Rapid changes in the chemistry of ash and underlying soil layers after pile burning in a subalpine forest

Pile burning is a common practice used to reduce woody debris generated by forestry operations. Combustion of the accumulated fuels generates high temperatures that penetrate into soils and produce highly alkaline, nutrient-rich ash. Little is known about the immediate influence of pile burns on underlying soils or the short-term chemical changes that may regulate post-fire soil microbiome and vegetation recovery. Here we present water-soluble solute concentrations extracted from ash, underlying burned organic matter and mineral soil of six burn piles. Sampling began within days of pile burning and continued throughout the first post-fire year. Fresh pile burn ash had pH of ~10 and was concentrated in plant nutrients (N, P, K) and other solutes derived from combusted wood and organic residue. The pH and solute concentrations in ash and burned soil layers declined significantly within months of burning. However, pH and concentrations of K, Ca, SO4, and NO3 in burn pile soils remained elevated relative to nearby unburned areas. Extreme heating and changes in soil structure following pile burns and severe wildfires coupled with the chemical changes we document here will likely have immediate and lasting impacts soil microbial and vegetation communities. The relatively uniform construction of burn piles and their widespread use creates a useful platform for evaluating the response and recovery of soil properties and processes following severe wildfire across a wide range of forest and soil types and climatic conditions.