Controls and relationships of soil organic carbon abundance and persistence vary across pedo-climatic regions.

One of the largest uncertainties in the terrestrial carbon cycle is the timing and magnitude of soil organic carbon (SOC) response to climate and vegetation change. This uncertainty prevents models from adequately capturing SOC dynamics and challenges the assessment of management and climate change effects on soils. Reducing these uncertainties requires simultaneous investigation of factors controlling the amount (SOC abundance) and duration (SOC persistence) of stored C. We present a global synthesis of SOC and radiocarbon profiles (nProfile = 597) to assess the timescales of SOC storage. We use a combination of statistical and depth-resolved compartment models to explore key factors controlling the relationships between SOC abundance and persistence across pedo-climatic regions and with soil depth. This allows us to better understand (i) how SOC abundance and persistence covary across pedo-climatic regions and (ii) how the depth dependence of SOC dynamics relates to climatic and mineralogical controls on SOC abundance and persistence. We show that SOC abundance and persistence are differently related; the controls on these relationships differ substantially between major pedo-climatic regions and soil depth. For example, large amounts of persistent SOC can reflect climatic constraints on soils (e.g., in tundra/polar regions) or mineral absorption, reflected in slower decomposition and vertical transport rates. In contrast, lower SOC abundance can be found with lower SOC persistence (e.g., in highly weathered tropical soils) or higher SOC persistence (e.g., in drier and less productive regions). We relate variable patterns of SOC abundance and persistence to differences in the processes constraining plant C input, microbial decomposition, vertical C transport and mineral SOC stabilization potential. This process-oriented grouping of SOC abundance and persistence provides a valuable benchmark for global C models, highlighting that pedo-climatic boundary conditions are crucial for predicting the effects of climate change and soil management on future C abundance and persistence.

Validation of a modified QuEChERS method for the quantification of residues of currently used pesticides in Cuban agricultural soils, using gas chromatography tandem mass spectrometry.

We present an analytical method to detect and quantify residues of currently used pesticides (CUPs), which include 31 active ingredients (ai) and seven transformation products (TPs) in tropical and agricultural soils of Cuba. Ten isotopically labeled analogous compounds served as internal standards (IL-IS). The novelty of this research is the inclusion of different tropical soils type scarcely studied for CUPs and TPs, based on the QuEChERS (quick, easy, cheap, effective, rugged and safe) method, followed by chromatography tandem mass spectrometry. All figures of merit proved to be satisfactory according to SANTE guidelines 2020 and 2021. Matrix effects (ME) calculated by the external standard method were significant (|ME| > 20% for almost all compounds; grand mean ± standard deviation (STD) 104 ± 108%) in all soils. The internal standard method compensated ME to non-significant levels (8 ± 50%), even for analytes with a non-structure identical IL-IS (STD, 13 ± 57%). Repeatability (relative standard deviation, RSDr) and reproducibility (RSDR) for skeletic regosol (SR) were 7.5 ± 2.8% and 11.7 ± 4.7%, respectively. Absolute (quantified for 11 analytes with structure identical IL-IS) and relative recovery from SR was 92 ± 13% (mean ± STD) and 90 ± 12%, respectively. Limits of quantification for SR ranged from 0.1 to 10 ng/g, except metalaxyl and oxyfluorfen (25 ng/g each). Linearity of matrix-matched (MM) calibration curves (5 to 100 ng/g) had an R2 of ≥ 0.99 for all soils and almost all analytes. The method was successfully applied to 30 real soil samples.

Chemodiversity of Dissolved Soil Organic Matter from Amazon Rainforest as Influenced by Deforestation.

Many biogeochemical processes are modulated by dissolved organic matter (DOM), but the drivers influencing the chemodiversity of DOM compounds in Amazonian soils are poorly understood. It has also been theorized whether deforestation controls the decline of DOM. In this study, we collected soil samples from thirty sites across different regions of Brazil's Legal Amazon, and we investigated the trade-offs among soil physical-chemical properties and DOM chemodiversity. We employed optical spectroscopy, Fourier transform ion cyclotron resonance, and multivariate analysis. Our results indicated that, despite variations in land use and soil physical-chemical properties, factors such as the deforested site, geometric mean diameter, weighted average diameter, and soil organic carbon were the main influencers of DOM chemodiversity variation. These findings highlight the importance of considering DOM chemodiversity as closely related to land use and its potential use in developing deforestation models for predicting soil quality decline in Brazil's Legal Amazon.

The potential for carbon dioxide removal by enhanced rock weathering in the tropics: An evaluation of Costa Rica.

Tropical environments show great potential to sequester CO2 by enhanced rock weathering (ERW) of powdered mafic rocks applied to agricultural fields. This study seeks to assess carbon dioxide reduction (CDR) potential in the humid tropics (1) by experimental weathering of mafic rock powders in conditions simulating humid tropical soils, and (2) from weathering rates determined from a Holocene tropical soil chronosequence where parent material is andesitic sediments. Experimentally determined weathering rates by leaching of basaltic andesites from Costa Rica (Arenal and Barva) for 50 t ha-1 applications indicate potential sequestration of 2.4 to 4.5 t CO2 ha-1 yr-1, whereas the USGS basalt standard BHVO-1 yields a rate of 11.9 t ha-1 yr-1 (influenced by more mafic composition and finer particle size). The chronosequence indicates a rate of 1.7 t CO2 ha-1 yr-1. The weathering experiment consisted of 0.6 mm of powdered rock applied atop 12 mm of Ultisol at 35 °C. To simulate a tropical soil solution, 100-mL aliquots of a dilute solution of oxalic acid in carbonated DI water were rained onto soils over a 14-day period to simulate soil moisture in the humid tropics. Solutions were collected and analyzed by ICPMS for concentrations of leached cations. A potential ERW scenario for Costa Rica was assessed assuming that one-half of lowland agricultural kaolinitic soils (mainly Ultisols, common crop and pasture soils, excluding protected areas) were to receive 50 t ha-1 of annual or biennial applications of powdered mafic rock. With an experimentally determined humid tropical CDR rate for basaltic andesite (3.5 t ha-1 yr-1) and allowances for carbon costs (e.g. emissions from processing and delivery) that reduce CDR to a net 3.2 t ha-1 yr-1, potential annual CDR of this tropical nation is ∼2-4 million tons, amounting to ∼25-50 % of annual CO2 emissions (mainly from transportation in Costa Rica).

Spatial distribution of Pb and Zn in soils under native vegetation in Southeast Brazil.

Heavy metals can play an important biological role as micronutrients but also as potentially toxic elements (PTEs). Understanding the natural concentrations of PTEs-Pb and Zn included-in soils allows for the identification and monitoring of contaminated areas and their role in environmental risk assessment. In this study, we aim to determine semi-total or natural and available concentrations of Pb and Zn in topsoils (0-20 cm depth) from 337 samples under native vegetation in the State of Minas Gerais, Brazil. Additionally, we sought to interpret the spatial geochemical variability using geostatistical techniques and quality reference values for these elements in soils were established. The semi-total concentrations were determined by flame and graphite furnace atomic absorption after microwave-assisted nitric acid digestion method. The available concentrations were extracted using the Mehlich-I extractor and determined by atomic absorption spectrometer. Spatial variability was modeled using semivariance estimators: Matheron's classic, Cressie and Hawkins' robust, and Cressie median estimators, the last two being less sensitive to extreme values. This allowed the construction of digital maps through kriging of semi-total Pb and Zn contents using the median estimator, as well as other soil properties by the robust estimator. The dominance of acidic pH and low CEC values reflects highly weathered low-fertility soils. Semi-total Pb contents ranged from 2.1 to 278 mg kg-1 (median: 9.35 mg kg-1) whereas semi-total Zn contents ranged from 2.7 to 495 mg kg-1 (median: 7.7 mg kg-1). The available Pb contents ranged from 0.1 to 6.92 mg kg-1 (median: 0.54 mg kg-1) whereas available Zn contents ranged from 0.1 to 78.2 mg kg-1 (median: 0.32 mg kg-1). The highest Pb and Zn concentrations were observed near Januária, in the northern part of the territory, probably on limestone rocks from the Bambuí group. Finally, the QRVs for Pb and Zn in natural soils were lower than their background values from other Brazilian region and below the prevention values suggested by Brazilian environmental regulations.

Mammal traits and soil biogeochemistry: Functional diversity relates to composition of soil organic matter.

Mammal diversity affects carbon concentration in Amazonian soils. It is known that some species traits determine carbon accumulation in organisms (e.g., size and longevity), and are also related to feeding strategies, thus linking species traits to the type of organic remains that are incorporated into the soil. Trait diversity in mammal assemblages - that is, its functional diversity - may therefore constitute another mechanism linking biodiversity to soil organic matter (SOM) accumulation. To address this hypothesis, we analyzed across 83 mammal assemblages in the Amazon biome (Guyana), the elemental (by ED-XRF and CNH analysis) and molecular (FTIR-ATR) composition of SOM of topsoils (401 samples) and trait diversity (functional richness, evenness, and divergence) for each mammal assemblage. Lower mammal functional richness but higher functional divergence were related to higher content of carbonyl and aliphatic SOM, potentially affecting SOM recalcitrance. Our results might allow the design of biodiversity management plans that consider the effect of mammal traits on carbon sequestration and accumulation in soils.

Soil carbon stocks in stable tropical landforms are dominated by geochemical controls and not by land use.

Soil organic carbon (SOC) dynamics depend on soil properties derived from the geoclimatic conditions under which soils develop and are in many cases modified by land conversion. However, SOC stabilization and the responses of SOC to land use change are not well constrained in deeply weathered tropical soils, which are dominated by less reactive minerals than those in temperate regions. Along a gradient of geochemically distinct soil parent materials, we investigated differences in SOC stocks and SOC (Δ14 C) turnover time across soil profile depth between montane tropical forest and cropland situated on flat, non-erosive plateau landforms. We show that SOC stocks and soil Δ14 C patterns do not differ significantly with land use, but that differences in SOC can be explained by the physicochemical properties of soils. More specifically, labile organo-mineral associations in combination with exchangeable base cations were identified as the dominating controls over soil C stocks and turnover. We argue that due to their long weathering history, the investigated tropical soils do not provide enough reactive minerals for the stabilization of C input in either high input (tropical forest) or low-input (cropland) systems. Since these soils exceeded their maximum potential for the mineral related stabilization of SOC, potential positive effects of reforestation on tropical SOC storage are most likely limited to minor differences in topsoil without major impacts on subsoil C stocks. Hence, in deeply weathered soils, increasing C inputs may lead to the accumulation of a larger readily available SOC pool, but does not contribute to long-term SOC stabilization.

Are the Brazilian prevention values for copper and zinc in soils suitable for protecting earthworms against metal toxicity?

The current Brazilian copper (Cu) and zinc (Zn) prevention values (PV) for soil quality do not take into account the ecotoxicological impacts on soil organisms, which suggests these guiding values may not be protective of soil ecological trophic levels. This study assessed the acute (mortality) and chronic toxicity (reproduction), as well as the cumulative (bioaccumulation) potential of Cu and Zn (pseudo-total and available fractions) for earthworms Eisenia andrei in a Tropical Artificial Soil (TAS) and two tropical field soils (Oxisol and Alfisol). Toxicity data based on pseudo-total fractions were compared to PV. The Lowest Observed Effect Concentrations (LOEC) for the mortality endpoint were found at Cu and Zn concentrations higher than their PV (60 and 300 mg kg-1, respectively), regardless of the soil type. However, concentrations lower than PV reduced the reproduction of E. andrei by 20% (compared to the controls) for Cu in all tested soils (EC20s from 31.7 to 51.2 mg kg-1) and by 50% for Zn in Oxisol and Alfisol (EC50s = 225 and 283 mg kg-1, respectively). In TAS, only the EC20 (273 mg kg-1) for Zn was lower than PV. Increases of Cu in earthworm tissues occurred at concentrations higher than PV in all tested soils (LOEC values from 70 to 107 mg kg-1). The same was observed for Zn in TAS (LOEC = 497 mg kg-1), while in the field soils, the increases of Zn in earthworm tissues were lower than PV (LOEC = 131 and 259 mg kg-1 in Alfisol and Oxisol, respectively). We suggest the following: (1) The current Brazilian PV for Cu and Zn are not protective for earthworms (E. andrei) in the field soils tested; (2) PV derived from ecotoxicological assays in artificial soil cannot be representative for Brazilian field soils; (3) Using PV based on the pseudo-total fraction, without a soil-type normalizing factor, may limit the representativeness of this threshold for different soil types.

New approaches to the effects of Si on sugarcane ratoon under irrigation in Quartzipsamments, Eutrophic Red Oxisol, and Dystrophic Red Oxisol.

BACKGROUND: C:N:P homeostasis in plants guarantees optimal levels of these nutrients in plant metabolism. H However, one of the causes to the effects of deficit irrigation is the loss of C:N:P homeostasis in leaves and stems that causes reduction in the growth of sugarcane. Being able to measure the impact of water deficit on C:N:P homeostasis in plants from the stoichiometric ratios of the concentrations of these nutrients in leaves and stems. This loss causes a decrease in nutritional efficiency, but can be mitigated with the use of silicon. Silicon favors the homeostasis of these nutrients and crop productivity. The magnitude of this benefit depends on the absorption of Si by the plant and Si availability in the soil, which varies with the type of soil used. Thus, this study aims to evaluate whether the application of Si via fertigation is efficient in increasing the absorption of Si and whether it is capable of modifying the homeostatic balance of C:N:P of the plant, causing an increase in nutritional efficiency and consequently in the production of biomass in leaves and stems of sugarcane ratoon cultivated with deficient and adequate irrigations in different tropical soils. RESULTS: Water deficit caused biological losses in concentrations and accumulation of C, N, and P, and reduced the nutrient use efficiency and biomass production of sugarcane plants cultivated in three tropical soils due to disturbances in the stoichiometric homeostasis of C:N:P. The application of Si increased the concentration and accumulation of Si, C, N, and P and their use efficiency and reduced the biological damage caused by water deficit due to the modification of homeostatic balance of C:N:P by ensuring sustainability of the production of sugarcane biomass in tropical soils. However, the intensity of attenuation of such deleterious effects stood out in plants cultivated in Eutrophic Red Oxisols. Si contributed biologically by improving the performance of sugarcane ratoon with an adequate irrigation due to the optimization of stoichiometric ratios of C:N:P; increased the accumulation and the use efficiency of C, N, and P, and promoted production gains in biomass of sugarcane in three tropical soils. CONCLUSION: Our study shows that fertigation with Si can mitigate the deleterious effects of deficient irrigation or potentiate the beneficial effects using an adequate irrigation system due to the induction of a new stoichiometric homeostasis of C:N:P, which in turn improves the nutritional efficiency of sugarcane cultivated in tropical soils.

Effect of selenium and soil pH on cadmium phytoextraction by Urochloa decumbens grown in Oxisol.

It has been speculated that selenium (Se) supply can affect cadmium (Cd) 'availability' and increase the Cd tolerance of plants used for phytoextraction, in a pH-dependent process. Thus, we evaluated the interaction Cd-Se and the effects of soil pH in this interaction on plant availability of Cd and phytoextraction efficiency of Urochloa decumbens cv. Basilisk grown in Oxisol. Two soil concentrations of Cd (0.93 and 3.6 mg kg-1) and Se (<0.2 and 1 mg kg-1) and two soil pH (0.01 mol L-1 CaCl2) conditions (4.1 and 5.7) were considered. At both pH, Se supply increased the exchangeable fraction of Cd and decreased the residual Cd fraction. At pH 4.1, the growth of U. decumbens was impaired by Se addition, regardless of Cd exposure. The lower root growth and tillering of U. decumbens exposed to Cd disappeared at pH 5.7 due to uptake of low Se concentrations. Thus, the toxic or beneficial effects of Se on growth of U. decumbens used for Cd phytoextraction depend on the amount of Se assimilated. The Cd phytoextraction efficiency of U. decumbens was not improved by Se supply, regardless of soil pH. Therefore, we cannot recommend the application of Se to increase Cd phytoextraction by this grass.

Adaptive neuro-fuzzy inference system (ANFIS) and multiple linear regression (MLR) modelling of Cu, Cd, and Pb adsorption onto tropical soils.

Soils interact in many ways with metal ions thereby modifying their mobility, phase distribution, plant availability, speciation, and so on. The most prominent of such interactions is sorption. In this study, we investigated the sorption of Pb, Cd, and Cu in five natural soils of Nigerian origin. A relatively sparsely used method of modelling soil-metal ion adsorption, i.e. adaptive neuro-fuzzy inference system (ANFIS), was applied comparatively with multiple linear regression (MLR) models. The isotherms were well described by Freundlich and Langmuir equations (R2 ≥ 0.95) and the kinetics by nonlinear two-stage kinetic model, TSKM (R2 ≥ 0.81). Based on the values delivered by the Langmuir equation, the maximum adsorption capacities (Qm*) were found to be in the ranges 10,000-20,000, 12,500-50,000, and 4929-35,037 µmol kg-1 for Cd, Cu, and Pb, respectively. The study revealed significant correlations between Qm* and routinely determined soil parameters such as soil organic carbon (Corg), cation exchange capacity (CEC), amorphous Fe and Mn oxides, and percentage clay content. These soil parameters, combined with operational variables (i.e. solution/soil pH, initial metal concentration (Co), and temperature), were used as input vectors in ANFIS and MLR models to predict the adsorption capacities (Qe) of the soil-metal ion systems. A total of 255 different ANFIS and 255 different MLR architectures/models were developed and compared based on three performance metrics: MAE (mean absolute error), RMSE (root mean square errors), and R2 (coefficient of determination). The best ANFIS returned MAEtest 0.134, RMSEtest 0.164, and R2test 0.76, while the best MLR returned MAEtest 0.158, RMSEtest 0.199, and R2test 0.66, indicating the predictive advantage of ANFIS over MLR. Thus, ANFIS can fairly accurately predict the adsorption capacity and/or distribution coefficient of a soil-metal ion system a priori. Nevertheless, more investigation is required to further confirm the robustness/generalisation of the proposed ANFIS.

Evaluation of temperature corrections for pesticide half-lives in tropical and temperate soils.

Temperature is a key factor that influences pesticide degradation. Extrapolating degradation half-lives (DT50) measured at a given temperature to different temperatures remains challenging, especially for tropical conditions with high temperatures. In this study, the use of the standard Arrhenius equation for correcting temperature effects on pesticide degradation in soils was evaluated and its performance was compared with that of alternative Arrhenius-based equations. To do so, a database of 509 DT50 values measured between 5 and 35 °C for 32 pesticides on tropical and temperate soils was compiled for the first time through an extensive literature search. The temperature correction models were fitted to the database using linear mixed regression approaches that included soil type and compound effects. No difference in the temperature dependence of DT50 between tropical and temperate soils was detected, regardless of the model. A comparison of the prediction performances of the models showed that constant activation energy (Ea) cannot be considered valid for the whole range of temperatures. The classical Arrhenius equation with an Ea of 65.4 kJ.mol-1, as recommended by the European Food Safety Authority (EFSA), was shown to be valid for correcting the DT50 only for temperatures ranging from 5 to 20 °C. However, for temperatures greater than 20 °C, which are common in tropical environments, the median Ea was significantly lower at 10.3 kJ.mol-1. These findings suggest the need to adapt the standard temperature correction of the European pesticide risk assessment temperature procedure when it is applied in tropical settings.

Arsenic pools in soils under native vegetation on a steatite outcrop in Brazil.

Pristine soils under native vegetation can present high levels of potentially toxic elements when developed from the weathering of some unusual parent materials, especially ultramafic rocks and some metal ores. Here, we used various selective extractions in order to study the partition and potential availability of As in eight soils developed from steatite (a talc-rich rock) on an ultramafic hill in Brazil. Soils varied from shallow Entisols on the summit to Inceptisols and Oxisols on slopes and footslopes, where total As contents (determined by X-ray fluorescence) reached levels as high as 225 mg kg-1, which might raise concerns about their potential agricultural use and occupation. Despite these high values for pristine soils, water- and Mehlich-available As were nil or negligible in all soils, whereas oxalate-extractable As reached a maximum 4.2 mg kg-1, and the highest semi-total (nitric acid digestion) was 9.3 mg kg-1. However, As relative availability (compared to total As) varied widely among soils, with one Inceptisol (with a total 11-19 mg kg-1) reaching 100% of its total As extractable by nitric acid, whereas an Oxisol showed <0.1% in nitric acid extract. Generally, we can conclude that, in soils with the highest total As concentrations, most As is contained within resistant, coarse phases such as primary magnetite, chromite and others, and a minor but still considerable part is bound to secondary Fe oxides. Thus, despite the unusually high As contents for soils under pristine savannic and forest native vegetations, the different As pools assessed here apparently do not raise immediate concerns where ultramafic rocks rich in Fe oxides give rise to soils under tropical climate. However, it is theoretically possible that subsoil saturation and Fe oxide reduction release some As in ground- and surface waters, which deserves further investigation.

Effects of nutrient (N, P, K) fertilization on the dynamics of fine roots in tropical rain forests with different soil texture in the Colombian Pacific region / Efectos de la fertilización con nutrientes (N, P, K) sobre la dinámica de raíces finas en bosques húmedos tropicales con diferente textura del suelo en la región del Pacífico colombiano

Abstract Introduction: Fine root dynamics include production, turnover and decomposition; they are crucial to forest health, affect the entire biogeochemical complex of the ecosystem, and consequently, they substantially affect carbon balance. However, the influence of environmental factors and soil nutrient limitation on fine roots presents considerable uncertainties and has not been studied in tropical forests with more than 7 000 mm annual rainfall. Objective: To measure the effect of fertilization on fine roots in the high precipitation Chocó forest. Methods: We worked in two sites of the Chocó region, Colombia (August 2014-May 2015), where rainfall exceeds 10 000 mm per year. We applied five fertilization treatments (N, P, K, NPK and Control) to two soil type plots. Soil cylinders were removed at pre-established intervals to measure roots. Results: Phosphorus applications increased fined roots; and more fine roots were produced in sandy than in loam soil. The effects of fertilization were related, but not clearly determined by edaphic conditions. Conclusions: In this Chocó forest, the production of fine roots was higher in sandy and nutrient-rich soils but belowground net primary productivity was limited by the content of edaphic Phosphorus.

Resumen Introducción: La dinámica de las raíces finas incluye producción, rotación y descomposición; son cruciales para la salud de los bosques, afectan todo el complejo biogeoquímico del ecosistema y, en consecuencia, afectan sustancialmente el balance de carbono. Sin embargo, la influencia de los factores ambientales y la limitación de nutrientes del suelo en las raíces finas presenta incertidumbres considerables y no se ha estudiado en bosques tropicales con más de 7 000 mm de precipitación anual. Objetivo: Medir el efecto de la fertilización en las raíces finas en el bosque chocoano de alta precipitación. Métodos: Se trabajó en dos sitios de la región del Chocó, Colombia (agosto 2014-mayo 2015), donde las precipitaciones superan los 10 000 mm anuales. Se aplicaron cinco tratamientos de fertilización (N, P, K, NPK y Control) a dos parcelas por tipo de suelo. Los cilindros de suelo se retiraron a intervalos preestablecidos para medir las raíces. Resultados: Las aplicaciones de fósforo aumentaron las raíces finas; y se produjeron más raíces finas en suelos arenosos que en francos. Los efectos de la fertilización estuvieron relacionados, pero no claramente determinados por las condiciones edáficas. Conclusiones: En este bosque chocoano, la producción de raíces finas fue mayor en suelos arenosos y ricos en nutrientes, pero la productividad primaria neta subterránea estuvo limitada por el contenido de fósforo edáfico.

Soil fertility in slash and burn agricultural systems in central Mozambique.

Slash and burn is a land use practice widespread all over the world, and nowadays it is formally recognized as the principal livelihood system in rural areas of South America, Asia, and Africa. The practice consists of a land rotation where users cut native or secondary forest to establish a new crop field and, in some cases, build charcoal kilns with the cut wood to produce charcoal. Due to several socio-economic changes in developing countries, some scientists and international organizations have questioned the sustainability of slash and burn since in some cases, crop yield does not justify the soil degradation caused. To estimate the soil quality in agricultural and forest soils at different ages of the forest-fallow period (25, 35, and 50 years), this survey investigated rural areas in three locations in Manica province, central Mozambique: Vanduzi, Sussundenga, and Macate. Soil profiles were trenched and sampled with a pedological approach under crop fields and forest-fallow. The chronosequence was selected to test the hypothesis that the increase in forest-fallow age causes an improvement of soil fertility. Results highlighted discrete variations among locations in mineralogy, Al- and Fe-oxyhydroxides, sand, silt, pH, total organic carbon, humic carbon, total nitrogen, available phosphorous, chloride, nitrate, fluoride, and ammonium. Few differences in mineralogy, Fe-oxyhydroxides, available P, chloride, and nitrate were detected between crop fields and forest-fallow within the same location. Such differences were mostly ascribed to intrinsic fertility inherited from the parent material rather than a longer forest-fallow period. However, physicochemical soil property improvement did not occur under a forest age of 50 years (the longest forest-fallow considered), indicating that harmonization of intrinsic fertility and agronomic practices may increase soil organic matter and nutrient contents more than a long forest-fallow period.

Zinc isotopic signature in tropical soils: A review.

The micronutrient cycling in tropical latitudes is an issue of great concern because tropical soils are not only suffering micronutrient deficiency, but also influencing the global cycling of trace metals. With the development of stable isotope techniques, Zn isotopic composition (δ66Zn) has been an powerful tool to interpret the Zn behaviour, signature, and cycling in soils. This review compiles δ66Zn ratios of ten types of soils from both tropical and non-tropical latitudes, to (i) discuss the Zn isotopic signature in tropical soils and at the interfaces of soil-plant-river-ocean, (ii) disclose the Zn mass balance in tropical latitudes, and (iii) provide an implication for the eco-environmental effects of Zn cycling in tropical latitudes. Zinc isotopic signature is constrained by soil constituents. Our review summarized that the precipitation of secondary Fe oxides and organic complexation in the aqueous phases are likely to result in the preferential preservation of light Zn isotopes in tropical soils. The extreme weathering and material leaching of tropical soils can remove large amounts of Zn and thus result in Zn deficiency in tropical latitudes and pose risks to plant growth. The removed Zn is likely to influence the instantaneous riverine δ66Zn heavier than that of the crustal average. However, the modern oceanic δ66Zn will ultimately approach those of the parent materials by mass balance, at large geological timescales. Future direction should be concerned with the isotopic studies on Zn speciation in tropical soils and the association of isotopic ratios with the flux of Zn to quantitatively estimate of the Zn mass balance in tropical regions. The prospect of this review is to help solve the issue of plant micronutrition, as well as riverine and marine bio-availablity.

Revamping highly weathered soils in the tropics with biochar application: What we know and what is needed.

Fast weathering of parent materials and rapid mineralization of organic matter because of prevalent climatic conditions, and subsequent development of acidity and loss/exhaustion of nutrient elements due to intensive agricultural practices have resulted in the degradation of soil fertility and productivity in the vast tropical areas of the world. There is an urgent need for rejuvenation of weathered tropical soils to improve crop productivity and sustainability. For this purpose, biochar has been found to be more effective than other organic soil amendments due to biochar's stability in soil, and thus can extend the benefits over long duration. This review synthesizes information concerning the present status of biochar application in highly weathered tropical soils highlighting promising application strategies for improving resource use efficiency in terms of economic feasibility. In this respect, biochar has been found to improve crop productivity and soil quality consistently through liming and fertilization effects in low pH and infertile soils under low-input conditions typical of weathered tropical soils. This paper identifies several advance strategies that can maximize the effectiveness of biochar application in weathered tropical soils. However, strategies for the reduction of costs of biochar production and application to increase the material's use efficiency need future development. At the same time, policy decision by linking economic benefits with social and environmental issues is necessary for successful implementation of biochar technology in weathered tropical soils. This review recommends that advanced biochar strategies hold potential for sustaining soil quality and agricultural productivity in tropical soils.

Potentially toxic elements and rare earth elements in sandy soils from the Brazilian Cerrado.

This study aims to determine the concentration of potentially toxic elements (PTEs) and rare earth elements (REEs) in Brazilian sandy soils under the Cerrado at the Parnaíba-São Francisco Basin transition. We also explored the geochemical correlation between these elements and pH, cation exchange capacity (CEC), total organic carbon (TOC), sand, clay, oxides from secondary minerals, and chemical index of alteration for each basin. Mineralogical, physical, and chemical analyses were used to examine PTE and REE geochemistry in six sand soil profiles from the Brazilian Cerrado. The background concentrations of these elements are low, but soils from the Parnaíba Basin have higher concentrations of PTEs than soils from the São Francisco Basin. In soils from the Parnaíba Basin, mainly Al2O3 has relevance in the V and Cr geochemistry, as these elements increase with increasing Al2O3 content. On the other hand, the REEs have CEC as a soil attribute of higher relevance in the geochemistry of those elements is soils from the Parnaíba Basin, and this relevance divides the TOC, Fe2O3, and TiO2 minerals from the clay fraction. In soils from the São Francisco Basin, the geochemistry of PTEs is possibly associated with kaolinite, especially Cu, V, and Zn. In contrast, the Ba concentration was associated with the presence of feldspar. Unlike soils from the Parnaíba Basin, the REEs do not correlate with the studied soil attributes, except for Ho and Lu. Ho had a positive association with Al2O3. Ho and Lu are negatively related to the presence of iron oxides.

Rare earth elements (REEs): geochemical patterns and contamination aspects in Brazilian benchmark soils.

Rare earth elements have been increasingly used in modern societies and soils are likely to be the final destination of several REE-containing (by)products. This study reports REE contents for topsoils (0-20 cm) of 175 locations in reference (n = 68) and cultivated (n = 107) areas in Brazil. Benchmark soil samples were selected accomplishing a variety of environmental conditions, aiming to: i) establishing natural background and anthropogenic concentrations for REE in soils; ii) assessing potential contamination of soils - via application of phosphate fertilizers - with REE; and, iii) predicting soil-REE contents using biomes, soil type, parent material, land use, sand content, and biomes-land use interaction as forecaster variables through generalized least squares multiple regression. Our hypotheses were that the variability of soil-REE contents is influenced by parent material, pedogenic processes, land use, and biomes, as well as that cultivated soils may have been potentially contaminated with REE via input of phosphate fertilizers. The semi-total concentrations of REE were assessed by inductively coupled plasma mass spectrometry (ICP-MS) succeeding a microwave-assisted aqua regia digestion. Analytical procedures followed a rigorous QA/QC protocol. Soil physicochemical composition and total oxides were also determined. Natural background and anthropogenic concentrations for REE were established statistically from the dataset by the median plus two median absolute deviations method. Contamination aspects were assessed by REE-normalized patterns, REE fractionation indices, and Ce and Eu anomalies ratios, as well as enrichment factors. The results indicate that differences in the amounts of REE in cultivated soils can be attributed to land use and agricultural sources (e.g., phosphate-fertilizer inputs), while those in reference soils can be attributed to parent materials, biomes, and pedogenic processes. The biomes, land use, and sand content helped to predict concentrations of light REE in Brazilian soils, with parent material being also of special relevance to predict heavy REE contents in particular.

Humic Acid Improves Zn Fertilization in Oxisols Successively Cultivated with Maize-Brachiaria.

Zinc (Zn) is an essential micronutrient for plant growth, and Zn deficiency is a global issue, especially in tropical soils. This study aimed to investigate the effects of humic acid (HA) and the Zn addition (Zn sulfate + HA) on the growth of maize and brachiaria in two contrasting Oxisols. The potential complexation of Zn sulfate by HA was evaluated by Fourier-transform infrared (FTIR) spectroscopy analysis. Zinc content and its availability in solution and the shoot and root biomass of maize and brachiaria were determined. FTIR spectroscopy revealed the complexation of Zn sulfate by HA through its S and C functional groups. In both Oxisols, solution Zn increased due to the combined use of Zn and HA. In a soil type-dependent manner, maize biomass and Zn in its shoots were affected only by the exclusive use of Zn fertilization. In the Yellow Oxisol, brachiaria growth and Zn accumulated in its shoot were positively affected by the combined use of Zn fertilization with HA. In the Oxisol with lower organic matter content, HA can assure adequate supplying of residual Zn, while increasing growth of brachiaria cultivated in sequence to maize.