Listening to tropical forest soils

Passive acoustic monitoring (PAM) has proven to be an effective tool for monitoring biotic soundscapes in the marine, terrestrial, and aquatic realms. Recently it has been suggested that it could also be an effective method for monitoring soil fauna, but has so far been used in only four studies in temperate and polar regions. We present the first study of soil soundscapes in tropical forests, using a novel analytical pipeline allowing for the use of in-situ recording of soundscapes with minimal soil disturbance. We found significant differences in soil soundscapes between burnt and unburnt forests and the first indications of a diel cycle in soil soundscapes. These promising results and methodological advances highlight the potential of PAM for large-scale and long-term monitoring of soil biodiversity. We use the results to discuss research priorities, including relating soil biophony to biodiversity, community structure and ecosystem functioning, and the use of appropriate hardware and analytical techniques.

Methods
We collected soil acoustic data in three municipalities: Santarém, Belterra and Mojuí dos Campos, in the state of Pará, (eastern Brazilian Amazon latitude ~ −3.046, longitude −54.947 WGS 84). We sampled seven sites unaffected by fire and three sites in which the forest has been burnt in 2015/2016. All sites were separated by a minimum distance of 2 km. Data were collected between 21 November and 06 December 2023. This period is the onset of the rainy season when soil biotic activity is likely at its highest although we avoided recording during periods of rain. 

Recordings were made at each point for 30 minutes between 09:20 and 14:00, with a minimum buffer of 3 minutes at the start and end of each recording to avoid including footsteps or other anthropogenic sounds associated with researcher presence. Recordings were made using a Zoom H5n recorder with JrF C-series Pro contact microphones and XLR impedance adapters in both the left and right channels. The input levels were set to the maximum (10). We chose to use contact microphones, as they are much less sensitive to above-ground sounds, and therefore reducing the likelihood of recording above-ground sounds that may obscure the soil soundscape pattern. Microphones were placed at the furthest distance apart the cables would allow, approximately 5 m, and small holes were dug so that the microphones would sit in the soil/clay layer just below the hummus layer.