Effects of wildfire on mercury mobilisation in eucalypt and pine forests

Abstract

Wildfires and subsequent rainfall play an important role in the redistribution of major, minor and trace chemical elements, which could be mobilised from burnt soils and ashes. In particular, the fire-induced release of mercury (Hg) into the environment is relevant to study due to its volatilisation and toxicity. However, the impact of wildfire followed by rainfall on mercury redistribution is poorly documented. Hence, eucalypt- and pine-covered hillslopes (1–3) were surveyed in two burnt areas (Ermida and S. Pedro do Sul, Portugal), as well as in a non-burnt eucalypt slope (Ermida). Top-soils (0–2cm) and ashes were sampled 4 and 14weeks after the fire, the latter following an episode of heavy precipitation. In order to clarify the effect of temperature on Hg release from the soil, a heating experiment was also conducted. Results revealed that 30% of the Hg retained in eucalypt soils was released by the fire, corresponding to a loss of 1.0–1.1gHgha−1 of burnt soil. Levels in burnt eucalypt soils were twice the values registered in burnt pine soils for both fire severities. Mercury in ashes pointed to similar trend between the two types of vegetation. Results from the heating experiment with soil suggest that temperature reached in the wildfire was insufficient to destroy the most stable bonds of Hg-ligands. Mercury concentrations were not correlated with organic matter content, both in soils and ashes. It was estimated that rainfall caused a loss of 1.0gHgha−1 from ashes and an uptake of 0.5gHgha−1 of soil washed out. The difference between the previous estimations, 0.5gHgha−1, corresponds to the quantity of Hg transported to the surrounding area or eventually introduced into aquatic systems. Hence, the results obtained in the present study highlight the importance of wildfire and subsequent rainfall in the mobilisation of Hg in the environment.