Molecular characterization of wildfire impacts on organic matter in eroded sediments and topsoil in Mediterranean eucalypt stands

Abstract

Wildfires can increase soil erosion due to the destruction of the vegetation and changes in soil organic matter (SOM) quantity and quality. This fact is especially dramatic in the case of Mediterranean ecosystems. Therefore, the major goal of the present study was to achieve a better understanding of the short-term effects of wildfires on the quantity and quality of organic matter (OM) in the topsoil of burnt Mediterranean eucalyptus plantations in north-central Portugal.

Wildfire had a large impact on slope-scale sediment losses, as they were approximately 30 times higher at the burnt site than at the unburnt site over the first 25 months after the fire. The pH and total nitrogen (TN) content of the burnt topsoil showed a noticeable increase immediately after the wildfire, whereas the total organic carbon (TOC) content revealed the opposite behavior. However, the TOC content showed a partial recovery during the subsequent two years after the fire. The sediments eroded after the fire were consistently more acidic and richer in TOC and TN than the burnt topsoil; they also revealed less pronounced changes with time-since-fire compared with the burnt topsoil.

Analytical pyrolysis analyses indicated that the fire resulted in an enrichment of aromatic compounds, nitrogen (N) constituents, lignin-derived compounds and polysaccharides. The latter is best explained with the input of partially charred OM, ash and decaying parts of plants killed by the fire. The thermal transformation of SOM caused the thermal breakdown and cracking of n-alkane compounds, as was revealed by the increase of the ratio of short-to-long chains and the alteration of the typical odd-over-even carbon predominance indexes. The relative abundance of specific biomarkers for vegetation and, in particular for Eucalyptus globulus, such as terpenoids eucalyptol and globulol decreased drastically or even disappeared from the pyrolysates of the burnt topsoil. These fire-induced changes in the quality of topsoil OM persisted during the whole post-fire period of 25 months. In general, the Py–GC/MS results pointed out that during the first two years after the fire the recovery of the topsoil OM quality was practically negligible.