Effects of forest fires on soil lead elemental contents and isotopic ratios

Abstract

This study described the behavior of Pb and its isotopes in forest soils affected by different temperature wildfires. We collected samples of burned (and unburned) soil and ash in Abiul, central Portugal, in areas affected by different temperatures. The different soil burned severities were assessed in situ.The high-temperature fires consumed all organic matter in the topsoil (down to 5 cm), while lower temperatures did not. All the soil and ash samples were analyzed for their lead (Pb) contents and Pb isotopic compositions using inductively coupled plasma mass spectrometry.The average Pb elemental concentration in the unburned topsoils was 10.7 mg kg−1, and the isotopic composition ranged from 206Pb/207Pb = 1.167 to 1.178. No significant accumulation of Pb was observed in the topsoil (and ash) affected by the high-temperature fire, but their respective ratios increased (soil 206Pb/207Pb = 1.197; ash 206Pb/207Pb = 1.180). However, there was a significant accumulation of Pb (15.3 mg kg−1) in the topsoil and especially in the ash (Pb avg. = 21.8 mg kg−1), affected by the low-temperature fire. The soil 206Pb/207Pb isotopic ratios remained stable but decreased in the ash (206Pb/207Pb = 1.174 and 1.166, respectively).According to the isotopic composition, Pb in the topsoils was of mixed origin (natural and anthropogenic). We assumed that lithogenic Pb (206Pb/207Pb > 1.19) occurred in stable mineral forms while anthropogenic Pb mainly originated from leaded gasoline (206Pb/207Pb < 1.16). Anthropogenic Pb was more easily mobilized due to the volatile nature of the emitted compounds of vehicular Pb (oxyhalogenides). Hence, during the high-temperature burning of the soil, Pb with a lower isotopic signature would be preferentially released from the soil Pb pool. Consequently, resulting soils and ash exhibited an increase in 206Pb/207Pb isotopic ratios. However, ash and soils affected by low temperatures, which did not volatilize anthropogenic Pb, retained their isotopic signatures.This work suggests that it may be possible to determine the temperature of a forest fire from the Pb isotopic signatures of the burned materials.