Review articleThe effect of fire on soil organic matter—a review
Introduction
Soil is the largest carbon pool on the Earth's surface (2157–2293 Pg). Of this C, ca. 70 % is organic and the remaining consists of carbonates (Batjes, 1996). Soil organic carbon pool doubles that present in the atmosphere (760 Pg) and is about two to three times larger than that in living matter in all terrestrial ecosystems Prentice et al., 2001, Post et al., 1990. Depending on their turnover rates in soil, three conceptual C fractions can be distinguished: active-labile and active-intermediate fractions—that may remain in soil for years or decades—and passive or refractory OM remaining in soil from centuries to millennia. In forest ecosystems, most C is stored in intermediate pools containing materials like wood fragments, litter, or partially decomposed OM that vary in their turnover time Belesdent and Mariotti, 1996, Schulze et al., 2000 (Table 1). Because of the large amount of C stored in soils, small deviations in the proportion of the above different forms may have a significant effect on the global C balance and therefore on climate change.
During the decomposition of the non-humified organic materials (remains of plants, animals and microorganisms) in soil, a variable proportion of organic C (60–80%) is reverted to the atmosphere as CO2. This is a rapid mineralization process that usually takes place during the first year. The remaining, non-mineralized C undergoes slower oxidation processes and, after complex transformations, it either turns into microbial biomass or is stabilized in the form of humic substances. The latter process, referred to as humification, implies severe changes in the stoichiometry and chemical complexity of the original macromolecular materials leading to humic substances with enhanced resistance to biodegradation and mineralisation (Tate, 1987). These changes are not only a consequence of the microbial activity but also of abiotic diagenetic transformations induced by external physical or physico-chemical factors, including fire (Hatcher and Spiker, 1988).
Vegetation fires have several ecological and environmental impacts. Biomass burning is a significant global source of atmospheric gases such as carbon dioxide (CO2) and methane (CH4), which are green-house gases contributing to global warming (Levine, 1994). During forest fires large amounts of CO2 are released to the atmosphere. As the vegetation in the burned ecosystems regrows, CO2 is again removed from the atmosphere via photosynthesis and incorporated into the new vegetative growth with a neat C balance that has been considered null Levine, 1996a, Levine et al., 1995.
The simplest model for biomass burning assumes that the ending products are H2O, CO2 and minerals contained in the ash. However, the complete oxidation of biomass requires conditions for oxygen availability during the combustion process which, in general, do not concur in nature. Under environmental conditions, the combustion is often incomplete and CO, CH4, a complex range of pyrolysis products including hydrocarbons and particulate OM fractions are also produced (Cofer et al., 1997). Therefore, vegetation fires also produce a considerable amount of newly formed C forms in addition to the thermal modification of the previously existing C forms in the ecosystem.
Only a small part of the heat generated during a forest fire is radiated to soil. The nature of the changes in soil depends on both the temperatures reached at different soil depths and the degree of heating that the different soil components can withstand before being altered. The degree of soil heating depends on factors such as the magnitude and duration of energy transferred from the fire to the soil, soil composition (including moisture), structure (porosity), etc. In general, below ground temperatures will rise very slowly due to the fact that dry soil is a very good insulator (DeBano et al., 1998), and to that in moist soils the evaporation of water will not allow a moist layer to go above the water boiling point (Campbell et al., 1995). Humphreys and Craig (1981) studied the soil temperature generated during different fire situations in Australian ecosystems finding that at 1 mm below the soil surface, the temperature only reaches 200 °C even under wildfire conditions whereas controlled burns reach only 125 °C and grasslands only 70 °C. By 2–3 cm, under their conditions, the soil temperature never exceeds the ambient temperature. However, the real effects of temperature on soil OM is a complex issue: Firstly, it is time-dependant (i.e. the effect of 10 s at 1000 °C could or could not be comparable to that for 30 min at 300 °C, this depends on the soil parameter under study, soil moisture and mineralogy and composition of soil OM) and no in-depth studies exists on this particular; secondly, soil temperature during fires is an average value (small OM rich particles may reach more than 1000 °C when burning on the topsoil); thirdly, even in very low intensity fires, the biological activity rebuild the soil aggregates and the OM particles burnt at different temperatures are distributed very quickly within the whole organic horizons.
Distillation of volatiles and loss of organic carbon in soils start at temperatures between 100 and 200 °C Kang and Sajjapongse, 1980, Giovannini and Lucchesi, 1997, above 200 °C the charring process starts. Between 130 and 190 °C lignins and hemicelluloses begin to degrade (Chandler et al., 1983). Higher temperatures, over 300 °C, exert structural changes, mainly decarboxylations in the soil macromolecules HAs and FAs and an increase in the proportion of aromatic structures Almendros et al., 1990, Almendros et al., 1992, Knicker et al., 1996. In general, changes on soil organic matter caused by fire or heat allow the definition of “pyromorphic humus”. This is composed of rearranged macromolecular substances of weak colloidal properties and enhanced resistance to biological degradation, as inferred from laboratory incubation experiments with natural or laboratory-heated samples Almendros et al., 1984b, González-Vila and Almendros, 2003.
Therefore, during a forest fire, a considerable rearrangement of C forms occurs and refractory and oxidation-resistant organic C forms of considerable residence time, including BC, are formed “de novo” (Schulze et al., 2000). Only a fraction of the total production of oxidation-resistant organic C produced from biomass burning is likely to be sequestered in the slow-cycling “geological” carbon reservoir (Bird et al., 1999). In fact, in some world's areas, like Mediterranean regions, where extreme environmental conditions alternate through the year, biological activity is not especially favorable to humification processes but to intense mineralization. Under such conditions, the relative importance of abiotic constraints such as fire and irreversible dehydration favored by intense solar radiation and drastic drying cycles, are important factors in the formation of stable OM in soil. Fire is considered the main disturbance in the Mediterranean basin Trabaud, 1984, Naveh, 1990, Barbéro et al., 1998.
Occurrence of wildfires in forest ecosystems has lasting effects on both the microbial composition and the OM, and hence on the whole soil dynamics. The alteration of natural ecosystems affects OM turnover and therefore productivity and community structure may be also affected (Pastor and Post, 1986). It is well known the global effect of forest fires in the release of CO2 and other greenhouse gases to the atmosphere, caused by the depletion of ecosystems' aboveground biomass, including the organic topsoil horizons, composed mainly of plant litter (Crutzen and Goldammer, 1993, Levine, 1996b and references therein), this represents a severe depletion of the ecosystem's largest portion of the labile or active C pool. Fire also acts as an evolutionary force for plants and other terrestrial vegetation species that, in the course of time, co-evolved with fires Ahlgren and Ahlgren, 1960, Pyne, 1996, Keely and Zedler, 1998. This force includes man kind (Pyne and Goldammer, 1997; Caldararo, 2002) and, from an holistic viewpoint, its effect and natural occurrence is greatest under ecophysiological formations where the availability of nutrients is suboptimum as a consequence of bioclimatic constraints or soil organo-mineral interactions. Less is known however about the effect that forest fires exerts on C stabilization in soil and on the more stable pool of C, represented mainly by the colloidal soil fractions and on the quality and quantity of soil humic materials.
Section snippets
Fires at a global scale
Unambiguous evidence for wildfires go back to the appearance of gymnosperms during the Famennian age, in the boundary between the Devonian and Carboniferous periods (Jones and Rowe, 1999). At this age (ca. 360×106 years b.p.) enough photosynthesis-derived atmospheric oxygen was available and, together with lightening, favored the ignition of biomass. Hence, the production of charcoal has often been considered as indicator of the occurrence of ancient fires. Most of today's fires are caused by
Effect of fire in soil microbes and C mineralization
After a forest fire there are several interactive factors that affect soil biota and, in turn, the evolution of OM may also be affected. These factors include direct sterilization, formation of ash, charcoal and fire-altered OM, and modifications of the soil forming factors and structure of the microflora and the whole trophic system i.e. changes in canopy and vegetation affecting soil properties.
In the short term, fire causes a drastic reduction in soil microbial biomass (Prieto-Fernández et
Organic matter content
The effect of fire on the total soil OM content is highly variable, and depends on several factors including fire type (canopy or aboveground, underground fires), intensity, and even slope. These effects may range from the almost total destruction of the soil OM to increases that may reach 30% in the surface layers as a consequence of external inputs, mainly from dry leaves and partially burnt plant materials in fires affecting the tree canopy (Chandler et al., 1983). Soon after a fire, a sharp
Effect of fire in the quality of soil organic carbon
As indicated above, wildfires are accompanied by the release of large amounts of CO2, CH4 and NOx to the atmosphere and a considerable production of severely and/or partly charred necromass, which is deposited on the site as fire remains or transported in the form of soot (Kuhlbusch, 1998a). Previous studies on natural ecosystems, as well as laboratory experiments simulating the effect of fire on natural or synthetic soils, demonstrated that this pyromorphic material consists of rearranged,
Conclusions
The effect of fire on soil OM is highly dependent on, among other factors, the type and intensity of the fire, soil moisture, soil type, and nature of the burned materials. Therefore, the effect on soil processes and their intensity influenced by fire are highly variable and no generalized tendencies can be suggested for most of the fire-induced changes in humus composition. Nevertheless, systematic changes in the combustion of humus after intense burning has been summarized by Almendros et al.
Acknowledgments
J.A. González-Pérez acknowledges the Spanish MCyT and the European Regional Development Fund (‘‘Ramón y Cajal’’ program) for financial support.
References (142)
- et al.
Effects of fire on humic and lipid fractions in a Dystric Xerochrept in Spain
Geoderma
(1988) - et al.
Characterization of synthetic carbohydrate-derived humic-like polymers
Sci. Total Environ.
(1989) - et al.
Solid state NMR studies of fire-induced changes in the structure of humic substances
Sci. Total Environ.
(1992) - et al.
Pyrolysis of carbohydrate-derived macromolecules: its potential in monitoring the carbohydrate signature of geopolymers
J. Anal. Appl. Pyrolysis
(1997) - et al.
Rearrangement of carbon and nitrogen forms in peat after progressive isothermal heating as determined by solid-state 13C- and 15N-NMR spectroscopies
Org. Geochem.
(2003) - et al.
Growth rate and response of bacterial communities to pH in limed and ash treated forest soils
Soil Biol. Biochem.
(1994) - et al.
Chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood
Org. Geochem.
(2002) Human ecological intervention and the role of forest fires in human ecology
Sci. Total Environ.
(2002)- et al.
Effects of charring on mass, organic carbon, and stable carbon isotope composition of wood
Org. Geochem.
(2002) The role of fire and soil heating on water repellency in wildland environments: a review
J. Hydrol.
(2000)
13C Nuclear magnetic resonance studies of kerogen from Cretaceous black shales thermally altered by basaltic intrusions and laboratory simulations
Geochim. Cosmochim. Acta
Evaluating levoglucosan as an indicator of biomass burning in Carajás, Amazônia: a comparison to the charcoal record
Geochim. Cosmochim. Acta
The Maillard reaction
Adv. Carbohydr. Chem.
Dehydration reactions of carbohydrates
Adv. Carbohydr. Chem. Biochem.
Organic matter changes inmediately after a wildfire in an Atlantic forest soil and comparison with laboratory soil heating
Soil Biol. Biochem.
Carbon mineralization dynamics in soils after wildfires in two Galician forests
Soil Biol. Biochem.
Black carbon in soils: the use of benzenecarboxylic acids as specific markers
Org. Geochem.
Black carbon in density fractions of anthropogenic soils of the Brazilian Amazon region
Org. Geochem.
The effects of vegetation and burning on the chemical composition of soil organic matter in a volcanic ash soil as shown by 13C NMR spectroscopy: I. Whole soil and humic acid fraction
Geoderma
The chemical structure of highly aromatic humic acids in three volcanic ash soils as determined by dipolar dephasing NMR studies
Geochim. Cosmochim. Acta
Black carbon—possible source of highly aromatic components of soil humic acids
Org. Geochem.
The formation and clay reactions of melanoidins
Geochim. Cosmochim. Acta
Effects of forest management on soil C and N storage: meta analysis
For. Ecol. Manag.
13C- and 15N-NMR spectroscopic examination of the transformation of organic nitrogen in plant biomass during thermal treatment
Soil Biol. Biochem.
The role of soot and primary oxidants in atmospheric science
Sci. Total Environ.
Ecological effects of forest fires
Bot. Rev.
An evaluation of some methods for oxidative degradation of the humic substances applied to carbohydrate-derived humic-like polymers
J. Soil Sci.
Contribución al estudio de la influencia de los incendios forestales en las caracterısticas de la materia orgánica del suelo. I: Transformaciones del humus en un bosque de Pinus pinea del centro de España
Rev. Écol. Biol. Sol.
Contribución al estudio de la influencia de los incendios forestales en las caracterısticas de la materia orgánica del suelo: II. Transformaciones del humus por ignición en condiciones controladas de laboratorio
Rev. Écol. Biol. Sol.
Fire-induced transformation of soil organic matter from an oak forest: an experimental approach to the effects of fire on humic substances
Soil Sci.
Laboratory appraisal of carbon sequestration and nutrient availability after different organic matter inputs in virgin and cultivated Zimbabwean soils
Commun. Soil Sci. Plant Anal.
Physical and chemical speciation of nitrogen in peat and derived humic fractions as revealed by 15N- and 13C-NMR under quantitative acquisition conditions
Postfire effects on soil properties and nutrient losses
Int. J. Wildland Fire
Pines of the Mediterranean Basin. Pinus
Total carbon and nitrogen in the soils of the world
Eur. J. Soil Sci.
Measurement of soil organic matter turnover using 13C natural abundance
Melanoidins and soil organic matter: evidence of strong similarities revealed by 13CP-MAS NMR
Soil Sci. Soc. Am. J.
Stability of elemental carbon in savanna soil
Glob. Biogeochem. Cycles
Long-term effects of fire on the composition and activity of the soil microflora of a subalpine, coniferous forest
Can. J. Bot.
The selective effect of heat treatment on the microflora of a greenhouse soil
Neth. J. Plant Pathol.
Lignin-degrading enzymes of commercial button mushroom, Agaricus bisporus
Appl. Environ. Microbiol.
Soil temperature and water content beneath a surface fire
Soil Sci.
Forest fire behavior and effects
Biomass burning emissions and the atmosphere
Degradation of coal by the fungi Polyporus versicolor and Poria monticola
Appl. Environ. Microbiol.
Translocation of hydrophobic substances into soil by burning organic litter
Soil Sci. Soc. Am. Proc.
The transfer of heat and hydrophobic substances during burning
Soil Sci. Soc. Am. J.
Fire's effects on ecosystems
The effect of slash burning on soil microflora
Plant Soil
Cited by (938)
Impact of wildfire recurrence on soil properties and organic carbon fractions
2024, Journal of Environmental ManagementThe effect of the synergistic thermal treatment and stabilization on the transformation and transportation of arsenic, chromium, and cadmium in soil
2024, Science of the Total EnvironmentMercury mobilization in shrubland after a prescribed fire in NE Portugal: Insight on soil organic matter composition and different aggregate size
2023, Science of the Total Environment