Mercury in Tropical Soils: modeling and speciation in the Amazon biome and physiological effects in plants

Abstract

Mercury (Hg) pollution poses global risks. Tropical soils, which are very weathered and rich in Fe oxides, have increased retention capacity of Hg, and high concentrations of Hg are found in some soils of the Amazon biome, even far from anthropogenic activities. The objective of this study was to evaluate the geochemistry of different soils of the Amazon biome, its relationship with the concentrations and species of Hg, and the use of the Random Forest algorithm (RF) to prediction the concentrations of Hg in soils. In addition, this study seeks to elucidate the physiological modifications of the species Avena sativa (oat) and Phaseolus vulgaris (common bean) cultivated in Oxisols contaminated by Hg. Soil samples from nine sites in the Amazon biome were collected and the physicochemical characterization and mineralogy was determined. The quantification of total Hg in soil (Hgtotal) and Hg species (Hg0 + Hg+ and Hg2+) was performed through direct mercury analyzer by thermosetting. As for the analysis of physiological modifications, the experiments were carried out in greenhouse in a Rhodic Acrudox (RA) and a Typic Hapludox (TH) with increasing concentrations of HgCl2. At the end of the experiment, the impact of Hg on photosynthesis, nutritional status, and oxidative stress was determined. The Amazon soils presented in the clay fraction: kaolinite, goethite, hematite, gibbsite, and quartz, besides mica in the soils of Acre and in a soil of Amazonas. Hgtotal in the studied soils ranged from 21.5 to 208.3 μg kg-1 (median of 103.9 μg kg-1). The RF algorithm determined was effective in the prediction of Hg. The most importance variables to the prediction of Hg were elevation, niobium and Al2O3 content, via portable X-ray fluorescence, and magnetic susceptibility. The main Hg species found in soils is Hg2+, quantified at temperatures above 300 ºC. In respect to the experiments with plants, the lower Hg sorption in the TH soil resulted in toxicity-like responses, whereas acclimation-like responses were observed in plants cultivated in the RA, confirming how the physicochemical properties of soils interfere with Hg sorption in tropical soils.