Underground, intricate networks of soil fungi underpin the functioning of terrestrial ecosystems. Yet despite their global importance, only 30% of global ecosystems have been sampled for these fungal partners, says research led by Justin Stewart, ecologist at VU Amsterdam and the Society for the Protection of Underground Networks (SPUN).
Arbuscular mycorrhizal (AM) fungi form important resource trade partnerships with plants. The fungi grow complex networks to help plants acquire nutrients, such as phosphorus and nitrogen. In exchange, plants provide carbon to the fungi, with roughly one billion tons of carbon transferred annually from plants to mycorrhizal fungal partners. Because these networks move massive amounts of carbon, nutrients, and water, they are often referred to as one of Earth’s circulatory systems.
In a study published in FEMS Microbiology Letters, researchers analysed environmental DNA from the largest global dataset of AM fungi compiled to date. Their findings reveal a striking gap: more than 70% of the world’s ecoregions have no sequencing data for AM fungi. By mapping these gaps, the researchers highlight how data collection has been heavily skewed toward just a few regions. The study was awarded “Best Study of 2025” in FEMS Microbiology Letters by the journal’s senior editors.
Limited set of regions
“AM fungi underpin the functioning of most terrestrial ecosystems, but our data are heavily skewed toward a limited set of regions,” says Justin Stewart. “If we want robust predictions about biodiversity, carbon cycling, and ecosystem resilience, we need far more representative global sampling.”
Large areas of Africa, parts of Asia, boreal systems, and drylands remain severely underrepresented. The consequences extend beyond biodiversity surveys. AM fungi influence plant productivity, soil aggregation, and carbon stabilisation. Without more representative sampling, projections of fungal distributions and their contributions to climate mitigation and restoration targets carry substantial uncertainty. Drawing attention to these severe data gaps can encourage researchers to focus future sampling efforts in key habitats.
Protecting Earth's mycorrhizal networks
The study is part of an ongoing global effort led by SPUN. Their mission is to map and advocate for the protection of Earth’s mycorrhizal networks. Last year, collaborators published global maps of mycorrhizal fungal diversity in Nature. These maps integrated nearly three billion DNA sequences, satellite imagery, and approximately 25,000 soil samples to generate predictive models of mycorrhizal fungal biodiversity and endemism globally.
While these models can predict biodiversity patterns globally – including in ecosystems without samples – they require validation. Uncertainty increases in areas with little or no empirical data. Underground AM fungal DNA data is still missing or not publicly available for more than 600 terrestrial ecosystems. Reducing this uncertainty will require targeted sampling and ground truthing campaigns across underrepresented regions.
Underground Explorers
To address this, SPUN launched the Underground Explorers Program, a decentralised and community-led initiative in which researchers and local communities collect soil samples in underrepresented ecosystems. By expanding sampling into overlooked habitats, the programme aims to reduce uncertainty in global maps and generate open access data that improve predictions and forecasts of fungal biodiversity.
Picture by Yevgeniy Lechshenko / SPUN