SEGH Articles

Studying the effects on soil organic matter of wildfires in central Portugal

01 January 2013
The main objective of this study is testing the efficacy of pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) as a fast analytical technique to detect wildfire-induced molecular alterations of the SOM in Mediterranean Leptosols.

 

Sílvia Regina Marques Faria, is a PhD student at the Universidade de Aveiro http:// www.cesam.ua.pt/silviaregina) undertaking a project entitled “WildFIRE effects on forest Soil Organic Matter stocks and losses by runoff”. She was a joint poster winner at the 9th ISEG meeting in Aveiro in July 2012. Her work is in collaboration with (IST-ITN), Sacavém, Portugal and (IRNAS-CSIC), Seville, Spain.

In Mediterranean ecosystems, forest fires are a common phenomenon and widely considered to be the main factor of disturbance. In the case of the Mediterranean Basin, the Iberian Peninsula has the highest risk of wildfire occurrence, and certainly in relative terms Portugal is affected most by wildfires, devastating on average 100,000 ha each year and dramatically larger areas in dry years like 2003 and 2005.

Wildfires result in the transformation of vegetation and litter, leaving charred residues, and thus influence the carbon cycle by changing: (i) the amounts of soil organic matter (SOM); and (ii) the proportions amongst the carbon pools with different degrees of stability. In addition to affecting the carbon cycle, fires also affect the amounts and availability of nitrogen within soil organic matter. Especially char and other highly stable soil carbon pools are dificult to determine with solvent-based methods, so that pyrolysis-based analysis methods offer several advantages.

The main objective of this study is testing the efficacy of pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) as a fast analytical technique to detect wildfire-induced molecular alterations of the SOM in Mediterranean Leptosols.

To this end, the topsoil of two slopes with adjacent patches of burnt and unburnt Maritime Pine (Pinus pinaster) and eucalypt plantations (Eucalyptus globulus) were sampled in the Serra de Lousã, central Portugal. Compounds released by analytical pyrolysis showed alterations in the SOM due to fire. At desorption step (300 ºC), the marked presence of labile carbohydrate- derived compounds and of long chain n-alkanes distribution pointed to the incorporation of fresh plant material and/or low wildfire severity. Pyrolysis at 500 ºC revealed an increase of low molecular weight molecules for certain homologous series in fire-affected soils, suggesting the occurrence of thermal breakdown and cracking of long-chain components. Also the presence of several thermo-labile markers indicated that the wldfire severity had been low. Elemental analysis indicated marked fire-induced increases in total organic carbon (TOC) and total nitrogen (TN) for the pine stand as opposed to noticeable decreases for the eucalypt stands. The pH and CEC results were consistent with the contrast in fire-induced changes in TOC and TN. Probably, this contrast between the two sites was not due to differences in direct fire effects (especially fire severity) but to indirect fire effects i.e. in particular needle/leaf fall from partially combusted tree canopies.

Sílvia Regina Marques Faria

Universidade de Aveiro www.cesam.ua.pt/silviaregina