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.

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.

Changes in soil organic carbon under perennial crops

This study evaluates the dynamics of soil organic carbon (SOC) under perennial crops across the globe. It quantifies the effect of change from annual to perennial crops and the subsequent temporal changes in SOC stocks during the perennial crop cycle. It also presents an empirical model to estimate changes in the SOC content under crops as a function of time, land use, and site characteristics. We used a harmonized global dataset containing paired-comparison empirical values of SOC and different types of perennial crops (perennial grasses, palms, and woody plants) with different end uses: bioenergy, food, other bio-products, and short rotation coppice. Salient outcomes include: a 20-year period encompassing a change from annual to perennial crops led to an average 20% increase in SOC at 0-30 cm (6.0 ± 4.6 Mg/ha gain) and a total 10% increase over the 0-100 cm soil profile (5.7 ± 10.9 Mg/ha). A change from natural pasture to perennial crop decreased SOC stocks by 1% over 0-30 cm (-2.5 ± 4.2 Mg/ha) and 10% over 0-100 cm (-13.6 ± 8.9 Mg/ha). The effect of a land use change from forest to perennial crops did not show significant impacts, probably due to the limited number of plots; but the data indicated that while a 2% increase in SOC was observed at 0-30 cm (16.81 ± 55.1 Mg/ha), a decrease in 24% was observed at 30-100 cm (-40.1 ± 16.8 Mg/ha). Perennial crops generally accumulate SOC through time, especially woody crops; and temperature was the main driver explaining differences in SOC dynamics, followed by crop age, soil bulk density, clay content, and depth. We present empirical evidence showing that the FAO perennialization strategy is reasonable, underscoring the role of perennial crops as a useful component of climate change mitigation strategies.