Soil organic matter alterations resulting from post-fire restoration actions

4 figuras.- 3 referencias.- Póster presentado en el 21st. International Symposium on Analytical and Applied. Pyrolysis, 9-12 May 2016, Nancy, France
Burned soils restoration/rehabilitation techniques include tillage, tree logging, reforestation with native or non-native species, and other management. In some cases, restoration may cause additional impacts on soils, producing more severe damages than those caused directly by fire. Triggered soil erosion processes, loss of soil fertility and alterations in the hydrological cycle are among these negative impacts. Analytical pyrolysis (Py-GC/MS) together with graphic interpretation tools like van Krevelen¿s diagram [1] may be used to shed light on alterations caused by burning and restoration plans to soil organic matter (SOM). The objective of this research is to study fire impacts on SOM in a sandy soil under pine (Pinus pinea) forest affected by a severe fire in August 2012 in Doñana National Park (SW Spain). mm) collected from burned adjacent areas before (B) and after restoration practices (BR) (logging and extraction of burned trees). An adjacent unburned (UB) area was used as a control. Bulk soil samples and its fractions were studied by Py-GC/MS and the results were interpreted taking advantage of a van Krevelen diagram using the H/C and O/C atomic ratios calculated from the chemical formulas of compounds as inferred from their mass spectra. The percentage of main compound families obtained by analytical pyrolysis, including alkane/alkene pairs, unspecific aromatic compounds (UAC), peptides, methoxyphenols, fatty acids, carbohydrate-derived, N-compounds and polycyclic aromatic hydrocarbons (PAH), show conspicuous differences among bulk samples from the B, BR and UB control areas. Compared to the UB site, the samples from the B site showed lower proportions of lignin methoxyphenols and increased ones of UAC and PAH. It could be hypothesized that fire produced defunctionalization of methoxyphenols, increasing the proportion of soil recalcitrant compounds. In all cases, the coarse fraction showed a high content of carbohydrate-derived compounds and methoxyphenols followed by fatty acids that could be explained by litter inputs after the fire [2]. While the coarse fraction from BR soil showed the highest proportion of methoxyphenols, the UB coarse fraction showed the major proportions of alkyl compounds. On the other hand, although the SOM composition from fine fractions varied largely from one area to another, SOM was found generally more altered than in the coarse fractions. In fact, SOM from the UB fine fraction shows a high proportion of alyl compounds and comparatively lower of carbohydrate- and lignin-derived compounds. In contrast, SOM from B fine fraction did not show a high contribution of alkyl compounds, which may be due to a thermal cracking of alkane/alkene chains [3]. In addition, a high contribution of UAC and PAH was observed in this B fine fraction. Finally, the fine fraction of BR samples showed a high contribution from labile compounds, possibly due to topsoil mixing caused by tilling during rehab actions. The van Krevelen diagram applied to pyrolysis compounds differentiates among chemical processes caused by fire and rehab practices. Among the main processes identified are condensation (oxidation and hydration), demethylation (oxidation and dehydrogenation) and oxidation-reduction. Figure: van Krevelen diagram based in the chemical formula of the Pyrolysis compounds as inferred from their mass spectra. Atomic ratios are mean values of detected compounds [1] DW. van Krevelen. Fuel 29, 269-284, 1950. [2] NT. Jiménez-Morillo et al. Land Degrad Develop. 2014. DOI: 10.1002/ldr.2314 [3] JA. González-Pérez et al. Org Geochem. 39: 940-944. 2008. DOI: 10.1016/j.orggeochem.2008.03.014.
Projects GEOFIRE (CGL2012-38655-C04-01) and POSTFIRE (CGL2013-47862-C2-1-R). The Spanish Ministry for Economy and Competitiveness and CSIC for their support. N.T Jiménez-Morillo is funded by a FPI research grant (BES-2013-062573)