A new tool for conservation?

For Jake Robinson, Greno Woods was an easy choice. This large, old forest in South Yorkshire county in the U.K. has historically undergone timber harvesting, quarrying and fires. Plantations of non-native conifers replaced parts of the forest several decades ago. At the same time, restoration efforts have tried to revive some of the degraded and deforested areas through the planting of native broadleaved and deciduous trees. This setting presented a unique opportunity: to listen to the sounds of restoration.

Robinson, a microbial and restoration ecologist at Flinders University in Australia, was particularly interested in two types of area in the forest: sites that had been cleared in the past three years, and previously deforested plots that restorers had planted with native trees more than 30 years ago.

Aboveground, the differences between the two were easy to spot. The recently cleared plots were mostly covered by bracken (Pteridium aquilinum), with a sprinkling of silver birch saplings (Betula pendula). By contrast, the restored areas had more diverse trees, including English oak (Quercus robur), sessile oak (Q. petraea), rowan (Sorbus aucuparia) and silver birch, along with a complex understory of bilberry (Vaccinium myrtillus), bramble (Rubus fruticosus agg.), holly (Ilex aquifolium) and bracken.

Field recording in Greno Wood. Sound can be used as a proxy in some cases — listening to soil can indirectly show what’s happening belowground without having to overturn every inch and disturb the denizens of the dirt. Image courtesy of Jake Robinson.

But Robinson wasn’t just interested in what he could see. He wanted to know if the restored and deforested sites were also different belowground.

After all, soil, while unremarkable to most of us at first glance, is home to more biodiversity per unit area than aboveground. This underground biodiversity — from worms to mites, and moles to bacteria to fungi — shapes the life you see on the surface.

“So many organisms in the soil play crucial roles in the functioning of the ecosystem like nutrient cycling, climate regulation, to plant and animal health,” Robinson said. “A lot of these things occur beneath the ground that we can’t actually see.”

The traditional ways of “seeing” under the ground, while critical, can be hard, laborious and expensive: you dig up the soil and sift through it to figure out what kinds of animals live there and how many. But recent research shows that sound can be used as a proxy in some cases — that listening to soil can indirectly show what’s happening belowground without having to overturn every inch and disturb the denizens of the dirt.

Sounds of soil

Marcus Maeder, an acoustic ecologist and sound artist at ETH Zurich, the Swiss Federal Institute of Technology, first got a taste of what the underground could sound like by accident. He was on vacation, sitting on a meadow in the Swiss alps, when, out of curiosity, he jabbed the probe of a special microphone he’d developed for another project into the ground. Immediately, his headphones were hit by a flurry of sounds.

“I was like, ‘Oh that’s crazy,’” Maeder said.

His interest piqued, Maeder eventually designed studies with other researchers to figure out what they could hear in soil. They installed special acoustic sensors at different points in a forest in Switzerland, and recorded sounds coming from the soil over different times of the day and seasons.

In the end, the researchers discovered that the underground world was a noisy one. From water moving through pores in the soil, to the vibrations of different animals moving and communicating, they could detect a wide variety of frequencies. And all of these sounds varied with the time of the day and season, according to a study published by Maeder and his colleagues in PLOS ONE last year.

They also found that soils that had a richer variety of sounds contained a greater diversity of species.

“This was a proof of concept to show that acoustic measurements of biodiversity are possible in soils,” Maeder said.

Eavesdropping on other life-forms isn’t new. Acoustics has become a part of several conservation researchers’ toolkit, where they’ve used recorders to track changes in the diversity of life inside forests and in the ocean.

Degraded plot in Greno Woods with audio sample of soil sounds from a degraded plot. Image and audio courtesy of Jake Robinson.

“We tried to adapt these methods to soil and that worked because we saw a close correlation between acoustic complexity measurements and species richness in our soil samples at the same time,” Maeder said.

In fact, in another study, Maeder and his colleagues recorded soils across different forms of land use. Intensively managed agricultural lands, they found, were eerily quiet.

“There were not many animals to hear. And that made us think, ‘OK, maybe we really hear management problems in the soil,’” Maeder said.

Inspired by the work of researchers like Maeder, Robinson wanted to see if he could hear the effects of restoration in soil as well. So he spent three months in 2022 listening to sounds coming from the soil samples of both the deforested and restored parts of Greno Woods. As it turned out, there were big differences.

The soils from the restored plots had a more diverse soundscape, or a greater variety of sounds, compared to soils from the recently deforested sites, Robinson and his colleagues reported in a recent paper published in Restoration Ecology. And this sound diversity was strongly related to the abundance of invertebrates; soil samples from the restored plots had many more invertebrates, like earthworms, than the ones from the deforested sites, a fact picked up on in the recordings.

What these differences mean for soil health isn’t immediately clear. But it’s generally thought that the more earthworms and other critters you have, the healthier the soil, unless the worms are not native to the region.

“So soil eco-acoustics, hopefully if we refine and optimize the techniques, can be used to get an indication of the health of the ecosystem over time,” Robinson added. “It can also tell us if the money we’re spending on ecosystem restoration is actually helping or not.”

But it’s still the early days of soil sound research. And many questions remain before soil acoustics is put to use for conservation efforts or to monitor restoration, Robinson said. “For example, can we infer some kind of ecological function from listening to the soil? Can we tell differences between individual species?”

Restored plot in Greno Woods with audio sample of soil sounds from a restored plot. Image and audio courtesy of Jake Robinson.

Detecting species

In Germany, Carolyn-Monika Görres has been attempting to solve one part of this puzzle.

Görres, a climate scientist at Geisenheim University, stumbled on soil acoustics while trying to solve a problem unrelated to restoration or conservation.

Her primary interest lies in figuring out how much greenhouse gas is emitted by soil-dwelling organisms. Termites, for instance, because of their abundance, are known to naturally produce considerable amounts of methane (termite mounds then act as sinks for much of this methane). But what about other insects with large numbers in soil?

To find out, Görres decided to study the larvae of two May beetle species: the forest cockchafer (Melolontha hippocastani) and the common cockchafer (M. melolontha). The larvae of these beetles, which can reach huge numbers in the soil, can turn into damaging pests when they feed on the roots of grasses, crops and trees, slowly killing them. They’re also large in size, making them fairly easy to observe.

But the challenge for Görres was figuring out how many May beetle larvae were under the ground, without having to dig up every bit of the land.

“So I Googled a little bit and found David Chesmore at the University of York, who was monitoring insects in aboveground plants and in wood using eco-acoustics. And I thought, ‘Hey, great, maybe we can also do that in soils,’” she said.

Eventually, Görres took some larvae-infested soil samples to her lab and listened to them. She hit the jackpot.

Not only could Görres and Chesmore hear the larvae moving and feeding through the soil, but they could also differentiate between the two species. Larvae of both species, they found, create their own distinct stridulation sounds — the scraping noise made as the larvae rub their mandibles or mouth parts together.

“Since the larvae make these sounds intentionally, they’re very rhythmic,” Görres said. “So they always have a special pattern to them. That’s why I can say, this pattern is from this species and this pattern is from that species.”

For Görres, the results from her lab are an exciting start. She’s now trying to see if she can refine her techniques to detect the larvae of the two May beetle species under meadows and forests directly. If successful, it could open up a lot of possibilities.

Putting sounds to use

Farmers, for instance, want to know where May beetle larvae infestations have occurred, Görres said. That would give them an opportunity to reduce the use of insecticides and specifically target the affected areas.

Listening to soil sounds could also help detect deteriorating soil health before its effects are visible aboveground, Maeder added. “We’ve talked to farmers about our recordings, where we’ve heard a problem, but they say their plants are healthy. And that shows how soil issues can take time to show up.”

Soil acoustics could also help foresters. Many of Germany’s forests, for example, are under severe stress from years of drought and infestations by various kinds of beetles.

Carolyn-Monika Görres installing acoustic sensors in the soil for continuous non-supervised audio recordings. Each green box contains a data logger and a battery, and is connected to one acoustic soil sensor. Image courtesy of Carolyn-Monika Görres.

 

“The challenge is that foresters can’t say which areas suffer from the effects of drought, and which ones from beetle larvae infestations by looking at the trees alone,” Görres said. “Their current method to locate infestation is to hire people who dig up the soil and count the larvae manually.”

But listening to forest soil could be a cheaper and quicker alternative. “It’s also nice to know where the larvae populations are and how old they are, so foresters can plant young trees in a way that have a higher chance of survival,” Görres said.

Soil sounds can also give researchers a window into how human activities affect life underground. “Especially noise from traffic and agricultural machinery,” Maeder said. “The first experiments that we are doing now are showing that this noise has a big impact on not only soil animals, but things like decomposition rate and growth of fungi.”

In this new field, researchers have barely scratched the surface of all that soil acoustics can be used for. And there are lots of teething issues that need fixing.

Soil is a difficult medium to work with and researchers are only beginning to tease apart what you can hear in it. Listening to soils outside the laboratory or controlled settings also has its challenges, like that of weather; the patter of rain on the ground or the howling of the wind can mask other soil sounds, making field recordings tricky.

Handling vast amounts of acoustic data is also a tough task. “That’s the bottleneck right now,” Görres said. “It’s easy to get the audio data, but there’s so much of it that it’s almost impossible to analyze them manually. What we need are automated algorithms for analysis.”

Despite these challenges, the researchers say they’re excited. For a long time, soil has been a bit of a black box, Görres said, with a fascinating world hidden from view. Perhaps listening to it will give us some answers.

Banner image: It’s generally thought that the more earthworms and other critters you have, the healthier the soil, unless the worms are not native to the region. Image by Nikola Johnny Mirkovic via Unsplash (Public domain).

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Citations:

Nielsen, U. N., Wall, D. H., & Six, J. (2015). Soil biodiversity and the environment. Annual Review of Environment and Resources, 40, 63-90. doi:10.1146/annurev-environ-102014-021257

Maeder, M., Guo, X., Neff, F., Schneider Mathis, D., Gossner, M. M. (2022). Temporal and spatial dynamics in soil acoustics and their relation to soil animal diversity. PLOS ONE, 17(3), e0263618. doi:10.1371/journal.pone.0263618

Maeder, M., Gossner, M. M., Keller, A., & Neukom, M. (2019). Sounding soil: An acoustic, ecological & artistic investigation of soil life. Soundscape Journal, 18(1), 5-14. Retrieved from https://www.academia.edu/download/60598560/SoundingSoil_Paper_Soundscape_Maeder20190914-126314-yaz882.pdf

Robinson, J. M., Breed, M. F., & Abrahams, C. (2023). The sound of restored soil: Using ecoacoustics to measure soil biodiversity in a temperate forest restoration context. Restoration Ecology, e13934. doi:10.1111/rec.13934

Korzekwa, K. (2015). Earthworm invaders have huge implications for forest health. Soil Horizons, 56(4), 1-3. doi:10.2136/sh2015-56-4-f

Brune, A. (2014). Symbiotic digestion of lignocellulose in termite guts. Nature Reviews Microbiology, 12(3), 168-180. doi:10.1038/nrmicro3182

Nauer, P. A., Hutley, L. B., & Arndt, S. K. (2018). Termite mounds mitigate half of termite methane emissions. Proceedings of the National Academy of Sciences, 115(52), 13306-13311. doi:10.1073/pnas.1809790115

Görres, C. M., & Chesmore, D. (2019). Active sound production of scarab beetle larvae opens up new possibilities for species-specific pest monitoring in soils. Scientific Reports, 9(1), 10115. doi:10.1038/s41598-019-46121-y

Wagenhoff, E., Blum, R., Henke, L., & Delb, H. (2015). Aerial spraying of Neemazal®-T/S against the forest cockchafer (Melolontha hippocastani, Col.: Scarabaeidae) in South-West Germany: The effects of two field trials performed in 2007 and 2008 on local populations. Journal of Plant Diseases and Protection, 122(4), 169-182. doi:10.1007/bf03356547