Short- and medium-term effects of three fire fighting chemicals on the properties of a burnt soil

The impact of three fire fighting chemicals (FFC) on 11 chemical soil properties and on soil recovery (0–2 cm depth) was evaluated 1, 30, 90 and 365 days after a prescribed fire. Five treatments were considered: unburnt soil (US) and burnt soil with 2 l m−2 of water alone (BS) or mixed with the foaming agent Auxquímica RFC-88 at 1% (BS+Fo), Firesorb at 1.5% (BS+Fi) and FR Cross ammonium polyphosphate at 20% (BS+Ap). At t=1 day, soil pH increases in the order USbBS≤BS+Fo, BS+FibBS+ Ap, which was most likely due to the accumulation of ashes, the reduction of organic acids and the cations supplied by FFC. In all burnt treatments, soil pH remained significantly higher than in US up until t=90 days. SOM richness remained similar and constant until t=90 days in all plots, but, probably due to fire-triggered erosion, at t=365 days it was significantly lower in BS+ Ap (C, N), BS and BS+Fo (C) than in US. Immediately after the fire, soil δ15N decreased in all burnt soils (significatively in BS+ Ap) due to the inputs of 15N depleted ashes from leguminous vegetation. Compared with US, soil δ15N increased significantly in all burnt plots between t=90 days (30 days in BS+Ap) and t=365 days, suggesting a medium-term fire-triggered increment of N outputs (15N depleted). As is habitually the case, there was a transient post-fire increase of NH4 +–N levels (significative for BS+ FFC plots) that lasted for 30 (BS, BS+Fo and BS+Fi) to 90 days (BS+Ap). The high initial NH4 +–N levels in BS+Ap (200× that of US; 9–18× those of BS, BS+Fo and BS+Fi), and its persistence can delay the post-fire vegetation recovery due to the toxicity of NH4 + to seeds and seedlings. NO3–N levels changed significantly only in BS+Ap between t=30 and t=90 days due to the nitrification of its large NH4 +–N pool. Except in BS+Ap, whose soil P levels were 70–140× (t=1 days) and 10–20× (t=365 days) higher than in the other treatments, available P content in BS and BS+FFC was not significatively higher than in US. The concentrations of available cations in BS and BS+FFC were higher (not always significatively, except for K) than in US until t=90 days, likely due to ashes- and FFC-derived cations. Contrarily to divalent cations, monovalent cations (more soluble and easily leached) decreased slowly until t=90 days.