ANALYTICAL PYROLYSIS (Py-GC/MS) FOR RAPID MONITORING OF SOIL ORGANIC MATTER RECOVERY IN A CHRONOSEQUENCE OF SEMIARID MEDITERRANEAN BURNED FORESTS

Wildfire is a frequent environmental perturbation in Mediterranean ecosystems, which severely affects physical and chemical soil properties. In particular, the composition and properties of soil organic matter (SOM) are those experiencing the most important transformations. In the short term, most fire‐induced alterations frequently contribute to the loss of soil quality and productivity. For these reasons, post‐fire soil management requires local information about the natural post‐fire evolution of the different soil types. Most recent studies have focused on the effects of fire in SOM composition, but research about progressive changes in the course of the restoration is scarce. In August 2012, a wildfire affected a forest area of ca. 90 ha in Montellano (Seville, SW Spain). The predominant vegetation consisted of Pinus pinaster, Pinus halepensis and Eucalyptus globulus. Soil samples were collected 1 month and 25 months after the fire. Sixteen months after the wildfire heavy machinery was used to remove burnt trees and plant residues as part of the post‐fire rehabilitation practices. The analysis of SOM molecular composition was done using analytical pyrolysis (Py‐GC/MS), i.e., a versatile on‐line analytical facility which requires no sample pretreatment. Pyrochromatograms of whole soil samples collected 2 years after the fire showed that SOM was still altered by fire, i.e., soil couldn’t be considered as restored. The evolution was illustrated by an improved Van Krevelen’s graphical‐statistical method, where the fire damage levels—or the soil recovery status—were visually compared as surface density plots in the space defined by compound‐specific atomic H/C and O/C ratios of the Py‐GC/MS molecules, either as autocumulative total abundances, or after subtracting the values at the different stages of the chronosequence. Our results indicate that rehabilitation practices carried out after the fire, which included the removal of burnt vegetation, far from helping s