Thermal Transformation of Soil Organic Matter by Natural Fires and Laboratory-Controlled Heatings

Abstract

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.

Effects of forest fires on water extractable organic matter and humic substances from Mediterranean soils: UV–vis and fluorescence spectroscopy approaches

“…Consequently, the fires have an impact on the quality of the HS, particularly on the most recalcitrant fractions (HA and FA), inducing an increase of the aromaticity of the compounds. This result is absolutely concordant with previous works (Almendros et al, 1990Baldock and Smernik, 2002;Gonzalez-Perez et al, 2004;Gonzalez-Vila and Almendros, 2003;Knicker et al, 2005;Vergnoux et al, 2009, in press). …”

“…These results on Hpi are in good agreement with those observed for SUVA 254 which are higher for NVR-Hpi than C-Hpi, indicating an increase of the aromaticity due to the fires. As it was found for the HS (Almendros et al, 1990Baldock and Smernik, 2002;Gonzalez-Perez et al, 2004;Gonzalez-Vila and Almendros, 2003;Knicker et al, 2005;Vergnoux et al, 2009, in press), the Hpi aromaticity is impacted by forest fires, leading to a significant increase. FEEM were recorded for each samples and we compared all the data.…”

“…For example, fire effects on humic substances have mainly shown an increase in aromaticity and a removal of oxygen-containing functional groups (Almendros et al, 1990Baldock and Smernik, 2002;Gonzalez-Perez et al, 2004;Gonzalez-Vila and Almendros, 2003;Knicker et al, 2005;Vergnoux et al, 2009Vergnoux et al, , 2010. The more recalcitrant and aromatic structures formed by fires derive from the alteration of carbohydrates (Almendros et al, 1997;Baldock and Smernik, 2002;Gonzalez-Vila et al, 2001), lipids, alkylated macromolecules and peptides . Consequently, some of these newly formed structures, initially non-humified, can become extractable like the humic fraction (Almendros et al, 1990;Fernandez et al, 1997).…”

Soil organic matter in a ponderosa pine forest with varying seasons and intervals of prescribed burn

“…Combustion and volatilization may result in losses of SOM and may also mineralize organically bound elements such as N, P, and base cations, which are then available for uptake by plants or leaching from the soil (DeBano et al, 1998). Residual unburned SOM altered by heating is left with disproportionate losses of hydrogen and oxygen relative to C and increased aromaticity (Almendros et al, 1990;Baldock and Smernik, 2002;Almendros et al, 2003;Gonzalez-Vila and Almendros, 2003;Gonzalez-Perez et al, 2004;Knicker et al, 2005). Increasing heating severity on SOM has been shown to decrease the oxygen content, thereby reducing FA and HA fractions while increasing aromaticity and humin content (Almendros et al, 1990;Fernandez et al, 1997Fernandez et al, , 2001Fernandez et al, , 2004.…”

Effects of two soil reclamation techniques on the distribution of the organic N compounds in a 15N labelled burnt soil

“…5A-D). Jointly, these contrasting tendencies in LS and BLS treatments slightly mitigated the huge differences that the soil combustion established between both treatments in this fraction, as was reported for this (Castro et al, 2006) and other soils (DeBano et al, 1979;Carballas et al, 1993;Sánchez and Lázzari, 1999;González-Vila and Almendros, 2003;Prieto-Fernández et al, 2004). The atom % 15 N in excess for the α-amino acid-N of both H1 and H2 steadily decreased during the Lolium and Pinus growing periods in the unburnt soil (LS N LS-L N LS-P) whereas in the burnt soil there was no a clear trend.…”