Short and medium-term effects of a wildfire and two emergency stabilization treatments on the availability of macronutrients and trace elements in topsoil

In NW Spain, a European region with very high fire incidence and erosion risks, the effects on soils of a medium-to-high severity wildfire and two emergency stabilization techniques were studied. In burned plots (control, BS; seeded with cereal, BSS; straw mulched, BSM) and adjacent unburned plots (US), the topsoil (0-2 cm) pH and thirteen NH4Ac-DTPA extractable elements were evaluated at t = 0, 4, 8 and 12 months after the fire. Compared to US, fire increased by 0.3-0.5 units the soil pH which decrease slowly over time, but remaining significantly higher at t = 12 (BS, BSM, BSS > US). Ammonium nitrogen (N) levels were higher (p < 0.05) in burned plots than in US, difference decreasing progressively from 48-fold (t = 0) to 25-fold (t = 12). Although no significant effect of fire was immediately observed, the extractable sodium (Na) and potassium (K) were higher (p < 0.05) in burned plots than in US at t = 4 and t = 8, probably due to cation leaching from the overlying ash. Fire did not modify the extractable magnesium (Mg), but at t = 0 the extractable calcium (Ca) and phosphorous (P) were transiently and significantly higher in burned plots than in US. Extractable aluminum (Al), iron (Fe), copper (Cu), cobalt (Co) and zinc (Zn) were lower and manganese (Mn) was higher in burned plots than in US. Neither seeding nor mulching significantly modified the topsoil concentrations of the elements considered. The PCA revealed that BS, BSM and BSS became more similar to US over the study period due to a rapid decrease in extractable Ca and Mg and a slow decrease in extractable Mn and NH4+-N. At t = 12, the most notable differences between burned plots and US were in the concentrations of extractable Al and Zn. Data suggest that at least another 4-8 months will be required for full recovery of the burned plots to unburned conditions.