Wildfire‐induced alterations of topsoil organic matter and their recovery in Mediterranean eucalypt stands detected with biogeochemical markers

Abstract

SummaryThis study addressed fire‐induced changes in topsoil organic matter (SOM) from a eucalypt plantation in Portugal over 2 years by using three complementary biogeochemical techniques: elemental analysis, analysis of biomarkers from the total extractable lipids (TLE) and solid state 13C nuclear magnetic resonance (NMR) spectroscopy. Direct wildfire effects included a marked decrease in soil total organic carbon (TOC) and total nitrogen (TN) content. However, both contents seem to have recovered during the 2 years. Fire also substantially reduced the TLE, producing noticeable changes in its composition. These included the thermal breakdown and cracking of n‐alkyl compounds. Ratios of short‐to‐long n‐alkanes and n‐fatty acid methyl esters (FAMEs) increased and typical carbon number predominance indices for n‐alkanes (odd‐to‐even) and n‐FAMEs (even‐to‐odd) were altered. Furthermore, the relative abundances of certain markers that are plant‐species specific were modified, especially by decreasing terpenoids such as epiglobulol, ledol and globulol, which are characteristic of Eucalyptus globulus. Other differences observed in the burnt soil were the appearance of levoglucosan, a typical marker for the thermal alteration of polysaccharides, larger relative abundances of lignin‐derived compounds (vanillin and methoxyphenols) and the presence of N‐heteroaromatic structures, which suggested the accumulation of ‘black nitrogen’. The 13C NMR spectra indicated that the wildfire produced a considerable increase in the aromaticity and aromatic condensation of the topsoil SOM. This was reflected in a broadening of the signal of aromatic compounds at the expense of O‐alkyl and alkyl‐C compounds. The continuation of these differences in SOM quality during the 2‐year study suggested a slow recovery of soil properties, possibly influenced by a limited recovery of the vegetation after the fire combined with the fire‐enhanced losses of soil.