FT-IR spectroscopy reveals that ash water repellency is highly dependent on ash chemical composition

After a fire, an ash layer is commonly present, which influences soil properties and hillslope hydrology. The wettability of ash, which can vary from highly absorbent to water repellent, is an important characteristic in this context. Recent work has suggested that ash wettability is related to its degree of combustion, which in turn, can be expected to determine ash chemical composition. In this paper we therefore examine the relationship between ash water repellency and ash chemical composition. Ten ash samples with different wettability were each taken from four burned Mediterranean forest sites located at Albaida (A), Llíber (L), Navalón (N), and Pinoso (P), in the east of Spain. The persistence of water repellency of samples was assessed by the Water Drop Penetration Time (WDPT) test and Fourier Transform-Infrared (FT-IR) analysis was applied to characterise sample chemical composition. Ash water repellency varied from wettable to severely water repellent and differed in terms of maximum WDPT and number of water repellent samples within the four locations. In all FT-IR spectra obtained, the absorbance bands assigned to organic matter and carbonates were dominant. They are subject to change during progressive combustion. The ratios of relevant peak areas were calculated, i.e. between aliphatic bands in the 3000–2800 cm− 1 region, other organic matter bands in the 1800–1200 cm− 1 region, and of the calcite band at 875 cm− 1. These ratios are related to organic matter and calcite contents in ash samples and proved to be closely linked to the measured WDPT values through an exponential relationship. Organic matter content in all water repellent ash samples was more than twice the carbonate content. From the results obtained it is concluded that the water repellency or wettability of wildfire ash is driven by the ratio of remaining organic matter to emerging carbonates, which in turn, is related to fire severity.

► Water repellency of ash samples corresponds inversely with the fire severity.
► FT-IR spectroscopy was applied to water repellent and wettable ash samples.
► Key changes were observed in the content of organic matter and carbonates.
► WDPT values were strongly sensitive to the organic matter/calcite ratio.
► Threshold demarcating water repellent behaviour is indicated by this ratio.