Listening to the Soil: Temporal Organization and Environmental Drivers of Soil Sonotopes Across Seasonal and Solar Cycles

Soil ecosystems generate a wide variety of biological and physical sounds, yet the temporal organization of soil acoustic environments remains poorly understood. This study investigated seasonal and daily dynamics of soil acoustic activity using continuous recordings collected over an entire calendar year at four soil stations subjected to different vegetation management regimes. Custom-built piezoelectric probes were used to record vibrations within the upper soil layer. Two complementary analytical approaches were applied. Conventional Acoustic Features (Root Mean Square, Zero Crossing Rate, Spectral Centroid, Spectral Bandwidth, Spectral Entropy, and Mel-Frequency Cepstral Coefficients) were used to characterize monthly and solar-phase variability, whereas Sonic Heterogeneity Indices (SHIft and SHItf) were employed to investigate seasonal organization and climatic forcing. Climatic variables and solar phases were analyzed using correlation analyses, clustering procedures, machine learning models, and seasonal and sinusoidal frameworks. Acoustic Features revealed a clear seasonal organization, with winter and early spring months forming coherent acoustic regimes across most stations. Responses to solar phases were detectable but strongly dependent on local site conditions. SHIft and SHItf metrics showed higher heterogeneity during autumn and winter than during spring and summer. Climatic variables emerged as important drivers of acoustic heterogeneity, while sinusoidal models generally described annual SHItf dynamics better than conventional seasonal classifications. These findings indicate that soil acoustic environments are structured by interacting seasonal, climatic, and astronomical processes operating across multiple temporal scales and support the development of soil ecoacoustics as a non-invasive tool for investigating ecosystem functioning.