Discussion

This was the first study on the arbuscular mycorrhizal fungi community in the rhizosphere of olive trees in Rio Grande do Sul. The data obtained from the collection of rhizosphere soil from the three olive cultivars studied Arbequina, Coratina, and Picual allowed for the characterization of the autochthonous AMF species associated with olive cultivation at the region Centro Agropecuário da Palma (UFPel), establishing an initial ecological basis for understanding the composition and structure of AMF communities associated with olive cultivation in southern Brazil.

Species-level identification revealed a community composed of the genera Glomus, Diversispora, Acaulospora, Gigaspora, and Scutellospora, groups frequently reported in perennial agricultural systems and previously associated with olive cultivation, as reported by Calvente et al. (2004), Kachkouch et al. (2012), and Montes-Borrego et al. (2014). The species richness observed was higher than that reported by Calvente et al. (2004), who evaluated olive orchards under Mediterranean conditions in southern Spain, and similar to that found by Melloni et al. (2020), who investigated multiple olive cultivars grown in clay soils under subtropical conditions in Brazil. In contrast, slightly higher diversity values were observed in the study by Ouledali et al. (2022), conducted in southern Tunisia along an aridity gradient, and Meddad-Hamza et al. (2017) conducted in Algeria with a large climatic distance from humidity to desert. These differences among studies may suggest that regional edaphoclimatic conditions and soil properties influence the structure of AMF communities in olive orchards. Variations in temperature and precipitation can affect sporulation patterns and species composition (Davison et al. 2021), whereas phosphorus availability and soil organic matter content are important determinants of AMF diversity. High phosphorus levels tend to reduce both root colonization and sporulation, whereas higher organic matter contents may promote microbial activity and enhance fungal diversity (Berruti et al. 2016; Kemmelmeier et al. 2022; Rui et al. 2022). Therefore, differences in climatic conditions and soil chemical characteristics likely contributed to the variation observed among studies.

Comparisons with AMF communities associated with native vegetation provide additional ecological context for interpreting the patterns observed in the olive orchards. Studies conducted in native grassland areas with sandy soils of the Pampa biome in southern Brazil reported the occurrence of the genera Glomus, Acaulospora_, _Gigaspor_a, and Scutellospora, including the species Acaulospora scrobiculata, _Gigaspora margarita, Glomus clarum and Scutellospora heterogama (Mello et al. 2006). Similar patterns were described in temperate grasslands of the Flooded Pampa biome in Argentina, where species of the genera Glomus and Acaulospora were reported (Escudero and Mendoza 2005). Although direct comparisons are limited by differences in host species and environmental conditions, the recurrent occurrence of some of these genera in both native ecosystems and olive orchards suggests that part of AMF community associated with olive trees may be derived from the regional species pool of the Pampa biome. However, some species recorded in the present study, such as G. ambisporum, D. globifera, G. decipiens, and G. rosea, were not reported in surveys conducted in natural Pampa areas, which may indicate that olive cultivation and the specific conditions of the agricultural environment influenced the composition of these fungi. Therefore, future studies incorporating direct comparisons between olive orchards and adjacent native vegetation are essential to test this hypothesis and understand whether agricultural shifts reshape autochthonous mycorrhizal communities.

The predominance of G. ambisporum in all cultivars, surpassing other species such as G. decipiens, D. globifera, and G. rosea, is consistent with literature. Similar patterns of dominance of the genus Glomus in olive groves in morphological and molecular characterization studies have already been widely documented (Bizos et al. 2020; Montes-Borrego et al. 2014; Palla et al. 2020). In Brazil, Melloni et al. (2020) also reported G. ambisporum as the most abundant arbuscular mycorrhizal fungal species associated with olive trees. This dominance pattern may be related to the high ecological adaptability of species in the Glomeraceae family, which exhibit greater tolerance to different agricultural management practices (Ouhaddou et al. 2025; Zhang et al. 2021) and adverse environmental conditions, including saline soils and more arid environments (Ouledali et al. 2022; Malik et al. 2025).

A possible explanation for the greater persistence of these species in agroecosystems is associated with their functional characteristics. Members of the Glomeraceae family exhibit ruderal characteristics, such as high sporulation capacity, rapid renewal of extraradical mycelium, the ability to allocate biomass within the root, and are able to colonize new hosts through hyphal fragments present in the soil (Cahyaningtyas and Ezawa 2024; Zhang et al. 2021). This set of strategies favors the rapid recolonization of the root system and contributes to the maintenance of these species in agricultural matrices. The prominence of Glomus in this inventory may suggest an evolutionary adaptation to managed perennial systems, leading to representatives of Glomeraceae being frequently reported as dominant in studies characterizing arbuscular mycorrhizal fungi in agricultural environments. However, further controlled and longitudinal studies are required to experimentally test this hypothesis and confirm the underlying mechanisms driving this adaptation in olive orchards.

In contrast, the occurrence of species belonging to the Gigasporaceae family, such as G. decipiens, G. rosea, and S. calospora, contributed to the functional diversity of the community, despite their low relative abundances. Their lower abundance may be related to the ecological strategy typically exhibited by members of this family, which allocate a substantial proportion of their biomass to the development of extensive extraradical hyphal networks and producing spores only in later stages of the life cycle (Arcidiacono et al. 2024; Cahyaningtyas and Ezawa 2024). In addition, these fungi preferentially colonize new hosts through intact hyphal networks rather than fragmented hyphae, which may reduce their persistence under managed agricultural conditions (Cahyaningtyas and Ezawa 2024; Dhumal and Shinde 2020; Johnny et al. 2025). Despite their lower abundance, species of this family are functionally important because they contribute to soil exploration and the acquisition of poorly mobile nutrients (Arcidiacono et al. 2024), which could increase the functional diversity of AMF communities in olive orchards.

Although climatic and physicochemical variables of the soil were not directly evaluated, previous studies demonstrate that pH, temperature, and humidity are determining factors in the structuring of AMF communities in olive groves, functioning as ecological filters for the species (Montes-Borrego et al. 2014; Meddad-Hamza et al. 2017). The soil pH observed in this study (5.4) likely acted as an important environmental filter, influencing the composition of the detected species, since variations in this parameter are frequently associated with the formation of distinct mycorrhizal communities (Davison et al. 2021; Snyder et al. 2025). Furthermore, the low organic matter content recorded in the soil (1.66%) may also have contributed to the structuring of the arbuscular mycorrhizal fungi community in the Palma Agricultural Center. Organic matter is recognized as a relevant factor for the diversity and abundance of these fungi (Shao et al. 2024; Vázquez-Santos et al. 2025), and its low availability can limit the occurrence of certain taxa. This effect can be observed in the low representation of the genus Acaulospora, whose species are frequently reported with greater abundance in soils with a high organic matter content (Vázquez-Santos et al. 2025).

Regarding olive genotypes, the present study observed significant variability in spore abundance among cultivars for three species: G. decipiens sporulated preferentially on Arbequina, D. globifera on Coratina, and G. rosea on Picual. This result is consistent with the findings of Montes-Borrego et al. (2014) and Meddad-Hamza et al. (2017). This cultivar-specific relationship indicates that the host genotype directly influences the structure of the AMF community, probably mediated by the release of different root exudates by each cultivar, which include organic compounds such as organic acids, sugars, amino acids, and phenols. These substances act in the signaling, growth, and development of specific rhizosphere microorganisms (Chen and Liu 2024; Li et al. 2023). Studies by Miho et al. (2021) demonstrate that the Coratina cultivar is particularly rich in oleuropein aglycones, which may have stimulated the greater presence of D. globifera in this cultivar, this hypothesis deserves experimental verification in future studies.

At the community scale, this genotype effect also manifested itself consistently. Multivariate analysis confirmed that cultivar identity was the main explanatory factor for the variation in composition, accounting for 56.2% of the total variability. The confirmation of the homogeneity of the multivariate dispersion indicates that the observed differences reflect real changes in community composition, and not statistical artifacts resulting from unequal variation between groups (Anderson 2001). Thus, it was observed that the Coratina cultivar showed greater internal cohesion, indicating that the cultivar may have created a more restrictive ecological filter on the AMF assemblage. These patterns are consistent with recent studies that highlight the determining role of the host genotype in the assembly of AMF communities in perennial agricultural systems under stress conditions (Frew et al. 2025; Mei et al. 2025; Wang et al. 2024).

The analysis revealed that species richness did not differ significantly among cultivars, indicating that AMF are equally accessible to the three olive cultivars. Simpson's index was the only one capable of discriminating cultivars in a statistically robust manner, with Coratina showing significantly lower evenness than Arbequina and Picual, which may be a direct reflection of the disproportionate dominance of G. ambisporum in this cultivar. In contrast, Shannon's index showed only a marginal trend indicating greater sensitivity to the observed differences in rare species, and the irregular occurrence of A. koskei and S. calospora in specific samples of Arbequina and Picual may have increased the internal variance of these cultivars, diluting the differences between groups.

This dominance pattern is consistent with the results showing that the species G. ambisporum and G. decipiens were primarily responsible for the dissimilarity between Arbequina and Coratina, while G. rosea was the main contributor to the difference between Coratina and Picual_._ Compositional differentiation was concentrated in two functionally distinct taxa: a generalist species from the family Glomeraceae and a competitive species from the family Gigasporaceae with an extensive extraradical mycelial network (Cahyaningtyas and Ezawa 2024). This result is ecologically relevant because it may indicate that differential signaling between cultivars is not restricted to a single functional group, but encompasses contrasting ecological strategies, suggesting that host-fungus selection mechanisms in olive trees act broadly across the functional spectrum of the community.

The absence of significant differences in species richness among cultivars suggests that the three olive genotypes are associated with a similar regional pool of AMF. However, differences in species abundance and evenness indicate distinct patterns of species dominance, suggesting that the host genotype may influence the relative abundance of particular taxa without substantially altering total species richness pattern, which also observed in olive-associated AMF communities, in studies reported by Meddad-Hamza et al. (2017) and Montes-Borrego et al. (2014). This pattern may partially explain the differences observed in the abundance of G. decipiens, D. globifera, and G. rosea among cultivars, although additional studies are required to clarify mechanisms that differentiate the community. These differences in community composition may have implications for ecosystem functions associated with the symbiosis. Members of the Glomeraceae family, which dominated all cultivars, are generally recognized for their rapid root colonization and greater tolerance to soil disturbances, whereas representatives of the Gigasporaceae family tend to allocate more biomass to the development of extensive extraradical mycelial networks, potentially contributing to greater soil exploration and nutrient acquisition (Arcidiacono et al. 2024; Cahyaningtyas and Ezawa 2024). Thus, the predominance of functionally contrasting taxa among cultivars may indicate differences in the functional attributes of AMF communities.

From an ecological perspective, differences in community composition among cultivars may potentially influence ecosystem functions associated with root colonization, nutrient acquisition, phosphorus cycling, soil exploration, and plant responses to environmental stresses (Rui et al. 2022). In addition, the identification of autochthonous AMF has agronomic relevance, since olive trees naturally benefit from this symbiosis for adaptation to low-fertility soils and adverse environmental conditions (Chenchouni et al. 2020; Robă et al. 2024; Tekaya et al. 2022). The species identified in the present study, such as G. ambisporum, D. globifera, and G. decipiens, together with the coexistence of different functional groups, may represent a promising source of propagules for future inoculation programs and sustainable orchard management. Generalist members of the Glomeraceae family are typically associated with rapid root colonization, whereas representatives of the Gigasporaceae family may contribute to greater soil exploration through extensive extraradical mycelial networks, potentially enhancing nutrient acquisition (Arcidiacono et al. 2024; Cahyaningtyas and Ezawa 2024). Thus, differences in these functional attributes among species could potentially influence plant–fungus interactions and the functioning of the symbiosis. However, these interpretations remain speculative, and additional experimental studies are required to determine whether the compositional differences observed among cultivars translate into differences in plant performance and ecosystem services associated with the symbiosis.

Importantly, no commercial AMF inoculants had been applied to the experimental orchard before or during the study period, indicating that the community characterized here was likely predominantly autochthonous. This aspect is ecologically relevant because the introduction of commercial isolates may alter the structure of native AMF communities. Previous studies have shown that introduced fungi can initially suppress the colonization of indigenous AMF, whereas long-term observations indicate that many introduced isolates gradually decline in abundance or are eventually excluded from the system, probably due to the greater ecological adaptation of native communities to local soil and climatic conditions (Basiru and Hijri 2022; Janoušková et al. 2017). In addition to posing a lower risk of ecological imbalance, native AMF frequently exhibit greater persistence and effectiveness under field conditions, suggesting that their use as propagules represents a promising strategy for the development of inoculants adapted to regional edaphoclimatic conditions (Basiru and Hijri 2022).

However, it is important to consider several methodological limitations of the present study. Although previous studies have demonstrated that climatic conditions and soil physicochemical properties are key drivers of AMF community composition (Davison et al. 2021; Meddad-Hamza et al. 2017; Montes-Borrego et al. 2014), these variables were not directly assessed in this work. Therefore, the hypothesis that environmental factors contributed to the prevalence of certain species should be interpreted with caution, requiring further studies to understand these interactions in this cultivation area. In addition, the study was limited to four trees per cultivar due to the size of the experimental area, which restricts the statistical power of the analyses and the generalizability of the results. Although the multivariate analyses applied are appropriate for the sample size, the limited number of replicates may have increased uncertainty, particularly for species with low frequency of occurrence. The identification was based exclusively on spore morphology, which captures only a fraction of the true diversity of AMF present in soil, since taxa that sporulate poorly under field conditions or exhibit cryptic diversity remain undetected (Palla et al. 2020; Stockinger et al. 2010). Consequently, some of the observed patterns in species abundance and composition may reflect sampling and methodological limitations rather than purely ecological processes. Nevertheless, morphology-based identification relying on spore wall structure and differential staining with Melzer’s reagent remains a well-established method in AMF ecology (Melloni et al. 2020; Oehl et al. 2011). Thus, the fungal assemblage described herein provides an important ecological baseline for arbuscular mycorrhizal fungal communities associated with olive cultivation in southern Brazil, a region for which no previous inventories are available. Future studies integrating greater sampling effort, root colonization assessments, molecular approaches, and direct evaluation of environmental variables, as proposed by Kemmelmeier et al. (2022) and Palla et al. (2020), will be necessary to disentangle the relative contributions of environmental conditions, plant genotypes, and sampling effects on the structure of AMF communities associated with olive trees in this region.