This study showed that both rhizosphere and bulk soil samples had a high relative abundance of beneficial microorganisms that could have positive effects in facilitating seedling establishment in burned old-growth N. pumilio forest in Los Andes Cordillera. Specifically, these microorganisms include saprophytic fungi, ectomycorrhizal fungi, ericoid mycorrhizal taxa, and N-fixing bacteria, all of which have the potential to positively influence plant establishment in burned soils due to mechanisms such as the production of phytohormones, improved acquisition of nutrients, tolerance to abiotic stresses, biocontrol against phytopathogens, and induction of systemic resistance (Farrar et al. 2014; Lou et al. 2023; Mirzaei et al. 2023; Siviero et al. 2008).
We found that eight years after fire, the bacterial and fungal taxa associated with N. pumilio plants did not differ greatly among sites (i.e., burned vs. unburned) or between soil types (i.e., bulk or rhizosphere; Table S1). Results from studies of fire effects on soil microorganisms largely vary, including positive or negative changes in community structure and/or no significant effects compared to unburned reference soils in the long-term (Longo et al. 2011; Mikita-Barbato et al. 2015). Fire intensity, severity, and frequency are some factors that can influence soil microbial community responses (Day et al. 2019), as well as time elapsed since fire (Bowd et al. 2022). On the other hand, the high relative abundance of microorganisms in burned soils can be attributed to several factors. These factors include the presence of fire-resistant structures, stimulation of proliferation due to increased soil temperatures resulting from reduced plant cover, and the introduction of microorganisms from undisturbed forest areas, among others (Kobziar et al. 2018; Whitman et al. 2019).
Proteobacteria, Actinobacteria and Acidobacteria were the most abundant phyla in each condition and soil type. This finding is consistent with several studies showing that members of these phyla are globally distributed (Barka et al. 2016; Eichorst et al. 2018; Nacke et al. 2011). The result from the LEfSe analysis show that some of the main biomarkers belong to Proteobacteria and actinobacteria (Fig. 5A). Proteobacteria is related to various functions involved in the C, N, and S cycles (Spain et al. 2009). Whereas, Actinobacteria and Acidobacteria have the ability to degrade a wide range of substances as the sole carbon source (Lladó et al. 2016; Rodriguez et al. 2022). Actinobacteria influence plant growth by mechanisms such as the production of phytohormones, nitrogen fixation and solubilization of nutrients such as phosphorus and are even involved in heavy metal bioremediation processes (Sathya et al. 2017). Similar results have been found in other studies in Chile. In a study performed by Almonacid et al. (2022) the main bacterial phyla in the N. obliqua forest were Proteobacteria and Acidobacteria. Similarly, Castañeda and Barbosa (2017), identified that Proteobacteria, Actinobacteria and Acidobacteria were the most abundant phyla in native forests.
The bacterial taxa, Xanthobacteraceae and Acetobacteraceae were some of the major bacterial families found. The Xanthobacteraceae family is related to nitrogen cycles in fixation, nitrification and denitrification processes and organic matter decomposition (Jang et al. 2020; Wang et al. 2016). Similarly, Acetobacteraceae consists of acetate-producing and non-acetate-producing bacteria with a wide distribution present in several environments including deserts and hot springs (Belova et al. 2009; Ming et al. 2016; Muhadesi et al. 2019; Yang et al. 2017). Some genera belonging to this family also have the ability to fix atmospheric nitrogen, and even exhibit plant growth-promoting traits, such as indole acetic acid (IAA) production and nutrient solubilization (Saravanan et al. 2007).
The bacterial genera with the highest relative abundance were Mycobacterium, Rhodoplanes, caballeronia, Bryobacter, Bradyrhizobium and Haliangium. Mycobacterium belonging to the phylum Actinobacteria have the ability to fix nitrogen (Sellstedt and Richau 2013), produce ACC deaminase (Nascimento et al. 2014) and also are capable of catabolizing aromatic compounds (Hennessee et al. 2016). These compounds include humic acids, fulvic acids, tannins and lignin present in organic matter, which can explain their presence in the reference sites (UBS, URS). Although polycyclic aromatic hydrocarbons were not quantified in this study, it can be hypothesized that they were present because wildfires increase the concentration of polycyclic aromatic hydrocarbons (Vila-Escalé et al. 2007). Here, further research is advised to shed light into these questions.
Rhodoplanes is an abundant phototrophic bacterium in forest (Chakravarthy et al. 2012), and like Caballeronia are involved in nitrogen fixation (Buckley et al. 2007; Rodriguez et al. 2022). A study performed by Puri et al. (2020) showed that Caballeronia sordidicola isolated from spruce trees has the ability to produce (IAA) and solubilize phosphorus under in vitro conditions as well as to promote the growth of spruce and pine seedlings. On the other hand, the genus Bryobacter play a key role in the carbon cycle, it is aerobic chemoorganotrophic bacteria with the ability to metabolize a variety of sugars, polysaccharides and organic acids (Dedysh et al. 2017). Similarly, Haliangium is recognized as a rhizobacterium that promotes the growth of plants and possesses the capability to generate an antifungal compound known as Haliangicin, this metabolite exhibits inhibitory effects on the growth of a broad spectrum of fungi (Qiu et al. 2012). The genus Bradyrhizobium, is commonly associated with leguminous plants capable of fixing atmospheric nitrogen. A study conducted by VanInsberghe et al. (2015) showed that Bradyrhizobium is also present in forests associated with other plant species and even had a greater potential to metabolize aromatic carbon sources, which could explain its presence with even a higher relative abundance in burned sites compared to unburned sites.
On fungal communities, the main phyla were Basidiomycota and Ascomycota (Fig. 2B). At genus level Cortinarius was abundant in the reference site bulk soil (UBS) and even more abundant in the burned bulk soil (Table S1). The genus Cortinarius is recognized as an early successional genus with relatively low host specificity (Dickie et al. 2013). It is widely distributed, primarily in temperate and subarctic climates. In general, the relative abundance of mycorrhizal fungi tends to increase with the recovery of aboveground vegetation (Yang et al. 2020). Additionally, a recent study by Fuentes-Ramirez et al. (2018) found greater abundance and biological activity of cultivable microorganisms as early as one year after a wildfire in burned soil in the same study area. Ectomycorrhizal fungi like Cortinarius are known to form resilient structures such as sclerotia that allow them to persist for long periods of time and withstand the effects of wildfires (Baar et al. 1999; Day et al. 2019). These structures can persist for decades (Bruns et al. 2009), and in the rhizosphere they promote the formation of symbiotic mycelium (Kohout et al. 2018). Moguilevsky et al. (2021) showed that the ectomycorrhizal symbiosis in N. pumilio persists even after a disturbance such as a volcanic eruption.
High relative abundance of Cortinarius was found in bulk soil, and these results are consistent with a study conducted by Wang et al. (2023) in a wildfire-affected forest in China, where fungal diversity was greater in the bulk soil compared to the rhizosphere soil. Similarly, Almonacid-Muñoz et al. (2022) demonstrated a higher relative abundance of bacteria and fungi in bulk soil compared to rhizosphere soil in Nothofagus obliqua forests in south-central Chile. A study by Glassman et al. (2016) in the Stanislaus National Forest in the Sierra Nevada, California, USA, found that the majority of the most common fungal species and many ectomycorrhizal fungi colonizing seedlings after a wildfire were derived from spore bank fungi that had persisted as propagules in the soil.
Another representative fungal genus from the burned soil was Oidiodendron (Table S1). It is a commonly accepted ericoid mycorrhizal taxa that interact with the fine hair roots of ericaceous plants to improve mineral nutrition and protect against stress (Vohnik et al. 2005). The ability of these species to grow after fire in burned have enhanced the growth of compatible plants has been reported in burned soils (Caifa et al. 2023). Therefore, the presence of ericoid mycorrhizal fungi in this context can be considered an important factor contributing to the establishment of Gaultheria spp. shrubs that commonly co-occur with N. pumilio forests in the study area. This, in turn, may have positive effects on soil health and further promote the successful establishment of N. pumilio seedlings within burned soils. Despite the common occurrence of ericaceous plants in the understory of native forests in the Andes, and their ability to colonize volcanic rock deposits, the diversity of mycorrhizal fungi associated with native ericaceous plants remains largely unexplored.
In areas affected by wildfires, ericaceous plants (i.e., Gaultheria spp.) have been documented as one of the species capable of regrowing shortly after a fire event (Arroyo-Vargas et al. 2019). This post-fire recovery capacity of ericaceous plants is attributed to a combination of their ability to regenerate from seeds and vegetative shoots, and the facilitative role of ericoid mycorrhizal fungi in supporting their establishment in burned soils (Lou et al. 2023; Ojeda et al. 2016). Therefore, the presence of Oidiodendron in this context can be considered an important factor contributing to the establishment of Gaultheria spp. populations after fire. This, in turn, may have positive effects on soil health and further promote the successful establishment of N. pumilio seedlings.
Another abundant fungal family found in the study was Aspergillaceae, with a notable prevalence of the genus Penicillium specially in burned areas (Fig. 2D). Penicillium conidia can be readily dispersed in nature through the atmosphere and soil (Radhakrishnan et al. 2014), suggesting that they may have arrived after the fire. Also, Penicillium is known for its ability to degrade aromatic compounds produced during combustion (Wang et al. 2012). These findings are consistent with a study by Whitman et al (2019), which identified Penicillium as one of the key fungal genera that exhibits significant positive responses to fire and has broad ecological importance in the boreal forests of northwestern Canada.
The importance of the relationship between plants and microorganisms for the nutrition, growth, and development of plant species is widely recognized. Our results showed that eight years after a wildfire, beneficial microorganisms such as saprophytic, ectomycorrhizal fungi, ericoid mycorrhizal taxa, and N-fixing bacteria are present in the burned area with high relatively abundance, all of which have the potential to positively influence plant establishment in burned soils. This is highly relevant considering the role that these beneficial soil microorganisms could play in natural or assisted restoration of burned native N. pumilio forests.
Currently, the native tree N. pumilio is completely excluded from severely burned areas, and we hypothesize that once seed dispersal and germination occur, the species will benefit from the high relative abundance of beneficial soil microorganism that now await for plant species. However, in order to corroborate this hypothesis, further research is needed. The microbial characterization could be the first step to isolation of microorganisms to explore their plant growth-promoting traits, re-inoculate and evaluate the effect on N. pumilio seedlings under controlled conditions. In the mid-to-long term, this will help to improve the establishment, growth, and survival of plant species, with the potential to be used in processes of forest restoration when planting native trees.