In the Mediterranean basin forest fires are considered among the main disturbance factors and are the cause of both immediate and lasting environmental impacts. This is in part caused by the transformation of soil physical, chemical and biological characteristics associated to changes in soil organic matter (SOM); the most reactive and functional soil fraction (González-Pérez et al., 2004, 2008 and references therein). In this communication the effect of wildfire in SOM isotopic signature is analysed in an experiment comparing fire-affected and nearby nonaffected sites. The soil type was an Arenosol (WRB 2006) in a Mediterranean climate (Doñana National Park, Andalusia, SW Spain) with cork oak (Quercus suber) as the main vegetation cover. Whole soil samples (composite samples from 4 replicates) taken under the canopy of cork oak stands and different size fractions (sieving) were studied. The fire event was severe and occurred in the summer of 2012. Carbon stable isotopic analysis (δ13C IRMS) of bulk samples (whole soil and particle-size fractions) was carried out in a Thermo-Scientific Flash 2000 HT elemental micro-analyser coupled to a continuous flow Delta V Advantage isotope ratio mass spectrometer (IRMS). Direct pyrolysis compound specific isotopic analysis (PyCSIA) of carbon (δ13C) and hydrogen (δD) was also conducted with a double-shot pyrolyzer (Frontier Laboratories, model 3030D) attached to a Trace Ultra GC system. At the end of the chromatographic column and in order to locate specific peaks within the chromatogram, the flux was divided and 10% diverted to the flame ionization detector (GC/FID) and 90% to a GC-Isolink System equipped with a micro-furnace for combustion (EA) and coupled via a ConFlo IV universal interface unit to the Delta V Advantage IRMS (Py-GC-(FID)-C-IRMS). The identification of specific peaks were inferred by comparing mass spectra from conventional Py-GC/MS (data not shown) with Py-GC/FID and Py-GC/IRMS chromatograms obtained using the same chromatographic conditions.

Projects CGL2012-38655-C04-01 and CGL2008-04296 and fellowship BES-2013-062573 given by the Spanish Ministry of Economy and Competitiveness to N.T.J.M. Dr. J.M.R. is the recipient of a fellowship from the JAE-Doc subprogram financed by the CSIC and the European Social Fund

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