Análisis del efecto del fuego y de la presencia de contaminantes sobre la actividad y biodiversidad microbiana del suelo

Resum

Soil health can be defined as its ability to function as a living system, sustaining biological productivity and promoting environmental quality. The concept of soil health is related with its biological, chemical and physical properties, which are essential for sustainable agricultural productivity in the long term, with minimal environmental impact. The study of the role of microbial biodiversity in soil health, soil's ability to respond to stress conditions and the relationship between the two factors, is key to understanding the limits of ecosystem functioning and resilience biological activity in disturbed ecosystems. The most common microbial indicators of soil health used are biomass, microbial activity and microbial diversity, which provide useful information on the multifunctional role that microbial communities have in the ecosystem. Soils, like any other ecosystem, may suffer disturbances which alter its structure. Disturbances can be natural (fire, snow avalanches, extreme weather, floods, etc.) or generated by man (pollution, introduction and extinction of species, etc.). The effect of disturbances on ecosystems depends on the magnitude of the perturbing agent and the susceptibility of the ecosystem. In this study we have analyzed the effect of wildfires and pollutants on the physical, chemical and biological (microbial activity and diversity) properties of different soils. The effect of fire was studied in three specific areas of our country, two belonging to the Autonomous community of Andalusia (Aznalcóllar and Cazorla) and another to the Autonomous community of the Canary Islands (Tenerife), who suffered one or more wildfires in different years. Moreover, we studied the effect of the presence of organic pollutants (hydrocarbons) and inorganic (heavy metals) in soils from two municipal solid waste landfills located in the Autonomous community of Madrid, specifically, the villages of Torrejon de Ardoz and Getafe respectively. Chemical and physical characterization of burned soils in comparison to their respective controls was carried out by estimating the following parameters: pH, water retention capacity, carbon and organic matter and nitrogen content. The observed increase in the total carbon content of oxidizable and/or total nitrogen in burned soils can be attributed to the impact of fires on vegetation cover. To study the effect of fire on microbial activity we analized the following biomarkers: biomass, substrate induced respiration (SIR) and activity of various enzymes involved in the carbon cycle (ß-glucosidase, ß-N-acetyl glucosaminidase and invertase), nitrogen (urease) and phosphorus (acid and alkaline phosphatase). From the results it should be noted that we detected in most burned soils an increased of bacterial and fungal biomass and respiratory activity relative to control soils. With respect to the enzyme activities tested, we could not establish a common pattern of behavior in the different samples studied, except in the case of the phosphatase activity, for the highest levels was detected in the burned soils. The study of the effects of fire on the microbial diversity of soils was performed by analyzing DNA band fingerprints corresponding to the domains Bacteria and Archaea, obtained by denaturing gradient gel electrophoresis (DGGE), and by analyzing the sequences of the clones obtained from cloning 16S rRNA gene for Bacteria. Both approaches required tuning of molecular techniques employed. From the DGGE fingerprint obtained, we calculated Shannon diversity index and did a hierarchical analysis. The Shannon index obtained showed a greater diversity in Bacteria than in Archaea in all soils analyzed. Aznalcóllar´s burned soils had slightly higher rates of diversity in both domains (Bacteria and Archaea). In Cazorla, in which there were three consecutive analyzes in different years in two different location areas, the results reflect Shannon index greater or lesser microbial diversity depending on the year of sampling and the area sampled. From the cluster analysis performed, we concluded that the effect of fire was more significant on microbial communities of Archaea than Bacteria domain. From the results for the Shannon index and cluster analysis obtained in Tenerife soils samples before and after the rains, we could not derive a clear pattern of the effect of fire on microbial diversity. From each of the soil samples genomic DNA (16S RNAr gene) was amplified through PCR and the fragments were cloned for its identification using Ribosomal Database Project (RDP). We studied 1,268 sequences with more than 95% similarity. After performing rarefaction curves of each of the libraries we concluded that in most samples it would have been required to analyze a greater number of sequences to estimate conclusively the microbial diversity present in soils. However, we found that most of the bacteria present in the soils correspond to the phyla Proteobacteria, Acidobacteria and Actinobacteria. Furthermore, the identification at genus level did not allow to differentiate between control and burned soil, because in most cases the same genera have been identified. In the second part of our study we analyzed the physical, chemical and biological properties of soils from two landfills (Torrejón de Ardoz and Getafe) contaminated with hydrocarbons and heavy metals, respectively. In the Torrejón de Ardoz landfill we detected a high concentration of total hydrocarbons (aliphatic and aromatic), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), particularly in samples T2 and T9. The results indicate a statistically significant negative correlation between the concentration of PAHs and PCBs and water retention capacity, number of viable bacteria and fungi, respiratory and enzyme activity, which is indicative of the toxic effect exerted by pollutants on soil microbial populations. This same effect was also observed after estimating Shannon index calculated from the DGGE fingerprint and clone libraries. Thus, it was observed that samples with less pollution (T2B and T8) had higher bacterial diversity than soils with higher concentrations of pollutants (T2 and T9). In these samples we were able to identify 180 sequences with more than 95% similarity using RDP. Rarefaction curves showed that in three of the four samples studied (T2B, T8 and T9), the number of sequences analyzed was sufficient to determine the phyla present in these areas, having been detected as major phyla Proteobacteria, Acidobacteria and Actinobacteria. Finally, in the Getafe landfill, we could not establish a statistically significant correlation between the concentration of heavy metals and any of the parameters analyzed. This could be justified on the basis that the microbial communities in these soils could have adapted to the concentration of heavy metals present or that the concentration does not reach the toxicity threshold necessary to cause a detectable effect on the selected indicators.