Short- and medium-term effects of fire and fire-fighting chemicals on soil micronutrient availability.

The impact of fire and three fire-fighting chemicals (FFC) on soil micronutrient availability was evaluated 1, 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 (BS) or water with foaming agent Auxquímica RFC-88 at 1% (BS+Fo), Firesorb at 1.5% (BS+Fi) and FR-Cross ammonium polyphosphate at 20% (BS+Ap). Pre-fire contents of available micronutrient were homogeneous among plots and high (Fe, Zn) or insufficient (Co, Cu, Mn) for plant nutrition. At t=1 day, Fe availability decreased greatly in burnt treatments, with significant differences in BS+Fi (-50%) and BS+Ap (-75%), contrasting with Fe richness of the ammonium polyphosphate. The fire induced a significant increase (9-16x) of available Mn in burnt treatments that lasted for at least three months; the FFC effect on soil available Mn was imperceptible, despite the noticeable amounts of Mn they supplied (especially Firesorb and ammonium polyphosphate). In burnt soils, the Fe/Mn ratio also decreased strongly (92-99%) and significantly till t=90 days. A high increase was also found, at t=1 day, for the available Zn in all burnt treatments and, although the ammonium polyphosphate provided more Zn than the Firesorb, the increment was only significant in BS+Fi plots (+100%). Neither fire nor FFC effects on soil Cu availability were found. The slight increase of Co availability in BS, BS+Fo and BS+Ap at t=1 day was followed by a transient decrease in all burnt treatments at t=90 days. Except the Mn and the Fe/Mn ratio in BS+Ap, which remained significantly higher and lower, respectively, the indices of available micronutrients at t=365 days in all burnt soils were similar to the pre-fire levels.