Organic matter and wettability characteristics of wildfire ash from Mediterranean conifer forests

• TGA allowed quantifying various pools of organic matter and inorganic carbonates.
• Ash OM represents more resistant C pool, and the IC represents a new C pool.
• Ash water repellency was positively correlated to OM, and negatively to IC.
• OM and IC contents have implications for carbon sequestration and carbon cycling.

Thermogravimetric analysis (TGA) and water repellency (wettability) determination of ash samples collected following wildfires at four different Mediterranean conifer forest sites in eastern Spain were conducted to provide insights into variations in key chemical and physical characteristics of ash. TGA allowed the evaluation of organic matter (OM) and inorganic carbonate (IC) characteristics and their role in ash wettability. Thermogravimetric profiles were recorded for 40 ash samples (ten from each site). The persistence of water repellency of all ash samples was assessed by the Water Drop Penetration Time (WDPT) test. OM contents decreased, whilst OM stability indices and IC contents increased with increasing combustion completeness. The highest carbon fixation in IC was determined at the site that had experienced the most severe fire. WDPT values were positively correlated with OM and negatively with IC contents. All strongly to severely water repellent ash samples had OM/IC ratio above 2. Pools of OM in ash formed during wildfires represent a more resistant pool of C compared to their unburned precursors. Together with the new IC pool formed in ash, both pools could represent a carbon sink triggered by wildfire.