Many physical, chemical, mineralogical, and biological soil properties can be affected by forest fires. The effects are chiefly a result of burn severity, which consists of peak temperatures and duration of the fire. Climate, vegetation, and topography of the burnt area control the resilience of the soil system; some fire-induced changes can even be permanent. Low to moderate severity fires, such as most of those prescribed in forest management, promote renovation of the dominant vegetation through elimination of undesired species and transient increase of pH and available nutrients. No irreversible ecosystem change occurs, but the enhancement of hydrophobicity can render the soil less able to soak up water and more prone to erosion. Severe fires, such as wildfires, generally have several negative effects on soil. They cause significant removal of organic matter, deterioration of both structure and porosity, considerable loss of nutrients through volatilisation, ash entrapment in smoke columns, leaching and erosion, and marked alteration of both quantity and specific composition of microbial and soil-dwelling invertebrate communities. However, despite common perceptions, if plants succeed in promptly recolonising the burnt area, the pre-fire level of most properties can be recovered and even enhanced. This work is a review of the up-to-date literature dealing with changes imposed by fires on properties of forest soils. Ecological implications of these changes are described.

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Effects of fire on properties of forest soils: a review

After vegetation fires considerable amounts of severely or partly charred necromass (referred to here as char) are incorporated into the soil, with long-term consequences for soil C and N dynamics and thus N availability for primary production and C and N transport within the soil column. Considering results reported in the pyrolysis literature in combination with those obtained from controlled charring of plant material and soil organic matter (SOM), it has become clear that common models claiming char as a graphite-like material composed mainly of highly condensed polyaromatic clusters may be oversimplified. Instead, I suggest a concept in which char is a heterogeneous mixture of heat-altered biopolymers with domains of relatively small polyaromatic clusters, but considerable substitution with N, O and S functional groups. Such a concept allows fast oxidation facilitating both microbial attack and dissolution. Although, char is commonly believed to degrade more slowly than litter, over the long term and under oxic conditions, char may degrade to an extent that it becomes indistinguishable from naturally formed SOM. Oxygen depletion or environments with low microbial activity may be necessary for char to survive without major chemical alteration and in considerable amounts for millennia or longer.

/pdf/how-does-fire-affect-the-nature-and-stability-of-soil-46wrs9zhej.pdf

How does fire affect the nature and stability of soil organic nitrogen and carbon? A review

Soil pH is often hypothesized to be a major factor regulating organic matter turnover and inorganic nitrogen production in agricultural soils. The aim of this study was to critically test the relationship between soil pH and rates of C and N cycling, and dissolved organic nitrogen (DON), in two long-term field experiments in which pH had been manipulated (Rothamsted silty clay loam, pH 3.5–6.8; Woburn sandy loam, pH 3.4–6.3). While alteration of pH for 37 years significantly affected crop production, it had no significant effect on total soil C and N or indigenous mineral N levels. This implies that at steady state, increased organic matter inputs to the soil are balanced by increased outputs of CO2. This is supported by the positive correlation between both plant productivity and intrinsic microbial respiration with soil pH. In addition, soil microbial biomass C and N, and nitrification were also significantly positively correlated with soil pH. Measurements of respiration following addition of urea and amino acids showed a significant decline in CO2 evolution with increasing soil acidity, whilst glucose mineralization showed no response to pH. In conclusion, it appears that changes in soil pH significantly affect soil microbial activity and the rate of soil C and N cycling. The evidence suggests that this response is partially indirect, being primarily linked to pH induced changes in net primary production and the availability of substrates. In addition, enhanced soil acidity may also act directly on the functioning of the microbial community itself.

PH regulation of carbon and nitrogen dynamics in two agricultural soils

This state-of-knowledge review about the effects of fire on soils and water can assist land and fire managers with information on the physical, chemical, and biological effects of fire needed to successfully conduct ecosystem management, and effectively inform others about the role and impacts of wildland fire. Chapter topics include the soil resource, soil physical properties and fire, soil chemistry effects, soil biology responses, the hydrologic cycle and water resources, water quality, aquatic biology, fire effects on wetland and riparian systems, fire effects models, and watershed rehabilitation.

https://www.fs.fed.us/rm/pubs/rmrs_gtr042_4.pdf

Wildland fire in ecosystems: effects of fire on soils and water

http://jdguez.webs.uvigo.es/wp-content/uploads/2011/10/Comparison-of-the-effectiveness-of-composting-and-vermicomposting.pdf

Comparison of the effectiveness of composting and vermicomposting for the biological stabilization of cattle manure.

A Humic Cambisol developed over granite under Pinus pinaster Sol. located in the Atlantic climate zone, which had been affected by a high intensity wildfire, was studied 1 month after burning. The soil had a very rich organic matter A horizon, 30 cm deep. The effects of the fire on the N status and N mineralization capacity were estimated comparing the surface (0–5 cm) and subsurface (5–10 cm) layers from the burnt soil with the corresponding layers from the same unbumt soil. N mineralization kinetics were determined by aerobic incubation at 28°C for 11 weeks. The fire increased the total N content in the surface layer but not in the subsurface. Total inorganic N, which was mainly in the form of NH+4-N, increased after the burning in both layers, whereas NO−3-N content, which was very low, only increased in the subsurface layer. The fire increased the N mineralization capacity, but did not modify organic N mineralization behaviour. Ammonification largely predominated over nitrification in both the unburnt and the burnt soils. N mineralization kinetics followed the first order equation Nm = N0((1 − e−kt) but the fire affected the kinetic parameters. The potentially-mineralizable N decreased and the kinetic constant increased in the burnt samples showing that the wildfire reduced the mineralizable organic-N reserves and increased the mineralization rate, thus predicting a rapid depletion of the labile organic N. The temporary ability of the burnt soil to supply available N is recommended to be used to grow an early crop to avoid physical soil degradation.

Short-term effects of a wildfire on the nitrogen status and its mineralization kinetics in an Atlantic forest soil

Inorganic and organic N pools in soils burned or heated: immediate alterations and evolution after forest wildfires

Physical and chemical characterization of four composted urban refuses

The N mineralization capacity of 41 temperate humid-zone soils of NW Spain was measured by aerobic incubation for 15 days at 28°C and 75% of field capacity. The main soil factors affecting organic N dynamics were identified by principal components analysis. Ammonification predominated over nitrification in almost all soils. The mean net N mineralization rate was 1.63% of the organic N content, and varied according to soil parent materials as follows: soils on basic and ultrabasic rocks < soils over acid metamorphic rocks < soils developed over sediments < soils over acid igneous rocks < soils on limestone. The N mineralization capacity was lower in natural soils than in cropped soils or pastures. The accumulation of organic matter (C and N) seems to be due to poor mineralization which was caused, in decreasing order of importance, by high exchangeable H-ion levels, high Al and Fe gel contents and, to a lesser extent (though more markedly in cropped soils), by silty clay texture and exchangeable Al ions.

M. Carballas

Papers

Short-term effects of a wildfire on the nitrogen status and its mineralization kinetics in an Atlantic forest soil

Inorganic and organic N pools in soils burned or heated: immediate alterations and evolution after forest wildfires

Physical and chemical characterization of four composted urban refuses

Nitrogen mineralization and its controlling factors in various kinds of temperate humid-zone soils

Carbon and nitrogen mineralization in an acid soil fertilized with composted urban refuses