Characterisation of dissolved organic matter in water extracts of thermally altered plant species found in box - ironbark forests

This study investigated the chemical structure of extracted dissolved organic matter (DOM) derived from thermally altered plant material found in the box–ironbark environments of central Victoria, namely: Eucalyptus tricarpa, Pinus pinaceae, Isolepsis nodosa, Acacia pycnantha, and Cassinia arcuata. Samples taken from the plants were burnt at 150, 300, and 400°C, under oxidising and reducing conditions in order to represent the variable combustion undergone by plants during bushfire events. The extraction of DOM from the burnt residues into water was conducted over 96 h. During the extraction period, solution concentrations of DOM were measured using HPLC and TOC methods, while the chemistry of the extracted DOM was characterised using ATR-IR. The nature of the extracted DOM depended on the originating species and whether burnt under oxidising or reducing conditions. DOM derived from the residues of the Australian native trees and shrubs (E. tricarpa, A. pycnantha, and C. arcuata) were generally more aromatic and conjugated than the materials extracted from herbaceous I. nodosa and introduced dendrophyte P. pinaceae. When burnt under reducing conditions, fire residues were less polar, indicated by the fact that they were significantly less soluble and ATR-IR spectra spectroscopy showed little OH stretching compared with the extracts obtained under oxidising conditions. Water extracts from the burnt residues sorbed strongly to the mineral goethite, but had less affinity for the kaolinite surface.