Soil Processes, Pedofeatures and Microscale Metal Distributions: Relevant Study of Contaminant-Dynamics Calls for Pedology-Based Soil-Depth Sampling Strategies

Abstract

Short-term variations of soil conditions affect the form, mobility and bioavailability of metal pollutants. Released metals migrate toward depth where they are intercepted or precipitate, leading to variable spatial metal distribution patterns, at a macro-, meso-and microscale. Studies at a mesoscale give access to trace metal (TM) associations induced by pedological processes. Although scarcely documented, such meso-scale studies represent an essential step for relevant environmental risk assessment, halfway between field-and molecular-scale investigations. We argued for such approach by performing optical microscopy and micro-X-ray fluorescence on thin sections from two soils, contaminated either by industrial zinc-smelter waste or by urban wastewater. Consistent correlation between key indicators of pedological processes (Fe, Mn, and Ca) and trace metals (Zn, Pb, and Cu) on some 20 elemental maps of TM-hosting soil constituents and pedofeatures reveal distinct coinciding localizations, illustrating TM-accumulation via interception or (co)-precipitation processes. Micromorphological interpretation of characteristic pedofeatures in subsurface horizons (crystals, argillans, ferrans, and mangans) containing significant amounts of TM provide valuable insight into the contaminant dynamics in terms of lixiviation, colloidal transport, redox conditions, or fungal activity. Our mesoscale approach stresses the importance of pedology-based sampling strategies, instead of systematic soil-depth sampling, for soil contamination research in natural ecosystems.

The transfer of trace metals in the soil-plant-arthropod system

“…Whether trace metals such as Cu, Ni, Cd or Zn enter soil as ions or as components of amendments such as fertiliser or sewage sludge, short-term pedological processes act to change their speciation and bioavailability (e.g., Smolders et al 2009;van Oort et al, 2018). Laboratory studies (e.g., Backes et al, 1995;Qiang et al, 2017) commonly demonstrate that, if metal ions react longer with soil components they desorb less and the desorbed fraction is released more slowly than for shorter reaction times.…”