Wildland fire impacts on temporal and spatial patterns of trace elements in smelter-affected semiarid soils

Abstract

Wildland fires affect the trace element cycles at the vegetation-soil interface. We studied the trace elements (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sb, V, Zn) patterns in grass, litter, and soils located near a non-ferrous metal smelter in semiarid Namibia, collected along a chronosequence of fire events occurring in 2012, 2021, and 2023. The soils are not excessively contaminated, but the Pb isotopic analysis confirmed that the legacy mining and smelting are responsible for the metal(loid)s dispersion in the studied area. Comparisons of unburned and burned plots indicated that changes in trace element concentrations were most pronounced in litter and topsoil. The ash color, changes in the trace element concentrations, and their enrichment factors suggest that the fires in the area were of low intensity with temperatures <400 °C. Whereas Hg, and to a lesser extent also As, were partly re-emitted back to the atmosphere, other smelter-derived contaminants concentrated in the ash. The high leaching of As observed in the litter and topsoils (up to 233 µg/L) exceeds water quality guidelines and may be further enhanced by a fire-induced increase in soil pH. Our findings suggest that even moderately contaminated environments in the semiarid savanna, which are vulnerable to burning during the dry period, pose a potential risk to local water resources by mobilizing contaminants following the first rainfall of the wet season.