Abstract
Ultramafic soils are characterized by severe edaphic conditions induced by a low content of essential nutrients, an adverse Ca/Mg ratio, a low water-holding capacity, and high contents of geogenic potentially toxic elements (PTEs), in particular Cr, Ni, and Co. These metals commonly exceed the content limits set by environmental agencies and governments, representing serious environmental risks for ecosystems and human health. In alpine environments, ultramafic soils are characterized by modest thickness and poor horizon differentiation. Several studies on ultramafic soils have shown that their properties may be directly related to the characteristics of the parent rocks, but most of these studies deal with soil chemistry, metal availability, isotopic composition, and pedological characterization. The aim of this research is to investigate how much the geotectonic characteristics of ultramafic bedrocks, such as the degree of serpentinization, metamorphic imprint, and deformation, may affect the mineralogical and chemical variations of ultramafic soils, including the occurrence and potential mobility of the PTEs. Using a multiscale and multi-analytical approach, we fully characterize the properties and mineralogical composition of soil profiles with different ultramafic parent rocks, i.e., partially serpentinized peridotite, massive serpentinites, and foliated serpentinites, sampled within the Voltri Massif High Pressure–Low Temperature (HP–LT) metaophiolite (Western Alps, Italy). Our results, related to soils located at comparable latitude, altitude, landscape position, and pedological environment, outline that the degree of serpentinization, the metamorphic imprint, and the deformation history of the ultramafic parent rocks are key factors influencing soil evolution, mineralogy, and chemistry, as well as PTEs distribution and mobility. Moreover, this study shows that the high content of Cr, Ni, and Co in the studied ultramafic soils has to be considered of geogenic origin and highlights the need for new approaches and methods to obtain indications on the potential contamination of natural or anthropogenic soils.
Highlights
Ultramafic soils developed on peridotite and serpentinite bedrocks are characterized by severe edaphic conditions mainly induced by the (i) low content of essential nutrients (e.g., Ca, K, P, N),(ii) adverse Ca/Mg ratio, (iii) low water holding capacity, and (iv) high contents of geogenic potentially toxic elements (PTEs), in particular Cr, Ni, and Co [1,2,3,4,5,6,7,8,9]
The ultramafic soils studied in this work, which were developed on partially serpentinized peridotites (PSP), massive serpentinites (MS), and foliated serpentinites (FS), revealed significant differences in mineralogy, micromorphology, chemistry, and PTEs distribution
Water regime, topography, and other soil forming factors play a pivotal role in the soil evolution and for their physicochemical properties, our results, related to soils located at comparable latitude, altitude, landscape position and pedological environment, outline that the degree of serpentinization, the metamorphic imprint, and the deformation history of the ultramafic parent rocks are important factors influencing soil evolution, mineralogy, chemistry as well as PTEs distribution
Summary
(ii) adverse Ca/Mg ratio, (iii) low water holding capacity, and (iv) high contents of geogenic potentially toxic elements (PTEs), in particular Cr, Ni, and Co [1,2,3,4,5,6,7,8,9] These metals commonly exceed, by up to one order of magnitude, the content limits laid down by environmental agencies and governments [7,10,11], posing several environmental threats for ecosystems and human health [12,13]. Ultramafic soils occur worldwide in almost all of the 12 orders [14], it is known that most such soils reach a limited development stage, especially in cold and temperate climatic conditions such as alpine environments [15] In these environments, ultramafic soils are commonly characterized by modest thickness and poor horizon differentiation (lithosols, regosols, and rankers) [5,16,17,18]. The bedrock type is the most important factor influencing the mineralogical, geochemical, micromorphological, and pedological properties of these poorly developed soils [5,9,10,20,21,22,23,24]
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