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Context The Cerrado , an area of savannah and wetlands in Brazil, is an important wet ecosystem characterised by organic-rich soils that has important environmental functions. In this work, we hypothesised that the total elemental composition obtained via portable X-ray fluorescence spectrometry (pXRF) could be useful a tool to predict soil organic matter. Aims We aimed to predict the total organic carbon (TOC) content and its fractions (carbon fulvic acid, C-FA; carbon humic acid, C-HA; carbon humin, C-HU) of soils occurring in representative Veredas biome from the Cerrado , Brazil. Methods We collected 144 soil samples from surface (0–20cm) and subsurface (40–70cm) layers from six representative Veredas environment located in the Cerrado bome, Triângulo Mineiro region, Brazil. These samples were analysed using a pXRF to obtain the total elemental composition. TOC, C-HA, C-FA, and C-HU fractions were determined by wet-oxidation and titration methods. The pXRF data was correlated with TOC and humic substances and later subjected to simple and multiple linear regression (MLR). Key results The major elements corroborated the mineralogy of wetland soils. TOC and humic substances, mainly C-HU, were reasonably predicted using pXRF data. All humic substances and TOC were strongly correlated to Pb. Thus, by parsimony, TOC could be reasonable predicted exclusively based on Pb concentration. Conclusions We demonstrated that pXRF can be an efficient tool to assess the elemental composition of the Cerrado wetlands soils contributing to still scarce inventory. Further, the elements germane to soil organic matter compounds can be used to predict TOC and its fractions. Implications A detailed characterisation of the Cerrado wetland soils in situ can be successfully performed using pXRF in further studies.

Selenium (Se) is essential for animals and humans, and studies assessing the influence of sulfur (S) and phosphorus (P) on Se availability and biofortification in tropical soils are required. This study evaluated the adsorption of selenate (SeVI) and selenite (SeIV) in two oxidic soils with contrasting soil texture and assessed effects of phosphate and sulfate as competitive anions on Se desorption. The study also examined Se biofortification of Panicum maximum cv. Mombaça grown in Se-treated clayey soil under increasing rates of P fertilisation. In the laboratory, different Se concentrations were added to soils for adsorption studies (0, 1, 2, 4 mg L–1 for SeVI and 0, 4, 8, 16 mg L–1 for SeIV), with Se desorption performed by adding different concentrations of P (0, 2.18, 4.36, 10.90, 21.90 mg L–1 ) or S (0, 3.75, 7.5, 15, 30 mg L–1 ). Mombaça grass was grown in pots for 43 days in the clayey soil fertilised with SeVI and increasing P rates. Selenium adsorption varied depending on Se speciation, Se rate and soil texture, with SeIV showing greater adsorption than SeVI. In general, amounts of SeVI desorbed increased with increasing addition of P and (mainly) S. However, only P had a positive effect for releasing SeIV from soils. At all SeVI rates added in the clayey soil, Se contents in the shoot dry matter of Mombaça grass increased with increasing P fertilisation rate, agreeing with the desorption results. Further studies combining desorption with biofortification data are recommended for assessing the influence of P in sandy clay loam tropical soils.

Proximal sensors have been increasingly used for a variety of purposes worldwide, with great interest on portable X-ray fluorescence (pXRF) spectrometry applications in Soil Science. pXRF provides rapid, inexpensive, non-destructive, and environmentally-friendly elemental analysis in soils, rocks, sediments, plant tissues, water, and other matrices. While temperate regions concentrate the largest number of studies involving pXRF for soil characterization, tropical regions have recently demonstrated increasing interests in—and acceptance of—pXRF applications for soil-related studies due to the wide range of purposes in which this equipment can be utilized. However, the lack of a standard methodology for tropical soil analysis via pXRF coupled with the recognized variation of results according to the procedures used during analyses underlie the need for routine protocols for improving pXRF analyses in tropical environments. Besides establishing such a standard methodology, this review elucidates the main differences between soils from tropical and temperate regions that are key in pXRF research, presents state-of-the-art achievements in pXRF analysis, and envisions applications of pXRF in tropical soils.

Variability of elemental composition in a toposequence of quartzite‐derived soils in a tropical environment was evaluated via portable x‐ray fluorescence (pXRF) spectrometry. The toposequence featured different degrees of weathering and variable soil formation processes. Specifically, this study aimed to: (i) use elemental composition obtained through pXRF as a tool to investigate the degrees of weathering–leaching and pedogenic processes; and (ii) correlate elemental data with physicochemical properties of soils. Four soil profiles (Lithic Ustorthent, Typic Ustorthent, Typic Dystrudept, and Xanthic Hapludox) were described and sampled. Laboratory analysis for soil physicochemical characterization and pXRF were performed. Soils featured high contents of SiO2due to the predominance of quartz in the rock. However, pXRF analysis revealed that the soils showed a decrease in such content with depth. Potassium contents tended to be greater in the C horizons since they are less weathered and retained greater amounts of K‐bearing minerals, such as muscovite. Al, Fe, Ca, Ti, Cu, and Sr increased their contents with increasing clay content in soils, while Zr generally remained stable. Ba was leached out of soils during weathering. Both CaO and Sr were positively correlated with exchangeable contents of Ca2+and Mg2+, base saturation, pH, and cation exchange capacity, while inversely correlated to Al3+saturation. Fe, Al, Ti, Ca, K, Cu, Rb, and Zr accumulated in soil in comparison with the parent material. Variability of elemental contents in soils with differential degrees of weathering–leaching can be easily accessed with pXRF and such data correlate well with several soil physicochemical properties. Toposequence of quartzite‐derived soils variability assessed by pXRF.Soils presented varying weathering degrees, from Entisols to Oxisols.pXRF elemental data correlated well with soil physical and chemical properties.Soil mineralogy and pXRF data elucidated soils variability with weathering.Al, Fe, Ca, Ti, Cu, and Sr increased with increasing clay content.

Core IdeasWe verified the effect of soil moisture on portable X‐ray fluorescence (pXRF) results with undisturbed soil samples.Heavy oxides (Fe and Ti) are much less affected by soil moisture than light oxides (Si and Al).The power model described the effect of soil moisture on pXRF results for SiO2and Al2O3wellA practical correction factor for SiO2and Al2O3was proposed for accurate in‐field measurements Portable X‐ray fluorescence (pXRF) has been used in many countries to assess the total elemental composition of soils and has agronomic, pedological, and environmental applications. This technique is still incipient in tropical developing countries like Brazil but has potential to improve and decrease the costs of research. Soil moisture is one of the most important factors influencing pXRF results. This pilot study evaluated how soil moisture affects elemental concentrations of oxides common in Brazilian soils (SiO2, Al2O3, Fe2O3, TiO2, and P2O5) as assessed by pXRF. Undisturbed soil samples were collected from the surface and subsurface horizons of six representative Brazilian soils (representing ∼40% of the country) with variable texture, weathering, or leaching status. Soil moisture ranged from complete saturation to oven‐dry. During drying, six pXRF measurements were made and oxide contents were recorded. In all soils, the P2O5content was below the pXRF detection limit, and SiO2and Al2O3were strongly influenced by soil water content. As soil moisture increased, SiO2and Al2O3contents decreased (described by a power model, y= a– bxc). The TiO2content was much less affected by soil moisture, whereas Fe2O3was not significantly affected by soil moisture. Soil texture and weathering status are closely related to variation in the SiO2and Al2O3contents determined by the pXRF with soil moisture. These findings are critical to the appropriate application of pXRF in tropical countries featuring oxide‐laden soils. For in‐field measurement of oxides affected by soil moisture (Si and Al), a practical correction factor was determined.