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Soil ecoacoustics is an emerging field and suite of tools that use sound and vibration to detect belowground biological activity. It offers a minimally invasive way to assess soil communities and ecosystem processes. Across biomes, we found that soil ecoacoustics is being used to detect organisms, quantify animal behaviour, monitor soil health, and assess restoration interventions. In this review, we show that ecoacoustic metrics reflect changes in soil fauna activity, disturbance impacts, and recovery trajectories. However, major challenges remain, including inconsistent terminology, limited understanding of sound propagation across soil types, difficulty separating biotic from abiotic signals, and a lack of standardised methods. Thus, we propose foundational standard operating procedures (SOPs) and identify how soil ecoacoustics could be integrated into global biodiversity monitoring frameworks.

Ants serve as ecosystem engineers that maintain important ecological processes within forests. Given their ecological importance, it is a clear scientific shortcoming that we lack non-invasive methods to survey their behaviour inside common opaque habitats such as mounds, litter, and soil. In this study, we assess if acoustic signals from red wood ant (Formica rufa) mounds are useful to infer temporal changes in ant activity within forested ecosystems. We found that acoustic indices used previously as a proxy for soil fauna in soil ecological studies (Acoustic Complexity Index, Bioacoustic Index) can indeed separate sounds generated by the ant's daily routines (biophony) from other forest sounds. Yet, we also show that these indices are problematic proxies for soil diversity as they increase not only due to an increased number of species but also due to an increased number of the same species. Acoustic measures that incorporated the strength of acoustic signals, Average Power Density (APD) and Peak Power Density (PPD) also increased with increasing ant abundance and constituted the conceptually best proxy for ant activity. For example, the PPD could i) track diurnal changes in Formica rufa activity with a high temporal resolution (minutes) and ii) detect altered behavioural responses to temperature changes. We conclude that microphones detecting biophony can provide high-resolution information about in situ ant behaviours in forested ecosystems. Thus, passive acoustics monitoring offers a promising avenue as a non-invasive monitoring tool for soil macrofauna studies.