Spread throughout the soil, bacteria, fungi and archaea make up 18 percent of the Planet’s biomass. Their services are critical for humans and plants, a new research explains. But some critical factors put their resilience at risk
by Matteo Cavallito
The soil microbiome is the link between soil health, animal health and human well-being. But some major phenomena, from the use of chemicals in agriculture to climate change, threaten this balance and suggest new strategies. That’s the message delivered by recent research published in the journal Nature. The study, by researchers Samiran Banerjee, of North Dakota State University in Fargo, USA, and Marcel van der Heijden, of the University of Zurich, focuses on the functions performed by soil microorganisms, highlighting their direct and indirect impacts.
““We identify microorganisms that are shared between different one health compartments,” the authors explain. “And (We) show that soil, plant and human microbiomes are perhaps more interconnected than previously thought.”
Soil Microbiomes and One Health. Our review in @NatureRevMicro is out.
We demonstrate that soils are a cornerstone of one health. We list over 40 soil microbiome functions that contribute to soil, plant, animal and human health. @sbanerjee2018 https://t.co/6RVfubdQMs pic.twitter.com/PhxRKlmowV
— van der Heijden Lab (@vandeHeijdenLab) August 23, 2022
A huge invisible biomass
Microorganisms are so extremely widespread in soil that they represent the second largest fraction of the Planet’s biomass after plants with a share of 18 percent (including bacteria, fungi and archaea). According to the authors, they are the common thread in the process that connects soil with living creatures in a relationship of continuous exchange.
“Microorganisms link soil, plant, animal and human health, and microbial communities connect the different ecosystems,” the research states.
Moreover: “Bulk soil is likely the largest contributor to plant endophytic microbiota, contributing more than two- thirds of the bacterial and fungal diversity. Diets play a major role in shaping the gut microbiome composition of both humans and animals.”
The “One health” approach
Recognition of this interdependence is the basis of an approach known as One Health (holistic view of health) that, as defined by FAO, “aims to sustainably balance and optimize the health of people, animals and ecosystems.” Moreover, this paradigm “is essential for progress to anticipate, prevent, detect and control diseases that spread between animals and humans, tackle AMR, ensure food safety, prevent environment-related human and animal health threats, as well as combatting many other challenges.”
Not surprisingly, in this sense, a similar idea has provided some inspiration for research in response to the global pandemic. Recently, for example, an Anglo-Chinese study identified a statistical correlation between the concentration of zinc, iron, copper and selenium in soil and the outcome of Covid cases. Where the presence is lower, the authors reported, the mortality rate is higher. The survey had thus suggested a link between the phenomena while recalling the need for further studies.
Plant health comes through the microbiome
Plants are the primary beneficiaries of the action of soil microorganisms. In fact, plant species derive as many as 18 of the 29 elements essential for their life from the soil. The microbiome of the rhizosphere – the portion of soil surrounding the roots – also strengthens the metabolic repertoire of plants and facilitates a number of processes, including seed germination, seedling establishment, nutrition, water uptake, growth, pathogen suppression and stress tolerance.
No less important is the contribution to animal health. Many insects, for example, require specific microorganisms for their growth and survival. These, the researchers explain, are transmitted not only from parents to offspring but also through nutrition by eating plants that are exposed, in turn, to the microbiome.
The link between soil and human well-being
Finally, our well-being. “Soils can influence human health and society in a multitude of ways,” the researchers write. “Humans deliberately ingest soil to supplement a nutrient- poor diet (a phenomenon observed in parts of Asia, sub-Saharan Africa and Latin America as well as some Pacific islands, ed.). Soils are also used as detoxifying agents for making some food products edible, as well as for medicinal reasons.”
In addition, “People with more exposure to natural environments are less likely to suffer from allergic reactions, which may be linked to soil microbiomes and inhalation of soil particles.”
Of course, not all soil microorganisms have a beneficial effect. “More than 300 soil fungal species are known to cause human disease,” the research further states. However, “Although there are reports of human diseases from soil- borne pathogens, it is unclear what proportion of the human microbiome is directly or indi-rectly linked to the soil microbial reservoir.” The link between soil and human health, in any case, is a widely recognized fact.
Critical factors and recommendations
However, the authors note, several global change factors directly threaten the contributions of microbes to ecosystem services and health. These include climate change that promotes drought, reduced soil organic matter and declining microbial populations. The list of threats also includes pollution (from microplastics, antibiotics and pesticides) and intensification of land use.
“The total area of cultivated land worldwide has increased by more than 500% in the past five decades with a 700% increase in fertilizer use and a several- fold increase in pesticide use,” the research states. “Such intensive practices can reduce the diversity and complexity of microbiomes and negatively influence beneficial microorganisms in roots and soils.”
Against this backdrop, the authors therefore suggest developing new frontiers of research to answer questions that are still open. Among the issues to be addressed, they point out, are soil’s propensity to act as a reservoir for pathogens, management practices designed to counteract this phenomenon, and the interactions of microorganisms with other entities including viruses, nematodes, earthworms and soil arthropods.