Secret sounds beneath our feet can help restore soil health

By Dr. Jake Robinson, Flinders University, australia

Soils are the foundation of terrestrial life. They are conglomerates of both abiotic (non-living) and biotic (living) entities that form dynamic ecosystems. An estimated 59% of planet Earth’s species live in soil and invertebrates have major roles in keeping soils healthy. However, 75% of the world’s soils are affected by degradation––a figure that could rise to 90% by 2050 if deforestation, overgrazing, urbanisation and other harmful practices persist. This poses a major problem for biodiversity and the ecosystem services that sustain human populations. Indeed, 98% of our calories come from soil, and earthworms alone underpin 6.5% of the world’s grain production. However, detecting, measuring and monitoring soil biota is challenging––it’s too costly and intrusive to do at scale. Therefore, improved cost-effective and non-destructive soil biota assessments to guide soil management and restoration are needed.

Cue ecoacoustics. Ecoacoustic tools are used to detect acoustic waves emitted by organisms (e.g., bats, cetaceans, birds, frogs, insects, trees). For instance, ecoacoustics is widely employed to monitor above-ground and aquatic soundscapes, revealing changes in biodiversity and ecosystem dynamics. By detecting variations in species' acoustic signatures, ecoacoustics provides valuable insights into the status and trends of ecosystems.

Early trials testing microphones in soil-filled buckets

All living things make sounds. After all, sound is a form of energy––a set of vibrations that are propagated as acoustic waves through a particular medium, be it, air, liquid or solid. Some organisms are known as ‘soniferous’, meaning ‘sound-producing’. This term is typically reserved for those life forms that deliberately generate sound, for things like communicating (think of bird song) and navigating (think of echolocating bats). Other organisms simply produce sounds as a by-product of their activity, for instance, moving around a given environment (such as soil) causes vibrations.

We apply ecoacoustics in our research to detect the sounds of soil. We use specialised microphones attached to probes which we place into the ground to record the organisms. The recorded acoustic data can be analysed to make inferences about the abundance and diversity of soil communities. For this, we use acoustic indices – quantitative measures of soundscapes based on their frequency, amplitude, and temporal patterns. In healthy soils, soil biota are abundant and active, which is reflected in their acoustic profiles, as we have recently shown in UK and Australian forests.

Recording soil ecoacoustics data in the field

Soil ecoacoustics can help us monitor soil health. For instance, it can help us track the effectiveness of soil restoration interventions, and farmers may use it to gain insights into areas where earthworm populations are deficient. My colleague at Flinders University (Australia) has likened it to going to the doctor. When a person has an ailment, their doctor often asks them lifestyle questions, takes a blood test, and listens to the sounds of their lungs and heart. Our ecoacoustics research aims to do the same for soil. We ask questions about restoration status, take soil tests, and listen to the soil’s metaphorical beating heart––the acoustic waves emitted by resident soil biota. This aids in assessing soil degradation levels and gauging the effectiveness of our efforts in restoring soil biota.

It's still in its early days, but we think ecoacoustics has great potential to improve soil biodiversity monitoring. Indeed, a ‘horizon scan of global biological conservation issues for 2024’ identified soil ecoacoustics as an emerging priority.

The next time you're walking over soil, tread lightly. You don’t want to interrupt nature’s secret symphony!

References:

Anthony, M.A., Bender, S.F. and van der Heijden, M.G. (2023). Enumerating soil biodiversity. Proceedings of the National Academy of Sciences, 120(33), p.e2304663120.

Kraamwinkel, C.T., Beaulieu, A., Dias, T. and Howison, R.A. (2021). Planetary limits to soil degradation. Communications Earth & Environment, 2(1), p.249.

Kopittke, P.M., Menzies, N.W., Wang, P., McKenna, B.A. and Lombi, E. (2019). Soil and the intensification of agriculture for global food security. Environment international, 132, p.105078.

Fonte, S.J., Hsieh, M. and Mueller, N.D. (2023). Earthworms contribute significantly to global food production. Nature Communications, 14(1), p.5713.

Shamon, H., Paraskevopoulou, Z., Kitzes, J., Card, E., Deichmann, J.L., Boyce, A.J. and McShea, W.J. (2021). Using ecoacoustics metrices to track grassland bird richness across landscape gradients. Ecological Indicators, 120, p.106928.

Linke, S., Gifford, T., Desjonquères, C., Tonolla, D., Aubin, T., Barclay, L., Karaconstantis, C., Kennard, M.J., Rybak, F. and Sueur, J. (2018). Freshwater ecoacoustics as a tool for continuous ecosystem monitoring. Frontiers in Ecology and the Environment, 16(4), pp.231-238.

Robinson, J.M., Breed, M.F. and Abrahams, C. (2023). The sound of restored soil: using ecoacoustics to measure soil biodiversity in a temperate forest restoration context. Restoration Ecology, 31(5), p.e13934.

Robinson, J.M., Taylor, A., Fickling, N., Sun, X. and Breed, M.F. (2024). Sounds of the underground reflect soil biodiversity dynamics across a grassy woodland restoration chronosequence. bioRxiv, pp.2024-01.

Sutherland, W.J., Bennett, C., Brotherton, P.N., Butchart, S.H., Butterworth, H.M., Clarke, S.J., Esmail, N., Fleishman, E., Gaston, K.J., Herbert-Read, J.E. and Hughes, A.C. (2023). A horizon scan of global biological conservation issues for 2024. Trends in Ecology & Evolution.