Distribution of charred organic matter in the humic fractions of a fire affected xerochrept

Abstract

The frequent occurrence of wild fires in the Mediterranean region is expected to have a long term effect on carbon sequestration in the affected soils. In order to reveal this influence, the quality and quantity of humic material from a fire-affected (FA) and a fire-unaffected (FU) A-horizon of Xerochrepts from the Sierra de Aznalcollar (Spain) were analyzed. The wild fire resulted in an increase of organic C and N in the mineral soil. Solid-state 13C NMR spectroscopy revealed enrichments for all carbon compound classes, including 0- and N-alkyl C, although it was highest for aromatic C. This allows the assumption that the necromass, incorporated into the A horizon after the fire event contained considerable portions of unburned and/or partly charred remains. Most of the charred and partly charred material accumulated in the fraction insoluble in NaOH. Although a higher yield of organic matter soluble in NaOH but insoluble at low pH was extracted from the soil at site FA than from that of site FU, no major fire-induced alteration of its carbon composition was detected. This may evidence that mainly organic matter derived from slightly charred or uncharred organic matter was isolated with this fraction. Considering the N-fraction, the fire changed the chemical composition of all humic extracts. Those alterations are expressed in an accumulation of pyrrole-type N, although amide N was found to dominate. This may indicate that not all peptide-structures of the necromass were transformed by the fire, but it is also likely that those amides are part of melanoidized compounds, known to be formed during thermal treatment of sugar and amino acid containing mixtures. The fact that pyrrole-type N remained undetected in the solid state 15N NMR spectra of fire-unaffected soil organic matter supports their pyrogenic origin. Thus, the identification of such compounds in soil organic matter by means of solid-state 15N NMR spectroscopy may be taken as a first indication for the presence of pyromorphic humic material, which often is difficult to obtain by solid-state 13C NMR spectroscopy, due to overlapping of signals derived from char and from biogenic humic substances.