Imagine a field left bone-dry for weeks. Dust swirls, cracks run across the surface, and everything appears lifeless. Then the first raindrops fall. For us, it is just rain. For the soil, it is a moment of awakening.

Beneath the surface, an explosion occurs: bacteria awaken, fungi stretch out, and the great machinery of life begins to turn again. This is all well known. But a new international study shows that something unexpected happens: a whole hidden world of so-called RNA viruses bursts into activity.

And the effect grows even stronger if the soil is given phosphorus, one of the most essential nutrients for plants.

A festival of viruses

The researchers behind the study compare it to a festival underground. Just a few days after rainfall, viruses that usually exist quietly and unnoticed begin to attack soil bacteria in enormous numbers. In fact, they estimate that between ten million and one billion bacteria per gram of soil can be infected.

“It’s a completely invisible boom, and we never imagined it could be this massive,” says Ella Sieradzki from the Department of Agroecology at Aarhus University.

The most striking discovery is that RNA viruses respond just like the more well-known DNA viruses.

“We know a lot about DNA viruses: they cause everything from colds to flu-like infections in plants and animals. RNA viruses, on the other hand, have been far harder to detect in soils, and their role has long been a mystery,” Ella Sieradzki says.

Phosphorus tips the balance

In the study, the scientists tested what happened when dry soil was watered, both with and without application of phosphorus. The result was clear: phosphorus turbocharged RNA viral activity.

Within just a week, the composition of the viral community had shifted dramatically, and differences between soils with and without phosphorus were still visible three weeks later.

“We know phosphorus is a nutrient that makes plants grow. Now we can see that it also changes the entire viral ecology of the soil,” explains Ella Sieradzki.

Why does it matter?

Talking about viruses in soil may seem remote. But the bacteria they infect are among the most important players in Earth’s natural cycles. They control how organic material is broken down, and how much carbon is released as CO₂ or stored in the soil.

If viruses can influence bacteria on such a scale, it means they may indirectly help determine how much carbon soil holds on to, and how much ends up in the atmosphere.

In other words: tiny, invisible RNA viruses could play a part in shaping the future of our climate.

A research field in its infancy

This study is among the first to shine a light on the role of RNA viruses in soil. Many of the viruses discovered were so unknown that they lacked the genes normally associated with virus particles. This suggests that many may live hidden inside bacteria and fungi, without ever producing free-floating virus particles.

There are still many unanswered questions: Do these results hold true across all soil types? Is the effect the same in farmland, forests, and grasslands? And how do viruses interact with plant roots and other microorganisms?

But one thing is clear: RNA viruses can no longer be considered invisible in the soil ecosystem.

More information

Partners: Department of Agroecology at Aarhus University, Université Claude Bernard Lyon, Lawrence Livermore National Laboratory, CNRS, University of California and Berkeley.

Funding: This work was supported by a Lawrence Livermore National Laboratory, Laboratory Directed Research & Development grant (21-LW-060) to GT and by LLNL’s U.S. Department of Energy, Office of Biological and Environmental Research, Genomic Science Program “Microbes Persist” Scientific Focus Area (#SCW1632). ETS was supported by a Marie Skłodowska-Curie postdoctoral fellowship “Divobis”. Work at LLNL was conducted under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344.

Conflicts of Interest: None.

Read more: The publication “Phosphate amendment drives bloom of RNA viruses after soil wet-up” was published in the journal Soil Biology and Biochemistry. Authors: Ella T. Sieradzki, G. Michael Allen, Jeffrey A. Kimbrel, Graeme W. Nicol, Christina Hazard, Erik Nuccio, Steven J. Blazewicz, Jennifer Pett-Ridge, and Gareth Trubl.

Contact: Assistant Professor Ella T. Sieradzki, Department of Agroecology, Aarhus University. Email: ellasiera@agro.au.dk