PNNL

Abstract

High-intensity wildfires alter the chemical composition of organic matter, which is expected to be distinctly different for low-intensity prescribed fires. Herein, we used pyrolysis gas chromatography/mass spectrometry (Py-GC/MS), in conjunction with solid-state 13C nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy, to assess chemical alterations from three wildfires and a long-term frequent prescribed fire site. Our results showed that black ash formed by moderate intensity wildfires (450 oC). Both 13C NMR and FT-IR confirmed a higher relative percentage of carboxyl carbon in white ash, indicating the potential for higher water solubility and more mobile carbon, relative to black ash. Compared to wildfires, ash from low-intensity prescribed fire (=315 oC) contained less ArH, PAH and Ntg and more LgC and PhC. Controlled laboratory burning trials indicated that organic matter alteration was sensitive to burn temperature, but not related to fuel type (pine vs. fir) or oxygen absence/presence at high burn temperatures (>400 oC). This study concludes that higher burn temperatures resulted in higher (poly)aromatic carbon/nitrogen and lower lignin/phenol compounds.