Abstract
A review of the effects of wildfires on Mediterranean soils was carried out with special emphasis on the biogeochemical processes involved in soil C sequestration. Modifications to the composition and properties of soil humic substances and related resilient C-forms in soil resulting from fires were also discussed. The systematic effects of fires on soil organic matter (SOM) mainly involve changes to its solubility and colloidal properties, selective thermal degradation, structural condensation and the cyclization of soil C and N-forms. These effects represent an increase in the biogeochemical stability and chemical diversity of the SOM after the thermal generation of structures that are not found in soils unless they are affected by fire. Non-systematic effects also depend on local environmental constraints and on the great differences in fire intensities and propagation patterns. The effects of fire were also discussed, taking advantage of the results of laboratory simulation experiments. This approach is indispensable for differentiating fire intrinsic effects on soil from those produced by fire-induced inputs of charred necromass and the subsequent effects of soil erosion. In some cases, the characteristic properties acquired by post-fire soils can be explained in terms of molecular-level features of humic substances and, in particular, those concerning resistance to biodegradation, cation exchange capacity, N-availability, soil water repellence and aggregate stability.
Highlights
On a planetary scale, it has been estimated that between 530 and 555 × 106 ha of forest lands are affected by fire every year, causing losses of about 50 Pg yr-1 of biomass (Levine 1996)
The subsequent soil loss associated with erosive processes is frequently considered to be the most severe environmental perturbation caused by wildfires in terms of C exportation from the ecosystem (Certini 2005; Bowman et al 2009)
Most of the current research on the impact of fire on soil focuses on the alteration of soil organic matter (SOM), with particular emphasis paid to the molecular-level analysis of the various forms of soil C, the so-called humic substances
Summary
It has been estimated that between 530 and 555 × 106 ha of forest lands are affected by fire every year, causing losses of about 50 Pg yr-1 of biomass (Levine 1996). The subsequent soil loss associated with erosive processes is frequently considered to be the most severe environmental perturbation caused by wildfires in terms of C exportation from the ecosystem (Certini 2005; Bowman et al 2009). Low- and medium-intensity fires induce structural changes in SOM not readily distinguishable from those due exclusively to biotic soil processes (Almendros et al 1990; Hernández and Almendros 2012). The molecular characterization of the SOM provides objective information on soil processes, quality and resilience, i.e., its potential to remain unchanged against environmental impacts. Structural studies on SOM are helpful in establishing the variable extent of the independent C-stabilization processes in the different types of soils. The systematic use of multianalytical approaches for these purposes is not frequent, since it requires timeconsuming preparative laboratory techniques as well as the availability of expensive instrumental techniques
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