Thermal transformation of soil organic matter by natural fires and laboratory-controlled heatings

48 pages, 20 figures, 11 tables, 150 references.

The results of a multi-approach analysis on the effects that wildfires exert on soil organic matter (SOM) properties are presented. The methods used included wet chemical oxidation and therrnal degradation by flash pyrolysis, as well as Fourier-transformed infrared (FT-IR) and solid-state l3C and 15N nuclear magnetic resonance (NMR) spectroscopies. Such destructive and non-destructive analytical techniques were used for the assessment of heat effects to a molecular level applied to representative Spanish forest ecosystems and for the study of the alteration of different organic materials in the course of progressive heating in laboratory simulation experiments. The material s studied included: whole soils and sapric peat, isolated soil humic fractions, lignocellulosic biomass and preparations of cellulose. Among our findings, it can be emphasized: (i) the importance of the abiotic transformation of aliphatic precursors into aromatic macromolecules under present-day environmental conditions, (ii) a therrnal neoforrnation of heterocyclic N-forms, (iii) changes in the solubility properties of the soluble and colloidal SOM fractions, (iv) a preferentialloss of oxygen-containing functional groups and O-alkyl aliphatic structures, and (v) changes in the SOM macromolecular structure, probably related to the accumulation of a resistant alkyl moiety. In general, our results from laboratory experiments agreed with those obtained from soils affected by wildfires in different Continental Mediterranean forest forrnations. The results are discussed in terrns of the natural stabilization mechanisms of the most refractory SOM forrns, which is of particular interest in the study of the global C and N biogeochemical cycles.

The authors wish to thank the Spanish CICyT fOflgrants AMB 1999-0907 and AMB 1999-0226, as well as to the Spanish-German action HA 2000-0059 from the Spanish MCyT for allowing meetings and the mobility of the scientists involved in this research.

Peer reviewed