Response of soil microbial communities to fire and fire-fighting chemicals
Introduction
Wildfires are the major disturbance in forest ecosystems mainly due to vegetation destruction and soil degradation, causing very serious ecological, economical and social problems around the world, particularly in Mediterranean landscapes of European countries (Moreno et al., 1998, Ekinci and Kavdir, 2005, Carballas et al., 2009). The growing concern about this topic has stimulated numerous studies showing that fires induce important changes in physical, chemical and biochemical properties, the impact being determined by fire regimen factors (intensity, duration and recurrence) as well as by local conditions such as vegetation composition, topography or regional climate (Raison, 1979, Chandler et al., 1983, Carballas et al., 1994, Debano et al., 1998, Neary et al., 1999, Johnson and Curtis, 2001, González-Pérez et al., 2004, Certini, 2005, Carballas et al., 2009, Mataix-Solera et al., 2009). In the future, warmer and drier conditions associated with Global Change are expected to increase the area affected by fire (Flannigan et al., 2000); consequently the improvement of wildfire fighting techniques is a global concern.
A variety of chemicals have been increasing its use worldwide to assist in the strategic containment of fires. By definition a fire-fighting chemical is an agent that, either alone or mixed with water, delays or even stops the combustion of a given fuel (Adams and Simmons, 1999). These substances, classified as short-term retardants (those that do not reduce the combustion after evaporation of the water from the fuel) and long-term retardants (those that form a long-term combustion barrier after evaporation of the water), are generally composed of fertilizer salts with a thickening agent or a mixture of surfactants, foams stabilisers and wetting agents or a mixture of surfactants, foams stabilisers, wetting agents and solvents. Most research on these compounds have been traditionally focussed on the evaluation of their effectiveness in aiding fire-fighting and relatively little information is available on the potential environmental effects, including impacts on soils (Giménez et al., 2004). These fire-fighting chemicals are used extensively in natural areas with high environmental value, and available data suggest that there is a significant potential for damage to terrestrial vegetation and to aquatic ecosystems from some of these compounds (Adams and Simmons, 1999, Little and Calfee, 2002, Luna et al., 2007). There is a need to quantify the impact of fire-fighting chemicals on native vegetation and verify their effects on soil properties.
Soil microorganisms are the main agents responsible for long-term sustainability of soil ecosystems since they control the breakdown of organic matter and, hence, the net fluxes and amounts of soil carbon and nutrients through decomposition, mineralization, and immobilization processes (Nannipieri et al., 2003). Changes in several aspects of soil microorganisms (number, mass, activity and diversity) caused by natural or human activities may disrupt the functioning of the soil–plant system and lead to soil degradation; thus, microbial parameters can be used as bioindicators of soil quality changes induced by different soil disturbances (Pankhurst et al., 1997). More than 2000 t of fire retardants are used each year to combat fires in the Mediterranean zones (Luna et al., 2007); there is, therefore, a concern about the effect of fire-fighting chemicals on soil microbial communities in order to preserve soil health. Microbial community-based measurements can be potentially useful for detecting changes in soil quality due to the application of fire-fighting chemicals and therefore to evaluate the environmental compatibility of these compounds. However, despite the common use of fire retardants, surprisingly there is no information on their effects on the microbial soil community. Recent studies performed in the temperate humid zone (NW Iberian Peninsula) have shown that under laboratory conditions microorganisms of both unheated and heated soil samples can potentially be affected at short-term by a fire retardant difficult to biodegrade such as an acrylic-based polymer (Basanta et al., 2002, Basanta et al., 2003, Díaz-Raviña et al., 2006). The data indicated that the fire-fighting chemical at field doses reduced significantly N mineralization and slightly modified microbial biomass and soil enzymatic activities; likewise, microbial community structure, determined by means of the phospholipids fatty acid analyses (PLFA pattern), was affected by the fire-fighting chemical addition. It should be noticed, however, that the microbial changes induced by the fire retardant were less marked than those provoked by the soil heating at 350 °C.
The main hypothesis underlying the present work is that the fire-fighting chemicals at field doses can induce long-term changes in soil microbial communities and hence in soil functioning and quality, in which case they should be used with caution. Therefore, the aim of this study was to examine, under field conditions, the short-, medium- and long-term impact of three fire-fighting chemicals, representative of the compounds often used to combat fires in Mediterranean countries (foaming agent RFC-88 from Auxquímica; terpolymer Firesorb from Stockhausen; and ammonium polyphosphate FR from Cross), at normal application doses on the microbial communities (mass, activity and community structure) of forest ecosystems of the NW Iberian Peninsula.
Section snippets
Experimental design
Soil samples were taken from an experimental field, with UTM coordinates 29 T 05182–46509, located at Alto da Pedrada (Tomiño, Galicia, NW Spain) at an altitude of 455 m a.s.l. The soil, developed over a parent material of paragneises and with a slope of 18–19%, has a vegetation cover dominated by Ulex, Chamaespartium and Erica 50–60 cm height and showed sandy texture, acidic pH and relatively high organic matter (SOM) and C/N ratio values. After a prescribed fire (with the fire extinguished but
Results and discussion
The evolution of the soil enzyme activities in the 0–2 cm layer of all soil treatments along 60 months after the prescribed fire is shown in Fig. 1. The results clearly indicate that the prescribed fire caused an initial decrease in the two soil enzymes studied since values in the burnt soils were 35–46% (urease) and 73–86% (glucosidase) of those in the unburnt soil. This agreed with previous results obtained in other Galician forest soils both heated under laboratory conditions and affected by a
Conclusion
Considering the results overall, we can state the importance of the prescribed fires as a perturbation agent in the dynamics of the forest ecosystems located in the Atlantic humid temperate zone (Galicia, NW Spain), particularly in several aspects of the soil microbial communities such as mass, activity and microbial structure composition. The microbial parameters measured showed a different sensitivity to detect the fire and fire-fighting chemical impacts. Short- and medium-term effects of the
Acknowledgements
This study was supported by the Consellería de Educación y Ordenación Universitaria de la Xunta de Galicia (08MRU002400PR) and by the Ministerio de Ciencia e Innovación (AGL2008-02823), Spain.
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2023, Science of the Total EnvironmentCitation Excerpt :The enzymatic activities in the unburned soils measured six months after mulching were generally greater compared to the burned sites, and this is expected (Lucas-Borja et al., 2021). The latter authors stated that the lower enzymatic activity in burned soils is a clear effect of wildfire, which, due to the high soil temperature, destroys a large amount of the enzymes (Barreiro et al., 2010). Moreover, again Lucas-Borja et al. (2021) attributed these differences to the nutrient cycling, climate regulation, waste decomposition, wood production, and water regulation functions, which were lower in the soils subject to wildfire.