contents remained high for the CCRs, while in BR, there was a decrease in available Na, and aluminum (Al3+) was not available because the potential acidity (H + Al) < 0.6. Finally, complementary analyses showed that with respect to mineralogy, BR is composed mainly of iron oxyhydroxides and aluminosilicate phases, while carbonate, sulfide and silicate phases dominate the CCRs. The neutralizing character, the presence of nutrients in the CCRs and the unavailability of Al3+ in BR are positive physicochemical characteristics for the management of Amazonian acid soils; the use of this residues would add to the circular economy and sustainability of the Amazon.

The effect of aluminum treatment on metabolism in oil palm seedlings

Due to rainfall and high temperatures, the Amazonian soil undergoes changes in its source material and leaching of base cations. This results in deep, infertile, and acidic soil. Aluminum present in acidic soil impairs plant growth and development by inhibiting root formation, enzymatic reactions, absorption, transport, and nutrient utilization. This study aimed to evaluate the effects of aluminum dosage on the metabolism of the oil palm Elaeis guineensis Jacq. The study was conducted in a greenhouse at the Federal Rural University of Amazonia. The experimental design was randomized, with five replications, in which dosages of 0, 10, 20, 30, and 40 mg L-1 aluminum chloride (AlCl3.6H2O) were administered. Electrolyte leakage, nitrate, nitrate reductase, free ammonium, soluble amino acids, proline content, and soluble proteins were analyzed in the leaves and roots of the oil palm. The highest concentration of aluminum was found in the roots. AlCl3 treatment at 40 mg L-1 increased electrolyte leakage, nitrate, ammonium, and proline concentrations in the roots, and amino acid concentrations in both the leaves and roots. Furthermore, a decrease in nitrate reductase enzyme activity was observed in the roots. This study demonstrates that the oil palm has mechanisms of tolerance to aluminum toxicity.

Amazonian soil fungi are efficient degraders of glyphosate herbicide; novel isolates of Penicillium, Aspergillus, and Trichoderma.

Pesticide residues that contaminate the environment circulate within the hydrological cycle can accumulate within the food chain and cause problems to both environmental and human health. Microbes, however, are well known for their metabolic versatility and the ability to degrade chemically stable substances, including recalcitrant xenobiotics. The current study focused on bio-prospecting within Amazonian rainforest soils to find novel strains fungi capable of efficiently degrading the agriculturally and environmentally ubiquitous herbicide, glyphosate. Of 50 fungal strains isolated (using culture media supplemented with glyphosate as the sole carbon-substrate), the majority were Penicillium strains (60%) and the others were Aspergillus and Trichoderma strains (26 and 8%, respectively). All 50 fungal isolates could use glyphosate as a phosphorous source. Eight of these isolates grew better on glyphosate-supplemented media than on regular Czapek Dox medium. LC-MS revealed that glyphosate degradation by Penicillium 4A21 resulted in sarcosine and aminomethylphosphonic acid.

Phylogenetic analysis, computer modeling and catalytic prediction of an Amazonian soil β-glucosidase against a soybean saponin.

Saponins are amphipathic glycosides with detergent properties present in vegetables. These compounds, when ingested, can cause difficulties in absorbing nutrients from food and even induce inflammatory processes in the intestine. There is already some evidence that saponins can be degraded by β-glucosidases of the GH3 family. In the present study, we evaluated, through computational tools, the possibility of a β-glucosidase (AMBGL17) obtained from a metagenomic analysis of the Amazonian soil, to catalytically interact with a saponin present in soybean. For this, the amino acid sequence of AMBGL17 was used in a phylogenetic analysis to estimate its origin and to determine its three-dimensional structure. The 3D structure of the enzyme was used in a molecular docking analysis to evaluate its interaction with soy saponin as a ligand. The results of the phylogenetic analysis showed that AMBGL17 comes from a microorganism of the phylum Chloroflexi, probably related to species of the order Aggregatinales. Molecular docking showed that soybean saponin can interact with the catalytic site of AMBGL17, with the amino acid GLY345 being important in this catalytic interaction, especially with a β-1,2 glycosidic bond present in the carbohydrate portion of saponin. In conclusion, AMBGL17 is an enzyme with interesting biotechnological potential in terms of mitigating the anti-nutritional and pro-inflammatory effects of saponins present in vegetables used for human and animal food.

Soil chemical attributes in areas under conversion from forest to pasture in southern Brazilian Amazon

The south of the Brazilian Amazon is one of the largest cattle-producing regions in Brazil, however, most of the pastures are in low fertility soils. Thus, cattle breeders compensate for the low production of pastures, increasing the size of the areas, generating more deforestation and burning. These practices increase the chemical degradation process of Amazonian soils, making them increasingly infertile when improperly managed. With this, the objective of the work was to evaluate the impacts caused in the chemical attributes of soils, in areas under forest-to-pasture conversion, in the south of the Brazilian Amazon. The study was carried out in the district of União Bandeirantes, in an area of forest and two areas with pastures (brachiaria and mombaça grass). In the field, soil samples were collected at two depths (0.00–0.10 and 0.10–0.20 m), to carry out chemical analyzes. Further, uni, bi and multivariate statistical analyzes were carried out, besides geostatistical analyzes were carried out to study spatial variability and management zones. The conversion of forest to pasture increased the pH and exchangeable bases levels, reducing the availability of exchangeable aluminum and potential acidity, however, it induces losses of phosphorus and organic carbon from the soil. Among the pasture environments, the mombaça grass area presented higher fertility. Greater spatial variability of chemical attributes was observed in the environment with mombaça grass, indicating greater heterogeneity in the distribution of attributes in the area. We attribute this behavior to the higher grazing intensity and the micro-reliefs in the area that direct the flow of water and nutrients.

Ecological co-occurrence and soil physicochemical factors drive the archaeal community in Amazonian soils.

We evaluated the co-occurrence of archaeal taxonomic groups and soil physicochemical characteristics in relation to the structuring of the archaeal community in Amazonian soil under different land use systems. Soil samples were collected in primary forest (PF), secondary forest (SF), agricultural systems (AG) and cattle pastures (PA). Archaeal community composition was revealed based on high-throughput amplicon sequencing of the 16S rRNA gene. The results revealed co-occurrence of archaeal classes, with two groups formed: Thaumarchaeota classes, including South Africa Gold Mine-Group 1 (SAGMG-1), Crenarchaeotic group (SCG) and Crenarchaeota candidate division YNPFFA, with predominance in PF and SF; and Bathyarchaeota_unclassified, Methanomicrobia and Methanobacteria (Euryarchaeota) with the FHMa11 terrestrial group, with predominance in PA. The number of co-occurrences between groups was lower in SF, AG and PA (approximately 30%) than in PF. The qPCR analysis revealed that PF also had the largest number of archaeal representatives. Soil texture may be a limiting factor of interactions between groups since the most representative groups, SAGMG-1 and the SCG (over 20% in all sites), were positively associated with coarse sand, the soil factor most correlated with the groups (33% of the total). These results suggest that interactions between archaeal classes belonging to different phyla may be dependent on the number of individuals in the soil environment. In this context, differences in soil physical structure among the land use systems can reduce the representatives of key groups and consequently the co-occurrence of Archaea, which could compromise the natural dynamics of this complex environment.

Influence of soil management for soybean production under microbial diversity in amazon soils

The tropical Amazon has a unique biodiversity that has been affected by the development of pastures and economically important crops, such as soybeans. In the Amazon soil, the communities of microorganisms are diverse and act in different biogeochemical activities relevant to their adaptation to the environment. The assessment of changes in soil microorganism communities is essential to consider the impact of agribusiness action in one of the wealthiest regions in diversity in the world. Thus, the soil microbial diversity of the Amazon forest, the north region of Brazil, was evaluated regarding the influence of soybean farming with regions with periods of two and 14 years of exploitation, with regions of pasture and forest area, through the metagenomics approach with new generation sequencing technology, in addition, it was considered chemical characteristics such as pH value, organic matter content, macronutrients, micronutrients, and cations. High microbial diversity was identified at all collection sites and, despite this, bacterial, archaeal, and virus communities were very diverse between sites, with higher identification of Enterobacter cloacae and species of Pseudomonas, Pseudoplusia includens, Methanosarcina barkeri in the farmed and pasture, whose microbial diversity is influenced by the presence of cations and the interaction of organic matter with clay. It was evident that there is a change in the communities of native microorganisms for others adapted in the areas that had their vegetal cover eliminated.

Gibbsite Crystallinity and Morphology in Ferralsols and Bauxites

Gibbsite is a common mineral, present in several soil classes around the globe, especially in the tropical belt. It is also a key mineral in bauxite, the main natural source of aluminum. The occurrence of gibbsite in the soil greatly influences its physical and chemical characteristics. However, little is known about the important structural properties of this mineral, which determines how these characteristics will be expressed. In this work, we investigated the variation in the morphology and crystallinity of naturally occurring gibbsites from soils and bauxites. The crystallinity of the gibbsite was assessed using full width at half maximum (FWHM), mean crystallite dimensions (MCD), and deoxydylation temperature (DT) measurements. The results of these analyses were convergent, indicating a better crystallinity for gibbsites from the evaluated Amazon soils and bauxites, and worse crystallinity for the remaining soil samples. SEM analysis was used to assess the gibbsite morphology, and X-ray diffraction data were used to propose an orientation index for this mineral. The results indicated that there is little to no isomorphic substitution of Al by Fe in natural gibbsites, there is a prevalence of larger crystals, with a more laminar morphology, with orientation capability for the gibbsites of Amazon soils and bauxites, and smaller crystals tending to spherical shape for the other samples. These results suggest that natural gibbsite crystals of good crystallinity have large crystals with a laminar shape, while crystals of lower crystallinity present a smaller size and approximately an isodiametric shape.

The Role of Phosphate Fertilization on Physiological Responses of the Young Bertholletia excelsa Plants Grown in a P-Deficient Amazon Ferralsol.

Phosphorus (P) reacts with soil minerals, which makes it less available to plants. Considering that Amazonian soils have a low pH and nutrient availability, both of these properties contribute to an increase in P limitation. Here, we investigate how the addition of P to the substrate affects morpho-physiological traits of Brazil nut trees (Bertholletia excelsa Bonpl.). The experiment was carried out in a greenhouse with 24-month-old saplings, and the P treatments consisted of a control (Ferrasol without P addition) and 100, 200, 400, and 500 mg P kg-1 of added to the soil. When B. excelsa saplings were fertilized with phosphate, the N:P leaf ratio reduced from 50 to 26. Addition of P favored the photochemical efficiency of PSII (FV/FM), and the application of 200 mg kg-1 increased photosynthesis (PN) by 50%. Furthermore, phosphorus enhanced light and nutrient use efficiency. An increase in B. excelsa dry biomass was observed when 200 mg P kg-1 was added, with maximum yield occurring at 306.2 mg P kg-1. Physiological parameters suggest robust responses by B. excelsa to P fertilization. In addition, our findings reveal the critical role of P on B. excelsa growth in Ferralsol, as well as the potential of P fertilization to improve functional traits of this important Amazonian tree.

Cover crops and concentrations of carbon and nitrogen in Amazonian soil

The present study aimed to evaluate the potential of dry matter production by cover crops and their effects on the dynamics of organic matter, organic carbon and nitrogen in the soil. This study contributes to minimize the effect of climate change and improve agricultural production systems. The experiment was conducted under field conditions, in the experimental area of the Federal Institute of Education, Science and Technology of Rondônia, Colorado do Oeste Campus, in the municipality of Colorado do Oeste, RO, Brazil. The experimental design was completely randomized, arranged in a factorial scheme, with four replications, consisting of six species of cover crops cultivated in pre-season (Crotalária ochroleuca, Crotalária spectabilis, Mucuna pruriens, Cajanus cajan, Canavalia ensiformis and Lablab purpureus) and four sampling depths (0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm). The production and macronutrient contents in the dry mass of shoots of the different cover crops were evaluated, as well as the dynamics of organic matter, organic carbon and N in the soil at different sampling depths. The species Crotalaria ochroleuca and Crotalaria spectabilis showed great potential for dry matter production, while Lablab purpureus bean stands out in the extraction of nutrients such as P, K and Ca at full flowering. Lablab bean and Jack bean as cover crops were the most efficient strategies to promote the increase in the input of OM, C and N forms in the soil. NH4+-N and NO3--N contents were higher in the surface layers of the soil. Management with cover crops promoted gradual accumulation of SOM, which made it possible to influence the maintenance and increase of C and N concentrations in the soil and contributed to minimizing environmental impacts and increasing the quality and sustainability of soils and agricultural production systems

Humic fractions from Amazon soils: Lifetime study and humification process by fluorescence spectroscopy

The Amazon Forest is an important and primordial ecosystem, as it controls the dynamics of soil organic matter (SOM) and is considered one of the crucial carbon reservoirs in the world. Based on that, this study evaluated the molecular properties of humic (HA) and fulvic (FA) acid from Amazon soil, with and without Cu(II) ions, by Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) and Fluorescence Spectroscopy (FS). Fluorescence excitation-emission matrix (EEM) was obtained by FS, and EEM spectra were treated by canonical polyadic/parallel factor analysis (EEM-CP/PARAFAC). The results showed that the humification index (A465nm) determined by FS is directly dependent on fluorophores’ CII/CI ratio in these soils with depth (r = 0.89), especially for HA samples in deep horizons, which demonstrated that the CII consists more complex and humified structure of the SOM. The lifetime decay showed the presence of static quenching in the humic fractions. From the lifetime data provided by TRLFS and the different correlations with carbon content and A465nm, it can be suggested that the fluorescence suppression modifies the molecular properties of the humic fractions with the addition of Cu(II) ions, such as altering the mobility and availability of the metal in the complexation reactions in these Amazonian Spodosols, mainly for the fulvic fraction.

Impacts of soybean agriculture on the resistome of the Amazonian soil.

The soils of the Amazon are complex environments with different organisms cohabiting in continuous adaptation processes; this changes significantly when these environments are modified for the development of agricultural activities that alter the chemical, macro, and microbiological compositions. The metagenomic variations and the levels of the environmental impact of four different soil samples from the Amazon region were evaluated, emphasizing the resistome. Soil samples from the organic phase from the different forest, pasture, and transgenic soybean monocultures of 2–14 years old were collected in triplicate at each site. The samples were divided into two groups, and one group was pre-treated to obtain genetic material to perform sequencing for metagenomic analysis; another group carried out the chemical characterization of the soil, determining the pH, the content of cations, and heavy metals; these were carried out in addition to identifying with different databases the components of the microbiological communities, functional genes, antibiotic and biocide resistance genes. A greater diversity of antibiotic resistance genes was observed in the forest soil. In contrast, in monoculture soils, a large number of biocide resistance genes were evidenced, highlighting the diversity and abundance of crop soils, which showed better resistance to heavy metals than other compounds, with a possible dominance of resistance to iron due to the presence of the acn gene. For up to 600 different genes for resistance to antibiotics and 256 genes for biocides were identified, most of which were for heavy metals. The most prevalent was resistance to tetracycline, cephalosporin, penam, fluoroquinolone, chloramphenicol, carbapenem, macrolide, and aminoglycoside, providing evidence for the co-selection of these resistance genes in different soils. Furthermore, the influence of vegetation cover on the forest floor was notable as a protective factor against the impact of human contamination. Regarding chemical characterization, the presence of heavy metals, different stress response mechanisms in monoculture soils, and the abundance of mobile genetic elements in crop and pasture soils stand out. The elimination of the forest increases the diversity of genes for resistance to biocides, favoring the selection of genes for resistance to antibiotics in soils.

Geochemistry signatures of mercury in soils of the Amazon rainforest biome

Mercury (Hg) toxicity in soils depends on Hg species and other physical and chemical attributes, as selenium (Se) hotspots in soils, particularly relevant in Amazonian soils. The study of Hg species and their relations in representative locations of the Amazon rainforest biome is critical for assessing the potential risks of Hg in this environment. This work aimed to determine the concentration of total Hg and its species (Hg0, Hg22+ and Hg2+), and to correlate Hgtotal concentration with total elemental composition, magnetic susceptibility, and physicochemical attributes of Amazon soils. Nine sites in the Amazon rainforest biome, Brazil, were selected and analyzed for their chemical, physical, and mineralogical attributes. The clay fraction of the studied Amazon soils is dominated by kaolinite, goethite, hematite, gibbsite, and quartz. Mica was also found in soils from the States of Acre and Amazonas. Hgtotal ranged from 21.5 to 208 μg kg−1 (median = 104 μg kg−1), and the concentrations did not exceed the threshold value established for Brazilian soils (500 μg kg−1). The Hg2+ was notably the predominant species. Its occurrence and concentration were correlated with the landscape position and soil attributes. Hgtotal was moderately and positively correlated with TiO2, clay, and Se. The findings showed that geographic location, geological formation, and pedological differences influence the heterogeneity and distribution of Hgtotal in the studied soil classes. Thus, a detailed characterization and knowledgment of the soil classes is very important to clarify the complex behavior of this metal in the Amazon rainforest biome.

Genome-resolved metagenomics reveals novel archaeal and bacterial genomes from Amazonian forest and pasture soils

Amazonian soil microbial communities are known to be affected by the forest-to-pasture conversion, but the identity and metabolic potential of most of their organisms remain poorly characterized. To contribute to the understanding of these communities, here we describe metagenome-assembled genomes (MAGs) recovered from 12 forest and pasture soil metagenomes of the Brazilian Eastern Amazon. We obtained 11 forest and 30 pasture MAGs (≥50% of completeness and ≤10 % of contamination), distributed among two archaeal and 11 bacterial phyla. The taxonomic classification results suggest that most MAGs may represent potential novel microbial taxa. MAGs selected for further evaluation included members of Acidobacteriota , Actinobacteriota , Desulfobacterota_B, Desulfobacterota_F, Dormibacterota, Eremiobacterota , Halobacteriota, Proteobacteria , and Thermoproteota , thus revealing their roles in carbohydrate degradation and mercury detoxification as well as in the sulphur, nitrogen, and methane cycles. A methane-producing Archaea of the genus Methanosarcina was almost exclusively recovered from pasture soils, which can be linked to a sink-to-source shift after the forest-to-pasture conversion. The novel MAGs constitute an important resource to help us unravel the yet-unknown microbial diversity in Amazonian soils and its functional potential and, consequently, the responses of these microorganisms to land-use change.

Land Use, Temperature, and Nitrogen Affect Nitrous Oxide Emissions in Amazonian Soils

Nitrous oxide (N2O) is one of the main gases emitted from soils, and the changes in land use in the Amazon may alter gas emission patterns. The objective of this study was to evaluate the effects of land use, temperature, and nitrogen on N2O emissions in soils in the Amazon. For this, three treatments randomized, with five repetitions, were incubated to quantify N2O emissions: (i) three different land uses (wet rainforest, pasture, and agriculture); (ii) different temperatures (25, 30, 35, and 40 °C); and (iii) different nitrogen additions to the soil (0, 90, 180, and 270 kg of N ha−1). Our results show that land use alters the flux of N2O, with the highest emissions observed in agricultural soils compared to that in forest and pasture areas. The change in soil temperature to 30 °C increased N2O emissions with land use, at which the emission of N2O was higher in the pasture and agriculture soils. Our results showed that the emission of N2O in the soil of the Amazon rainforest was low regardless of the temperature and nitrogen treatment. Therefore, the change in land use alters the resilience of the ecosystem, providing emissions of N2O.

Geochemical Background for Potentially Toxic Elements in Forested Soils of the State of Pará, Brazilian Amazon

The objectives of this study were to establish geochemical background values of aluminum (Al), iron (Fe), arsenic (As), barium (Ba), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), vanadium (V), and zinc (Zn) for eastern Amazon soils and to assess the relationship between soil chemical and granulometric attributes using univariate and multivariate procedures. Samples from the topsoil (0–0.2 m) were collected in several areas with minimal or no human intervention. Pseudo total concentrations of potentially toxic elements were extracted via acid digestion and quantified via inductively coupled plasma optical emission spectrometry (ICP-OES). The results revealed that the soils present high acidity, low levels of cations and organic carbon, and highly heterogeneous geomorphological characteristics. The elements Fe (9300 mg kg−1) and Al (8200 mg kg−1) showed the highest mean concentrations in the study, while As (0.8 mg kg−1) and Cd (0.1 mg kg−1) presented the lowest levels. The results of this study indicate that the 90th percentile can be used to formalize quality reference values for the State of Pará and that the 98th percentile can be considered for areas rich in mineral deposits.

Geochemistry of selenium, barium, and iodine in representative soils of the Brazilian Amazon rainforest

The Amazon rainforest is a heterogeneous ecosystem and its soils exhibit geographically variable concentrations of trace elements. In this region, anthropic activities - e.g., agriculture and mining - are numerous and varied, and even natural areas are at risk of contamination by trace elements, either of geogenic or anthropogenic origin. A reliable dataset of benchmark values for selenium (Se), barium (Ba), and iodine (I) concentrations in soils is needed for use as a reference in research and public policies in the region. In this study, 9 selected sites in the Brazilian Amazon rainforest within areas represented by Oxisols and Ultisols were assessed for relevant soil physicochemical characteristics, along with the concentrations of total Se (SeTot), total Ba (BaTot), and sequentially-extracted soluble Se (SeSol) and adsorbed Se (SeAd) in 3 different soil layers (0–20, 20–40, and 40–60 cm). In addition, organically bound-Se (SeOrg) and total I (ITot) concentrations in the surface layer (0–20 cm) were measured. Soil Se concentrations (SeTot) were considered safe and are likely a result of contributions of sedimentary deposits from the Andes. Available Se (SeSol + SeAd) accounted for 4.5% of SeTot, on average, while SeOrg in the topsoil accounted for more than 50% of SeTot. Barium in the western Amazon (state of Acre) and central Amazon (Anori, state of Amazonas) exceeded national prevention levels (PVs). Furthermore, the average ITot in the studied topsoils (5.4 mg kg−1) surpassed the worldwide mean. Notwithstanding, the close relationship found between the total content of the elements (Se, Ba, and I) and soil texture (clay, silt, and sand) suggests their geogenic source. Finally, our data regarding SeTot, BaTot, and ITot can be used to derive regional quality reference values for Amazon soils and also for updating prevention (PV) and investigation (IV) values established for selected elements by the Brazilian legislation.