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Soils are the most species-rich ecosystems on Earth

A recent study has shed new light on the of our planet's ecosystems. While coral reefs, deep sea regions, and rainforest treetops are well-known hotspots for biodiversity, a surprising leader has emerged: soils. Researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, the University of Zurich, and the Agroscope agricultural research station have revealed that soils are the most species-rich ecosystems globally.

Their groundbreaking study, published in Proceedings of the National Academy of Sciences, unveils a remarkable discovery. It turns out that a staggering two-thirds of all species thrive within soil habitats. This astonishing figure is more than twice the previous estimates of soil biodiversity, which had placed soil-dwelling species at a mere 25%.

Among the diverse inhabitants of soils, fungi reign supreme, with a striking 90% of fungal species calling soil their home. Following closely are plants and their intricate root systems, encompassing 86% of the soil's biodiversity. Earthworms and mollusks, including snails, contribute to around 20% of soil life. Interestingly, the study highlights that the smallest organisms—such as bacteria, viruses, archaea, fungi, and unicellular organisms—have yet to be accurately estimated in terms of diversity.

Mark Anthony, the lead author from WSL, underscores the vital role of these tiny organisms. They play a crucial part in nutrient cycling, carbon storage, and even function as both pathogens and allies to trees. As we come to appreciate the significance of soil biodiversity, it becomes increasingly urgent to address the growing degradation and destruction of soils on a global scale.

Diversity of the major life forms found in soil. A) bristletail (© F. Ashwood), B) springtail (© H. Conrad), C) nitrogen-fixing bacteria-containing nodules on clover root (© M. van der Heijden), D) predatory mite (© H. Conrad), E) isopod (© F. Ashwood), F) scots pine root colonized by ectomycorrhizal fungi (yellow) (© M. Anthony), G) earthworm (© G.Brändle), H) nematode (© A. Murray), I) corn root colonized by arbuscular mycorrhizal fungi (blue) (© F. Bender), J) springtail (© F. Ashwood), K) a common soil bacterium Bacillus (Creative Commons Attribution-Share license, photo by M. Das Murtey and P. Ramasamy), L) horned mite (© H. Conrad), M), pseudoscorpion (© F. Ashwood), N) phage infecting a soil bacterium (© T. de Carvalho), O) centipede. Credit: Ashwood

Promote better protection of soils

The study's findings highlight the challenge of obtaining comprehensive data on soil diversity, especially in regions of the global South. This has led to significant variability in the results. For instance, when it comes to bacteria, the average percentage of species residing in soil is around 40%. However, the range spans from 25% to a high of 88%. Similarly, uncertainties abound for viruses, which have predominantly been studied in the context of human pathogens.

Recognizing these potential limitations, the authors of the study are prepared for potential critique of their methodologies and conclusions. Mark Anthony, the lead author, acknowledges that their work represents a pioneering effort to estimate the proportion of global biodiversity present within soil ecosystems.

Graphical overview of the share of species living in soil. Doughnuts reflect the percentage of species in soil versus all other ecosystems combined (e.g., marine, freshwater, built , host organisms such as humans). The larger doughnut on top shows the total share of species, and smaller doughnuts show individual shares for the most speciose and well-known groups ordered from greatest to least specialized in soil. Credit: Michael Dandley

The ultimate objective, as Anthony explains, is to lay the groundwork for essential decisions aimed at safeguarding soils and their inhabitants on a global scale. Soil environments face mounting pressures, ranging from intensified and climate change to the spread of . Anthony underscores the significance of their study, emphasizing that the vast diversity found within soils warrants greater attention and prioritization in efforts.

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