Transformation of the chemical composition of soil organic matter during and after vegetation fires

Comunicación oral presentada a la citada conferencia, celebrada del 6-10 de octubre, 2003, en Schloss Hohenkammer, Munich (Germany).

After wildfires, charcoal together with charred plant residues, introduced into the soil system, can lead to a substantial increase of the soil organic matter pool. The charring process is accompanied by drastic chemical alterations of the litter material, manifested in the transfer of biologically available structures into relatively inert aromatic macromolecules. The incorporation of such charred materials into the soil organic matter pool is expected to have a long term impact on C- and N-sequestration, but also due to its differing chemical composition with respect to fresh plant litter, to influence the post-fire humification processes. In order to elucidate the medium term effect of wild-fires on soil organic matter composition, we first examined the chemical alterations induced to soil organic matter by controlled thermal stress. Therefore, a peat sample and several mineral soils were subjected to controlled heating at 350°C for up to 600 s and the changes in distribution patterns of the different N and C forms were characterized by solid state 13C and 15N NMR spectroscopy. The solid-state 13C NMR spectra showed that heating increased the aromaticity of the original samples by conversion of alkyl C and carbohydrates. While for the organic matter rich peat material, aromaticity values typical for heavily charred material were already reached after 150 s of thermal stress, a treatment time of 600 s was needed to reach the same pattern for the mineral soils. Almost all of the organic N in the original samples were attributable to peptide-like material. Solid-state 15N NMR of the heated samples indicated their heat induced progressive conversion into pyrrole- and indole-type compounds. The second part of the study included the comparison of organic matter composition in soils from fire-unaffected and affected sites in Southern and Central Spain. They were taken from one to five years after the most recent fire event. At most sites an increase of C and N content indicated the accumulation of necromass or an enhanced production of litter after the fire event. The determination of the chemical composition of the additional organic material, demonstrated that the fire-induced increase in soil organic matter is mostly attributed to charred residues rich in aromatic structures, but is also caused by the input of compounds derived from fire-unaffected plant residues. For all samples a higher carboxyl C content was found for the additional organic matter than for the laboratory produced charred residues. The latter indicates that after the fire event, oxidation either by microbial activity or abiotic chemical reactions occurred. This may increase its availability for microbial attack and thus decrease the recalcitrant nature of this material. On the other hand, the increasing amount of polar functional groups of those soil constituents could also contribute to an enhancement of their stability due to adsorption to the mineral phase. Characterization of the organic matter of two subsoils from a fire-affected and a control site showed a relatively efficient translocation of the char-derived aromatic structures into deeper horizons. This may occur as colloidal structures with the soil solution or in association with clay. Therefore, although the top horizons are affected by fire to a much higher extent, the translocation of charred remains within the soil asks for a more detailed analysis of subsoils, if a better understanding of the impact of fire on soil organic matter is wanted.

Peer reviewed