Heat-induced changes in water repellency of litter ash: role of organic functional groups as determined by FTIR spectroscopy

Hydrophobic substances in organic matter (OM) produced by various plant materials cause water repellency (WR). Waxy or oily hydrophobic plant materials are highly prone to wildfire, converting plant litter into ash, and altering the chemical composition of OM and WR. This study examined how temperature and exposure durations to heat affect the chemical composition of hydrophobic plant materials and their influence on WR of the respective burnt ash. Hydrophobic leaf litter samples collected from Japanese cedar (CED) and cypress (CYP) were exposed to a series of temperatures (100, 200, 300, 400, 500, 600 °C) for 20 and 40 min separately (control: 28 °C). WR, OM, and the presence of functional groups with Fourier transform infrared (FTIR) spectroscopy, in the burnt ash were tested. Ash WR and OM content decreased with increasing temperature. WR showed strong positive correlation with OM content. FTIR spectra analysis showed that absorbance peaks change with temperature, exposure duration, and plant species. Absorbance peak A (wavenumbers 3340 cm⁻¹) that disappeared quickly with exposure to heat was identified not to have high contribution to developing WR. Absorbance peaks B (wavenumbers 3020–2800 cm⁻¹), C (wavenumbers 1640–1600 cm⁻¹), and E (wavenumbers 1170–950 cm⁻¹), which disappeared after exposure to 300–400 °C concurrently with the disappearance of extreme WR, were noted as highly related with ash WR. Absorbance peaks D (wavenumbers 1400 cm⁻¹) and F (wavenumbers 872 cm⁻¹) appeared in concurrence with increasing ash wettability. Considering the correlation analysis, functional groups related to peaks B, C, and E were noted as the major contributors to the ash WR of the tested plant species. The findings of this research provide information on how litter ash becomes water-repellent at various forest fire temperatures and give an indication on how water-repellent ash produced through forest fires can influence the surface hydrological parameters of forest floors.