A new study reveals the microbial biodiversity of dehesa soil

A study reveals the underground interactions between fungi and oomycetes in twenty Andalusian dehesas, wooded pasturelands typical of the Iberian Peninsula, making it possible to identify the role of water as the main driver of microorganism diversity and to shed new light on the pathogen responsible for la seca, a disease greatly affecting the holm oak

Dehesas are home to much more than meets the eye. Under those open landscapes of holm oaks and grasslands typical of rural Spain lies a microbial world hidden in the soil, affecting the health and stability of the ecosystem. Although these communities are vital for the sustenance of life, their composition and the factors shaping their presence in these agroforestry systems remain little explored.

In order to better understand into this underground world, the Agricultural and Forestry Systems Evaluation and Restoration (ERSAF) research group at the University of Cordoba characterized these microbial populations in twenty Andalusian dehesas located in different climatic zones, such as the Valle de los Pedroches, a valley in Cordoba, and the Sierra de Aracena, a mountain range in Huelva. The study specifically analyzed the interactions between fungi and oomycetes (a group of differentiated organisms, but similar to fungi) present in the rhizosphere of holm oaks; that is, the part of the soil immediately adjacent to the roots and exposed to their influence.

Katherine Onoszko, the study’s lead author, explained that the team used a high-performance DNA sequencing technique called metabarcoding, which allows for the simultaneous identification of multiple organisms within the same sample using specific genetic markers and bioinformatics tools. By analyzing all these samples, the research group has been able not only to establish patterns and make comparisons between different areas, but also to relate climatic and topographic variables with this entire microscopic underground universe in which just two grams of soil can harbor between 800 and 1,200 species of fungi.

Water, the key driver of fungal biodiversity

Seeing the soil through this technique is what has allowed the research group to confirm several hypotheses. Beyond the influence of dominant plant species and other factors, water availability is the main driver of microbial diversity, especially fungi.

“Within a more or less similar climatic region and with the same soil composition, fungal communities can be expected to be very similar. However, we have observed that even small differences in rainfall and variations in the availability of water in the soil make a difference and greatly influence fungal communities,” says researcher Pablo González, another of the study's authors.

The pathogen that causes la seca: a destructive competitor

While water is the most decisive factor in the case of fungi, the composition of oomycetes is more influenced by interactions between species. An example of this is Phytophthora cinnamomi, best known for being the cause of la seca in holm oaks and cork oaks, a pathogen that is of particular concern to the scientific community due to its high incidence in dehesas, and that the team has followed closely in its study.

According to the results of the work, and as previous research by the group has already pointed out, the microorganism that causes la seca develops more easily in poor soil where there are fewer species, while in richer soils with greater diversity its infection capacity is reduced.

The fact that this pathogen is actually a destructive competitor opens the door to future research to combat la seca through other beneficial microorganisms that increase biodiversity, especially in a context “in which the use of phytosanitary products is increasingly restricted and the trend is towards the implementation of biological solutions,” explained Francisco Ruiz, another of the authors who participated in the study. Beyond this, as the researcher points out, designing effective strategies requires knowing the specific microbial communities. Based on this knowledge, measures could be applied meeting the specific needs of each dehesa, spaces of special importance due to their ecological value, and whose health begins with their soil.

Reference:

Onoszko, Katherine & González-Moreno, Pablo & Ruiz-Gomez, Francisco. (2025). Unravelling the diversity of soil fungal and oomycete communities in the Quercus ilex L. rhizosphere of dehesa grasslands: a metabarcoding approach. Plant and Soil. 1-30 10.1007/s11104-025-08019-9.