Field availability and mobility of metals in Ferralsols developed on ultramafic rock of Niquelândia, Brazil

Jeremie Garnier,Cecile Quantin,Sophie Raous,Edi Guimarães,Thierry Becquer

doi:10.1590/2317-4889202120200092

Abstract

Ultramaﬁc (UM) rocks are deﬁned as igneous rocks that contain more than 90% of maﬁc minerals. Soils derived from ultramafic rock are generally nutrient-deficient and have concomitant high concentrations of potentially phytotoxic trace elements (Ni, Cr, Co, Mn). Consequently, to assess the dynamics of nutrients and metals in the ultramafic complex of Niquelândia (Brazil), soil solutions have been sampled in soils characterized by high Cr(VI) availability. The metal contents in surficial water have also been analyzed to investigate the metals’ leaching and mobility. Soil solutions featured low nutrient contents, a large Ca:Mg imbalance, and high Ni and Cr concentrations. Chromium was present in its toxic dissolved form (Cr(VI)) in the soil and surficial solutions. Metals concentrations were often above the toxic limit for biota and were therefore able to affect soil functioning. Ni behavior in the topsoil appeared to be primarily controlled by organic matter, while Cr was more likely to be released from Fe-oxides by anionic exchange. This result agreed with the Cr(VI) lability assessed using isotopic exchange kinetics in a companion study. In these serpentinic tropical soils, the highly leached Fe-oxide horizons appear to play a large role in the sequestration and diffuse leaching of labile Cr(VI) and Ni, respectively, in the deeper part of soil profiles and the topsoil. At the catchment scale, surficial solutions results suggest that metals may be exported to surrounding ecosystems that are not adapted to these metals.

Highlights

Ecosystems developed on ultramafic (UM) rocks have long been known as unique (1)
The soil solution chemistry was slightly acidic and poorly mineralized in the (B) horizon, the solution was slightly more mineralized inside the topsoil due to the biodegradation of soil organic matter and the surface recycling
Soil solutions were free of Fe, Al, and Mn, which is in agreement with the immobilization of such elements as oxides in unsaturated soil profiles

Summary

Introduction

Ecosystems developed on ultramafic (UM) rocks have long been known as unique (1). Serpentinitic or ultramafic substrates are harsh environments for biota due to their low levels of nutrients (N, P, and K), the imbalance between calcium (Ca) and magnesium (Mg), and the presence of metals, such as nickel (Ni), chromium (Cr) and cobalt (Co) (Whittaker 1954, Brooks 1987, Kazakou et al 2008, Pędziwiatr et al 2018). Tropical soils developed in ultramafic rock have been widely studied due to their edaphic particularities and lateritic Ni mining. During the past few decades, the availability of Ni and hexavalent Cr has been highlighted by laboratory investigations of tropical soils developed in ultramafic rock from New Caledonia (Quantin et al 2002, Becquer et al 2003, Becquer et al 2006). Metal concentrations in “ultramafic” soil solutions remain incompletely and poorly documented (Anderson et al 1973, Johnston and Proctor 1981, Gasser et al 1994), under tropical conditions. A recent investigation of soil solutions from New Caledonia has shown significant concentrations of metals in toxic forms, i.e. free-ion species (Becquer et al 2010).

Methods

Results

Discussion

Conclusion