Long-term response of soil microbial communities to fire and fire-fighting chemicals

Wildfires are a global problem that can require the application of fire-fighting chemicals for combating them. These compounds can have negative effects on forest ecosystems; however, there are few studies about their influence on the soil microbial community. The aim of this work is to analyse under field conditions the impact of a prescribed fire and the addition at normal field doses of three different fire-fighting chemicals (foaming agent, ammonium polyphosphate, Firesorb) on different soil properties, 10 years after the fire and retardant addition. The study was performed in a Humic Cambisol under scrubland located in Galicia (NW Spain). Several physical, physicochemical, chemical (water holding capacity, pH, electric conductivity, total C and N, hydro-soluble C and carbohydrates), biochemical and microbiological [biomass C, enzyme activities, respiration, bacterial growth, fungal biomass and growth, biomass and community composition by the phospholipid fatty acid (PLFA) pattern] properties were analysed in the 0–2- and 2–5-cm soil layers. A marked effect of soil depth and no effects of prescribed fire on most analysed properties were observed, suggesting that soil chemical quality was recovered after 10 years, although changes in microbial community composition were still detected 10 years after the prescribed fire and the retardant addition. The PLFA pattern combined with principal component analysis allows us to differentiate the microbial communities according to both soil depth and soil treatment. The ammonium polyphosphate was the fire-fighting chemical with the strongest effects on the composition of soil microbial communities, which is in accordance with marked changes observed in vegetation.