There appears to have been a conference on the topic:
Effects of a low severity prescribed fire on water-soluble elements in ash from a cork oak (Quercus suber) forest located in the northeast of the Iberian Peninsula
By: Pereira, Paulo; Ubeda, Xavier; Martin, Deborah; et al.
Conference: International Meeting on Fire Effects on Soil Properties Location: Marmaris, TURKEY Date:FEB 11-15, 2009
Sponsor(s): Hacettepe Univ, Dept Biol; Ankara Univ, Dept Soil Sci
ENVIRONMENTAL RESEARCH Volume: 111 Issue: 2 Pages: 237-247 Published: FEB 2011
I found several papers with asearch of Web of Science. You might want to read :
Short and medium-term effects of a wildfire and two emergency stabilization treatments on the availability of macronutrients and trace elements in topsoil
By: Xesus Gomez-Rey, Maria; Jesus Gonzalez-Prieto, Serafin
SCIENCE OF THE TOTAL ENVIRONMENT Volume: 493 Pages: 251-261 Published: SEP 15 2014
and
Wildfire effects on extractable elements in ash from a Pinus pinaster forest in Portugal
By: Pereira, Paulo; Ubeda, Xavier; Martin, Deborah; et al.
Wild fire impact on copper, zinc, lead and cadmium distribution in soil and relation with abundance in selected plants of Lamiaceae family from Vidlic Mountain (Serbia)
By: Jovanovic, V. P. Stankov; Ilic, M. D.; Markovic, M. S.; et al.
Certini (Oecologia, vol. 153, p. 1-10, 2014) published a review paper in which the author describes the effect of fire on properties of forest soils. The fire effects in soil organic carbon, physical, physico-chemical and mineralogical properties, and nitrogen, phosporous and other nutrients is discussed by the author.
Pivello et al. (Plant and Soil, 2010) published and interesting article in which the authors have analysed the effect of fires on soil nutrient availability in an open savanna in Brazil. The authors observed that in burned plots the ash releases basic cations, which reduce exchangeable Al, available Fe and Mn.
In these two papers you will find other interesting references that can help you.