Introduction

Arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota, establish symbiotic associations with approximately 80–90% of terrestrial plant species (Wang et al. 2023; Wu et al. 2024). The evolutionary success of these fungi is largely attributed to their ability to form mutualistic interactions with plant roots in diverse ecosystems, contributing to their cosmopolitan distribution (Bonfante 2022; Rosling et al. 2024). Inside the root cortex cells, AMF develop highly branched structures called arbuscules, which constitute the main site of nutrient exchange between the fungus and the host plant (Prates Júnior et al. 2021). These fungi also produce vesicles that serve as lipid-rich storage structures, providing energy reserves for fungal development, while extracellular vesicles can contribute to molecular communication between plant and fungus (Holland and Roth 2023; Kameoka and Gutjahr 2022). Furthermore, extraradical hyphal networks extend beyond root depletion zones, expanding soil exploration and favoring the absorption of water and low-mobility nutrients such as phosphorus and zinc (Diagne et al. 2020; Hanane et al. 2020; Smith and Smith 2011).

In addition to their nutritional function, AMF contribute to plant tolerance to abiotic stresses, including drought, salinity, and heavy metal toxicity, promoting greater resilience in adverse environmental conditions (Dhalaria et al. 2020; Ennajeh and Ouledali 2024; Matos et al. 2022). Functional mycorrhizal associations have also been linked to better establishment and greater survival of transplanted seedlings, a critical factor for perennial agricultural systems (Ahmed et al. 2025; Meddad-Hamza et al. 2017). At the ecosystem level, AMF influence soil aggregation processes and structure rhizosphere microbial communities, contributing to the maintenance of biodiversity and the functioning of ecosystems (Shi et al. 2022; Shukla et al. 2025).

However, the composition and sporulation dynamics of AMF communities are also influenced by the plant species and the phenological stage of the host (Coelho et al. 2024; Deepika and Kothamasi 2021; Wang et al. 2024). Previous studies have shown that spore abundance and root colonization may vary throughout plant development, with higher sporulation frequently observed during reproductive and maturation stages, whereas the flowering stage may be associated with reduced spore production due to the high phosphorus demand of the host plant (Bohacz et al. 2022; Coelho et al. 2024). Furthermore, environmental management and edaphoclimatic characteristics such as seasonal fluctuations in precipitation and temperature also strongly affect AMF dynamics (Dong et al. 2025; Sakha et al. 2025). In tropical and subtropical environments, dry periods are generally associated with higher spore densities, under conditions of water limitation and elevated soil temperatures, while during the rainy seasons the concentration of spores is low and soil moisture limits colonization and symbiosis but not the viability of AMF (Coelho et al. 2024; Tenzin et al. 2022). Thus, the characterization of native AMF communities has been increasingly used as an ecological indicator of soil health and agroecosystem functioning (Bohacz et al. 2022).

In the context of olive growing, the plant benefits from mycorrhizal association through increased nutrient uptake and greater tolerance to environmental stresses (Chenchouni et al. 2020; Robã et al. 2024; Tekaya et al. 2022). Olive cultivation in Brazil has been increasing due to its economic potential and benefits for human health, which contain bioactive compounds that help prevent cardiovascular disease and diabetes (Bucciantini et al. 2021; Dos Santos et al. 2023; Sá 2024). Brazil has approximately 4,266 ha devoted to olive cultivation, with an average productivity of 1,054.6 kg ha−1 of olives (FAOSTAT 2024). Rio Grande do Sul is currently the leading olive-producing state in the country, generating more than BRL 10.462 million in production value (IBGE 2024). The expansion and consolidation of olive cultivation in the state have been fostered by initiatives such as the Programa Estadual de Desenvolvimento da Olivicultura (PRÓ-OLIVA), among the most widely cultivated olive cultivars in Rio Grande do Sul are Arbequina, Coratina, and Picual, which differ in key agronomic traits: Arbequina is a Spanish cultivar with early bearing and wide climatic adaptability; Coratina is an Italian cultivar valued for its high polyphenol content and antioxidant activity; and Picual is the world's most widely cultivated Spanish cultivar, known for its high oil yield and cold tolerance (Coutinho et al. 2009; Grompone and Villamil 2013; Miho et al. 2021). which promotes the adoption of technologies aimed at olive and olive oil production, contributing to increased productivity and enhanced income for family farmers through the establishment of new orchards (SEAPI 2025). However, the success of implementing and producing new orchards depends on the use of fertilizers, which contributes to Brazil's economic dependence on importing 85% of the mineral fertilizers used in the country (Al-Din and Alalaf 2025; Roma et al. 2023; Russo and Figueira 2023).

Given this scenario, the use of bioinputs such as arbuscular mycorrhizal fungi emerges as a strategic alternative to reduce dependence on chemical fertilizers (Ouhaddou et al. 2025), contributing to a more sustainable and accessible production model for family farming.

However, for these bioinputs to be effective, it is necessary to develop inoculants adapted to local conditions, since the composition of AMF communities is strongly influenced by edaphoclimatic factors, agricultural management, and plant species (Davison et al. 2021; Frew et al. 2025; Robã et al. 2024). Thus, characterizing the species naturally associated with established crops represents an essential first step in the development of bioinputs. Furthermore, the recovery of AMF spores from the rhizosphere may indicate the presence of infectious propagules and reflect the sporulating fraction of the community, suggesting that the taxa present constitute potential symbiotic partners for the host plants, although spore occurrence does not directly indicate active root colonization (da Silva et al. 2022; Kebede et al. 2023; Sivakumar 2013). No published data on root colonization rates in olive plantations within the study area are currently available, which further highlights the relevance of the present study as a baseline characterization.

In this context, it is hypothesized that the genotype of olive cultivars acts as a determining ecological filter in the assemblage of arbuscular mycorrhizal fungal communities, modulating not only the taxonomic composition but also the dominance structure and functional spectrum of associated taxa, with potential implications for symbiosis efficiency and nutrient acquisition. The objectives of this study were to characterize the arbuscular mycorrhizal fungi community associated with the rhizosphere of three olive cultivars cultivated in southern Brazil, to determine spore density and diversity indices for each cultivar, and to evaluate whether cultivar identity influences AMF community composition. In addition, this study provides the first inventory of AMF associated with olive orchards in Rio Grande do Sul, based on samples collected at the Palma Agricultural Center of the Federal University of Pelotas (UFPel). This study represents a first step towards future investigations aimed at elucidating the diversity and ecological functioning of arbuscular mycorrhizal fungi (AMF) communities in olive agroecosystems. Furthermore, knowledge of the native AMF community could support future studies focused on the development of regionally adapted bioinoculants and other mycorrhiza-based bioinputs.