Subsoils - defined as soil layers below 30 cm or mineral soils - contribute to more than half of the total soil carbon stocks and store substantial amounts of nutrients and water. Despite the critical ecosystem services they provide, including long-term carbon storage, water and nutrient supply to plants, and habitat for biological communities that can differ from the topsoil, they remain under-represented in research. The under-sampling is usually justified by assuming a negligible contribution to ecosystem services. A key factor underpinning many of these ecosystem services is the structural connectivity between topsoil and subsoil, particularly the continuity of macropores, mainly biopores (e.g. earthworm burrows and root channels), which influences water infiltration, root access to deeper horizon and nutrients resources, and the transport of solutes. Compared with the topsoil structure, which can be regularly disturbed (e.g. tillage) and reformed, the subsoil structure might persist over longer timescales and changes slowly unless woody perennials are present, making it a potentially important lever for soil management and climate change adaptation under increasing climate variability and uncertainty. Yet, subsoil properties, functions and their dynamics remain poorly characterized and quantified.

In this session, we aim to bring together studies across disciplines, including soil physics, soil biology, hydrology and biogeochemistry and the connectivity to land use and land management to shed light on the subsoil structure and dynamics and, more broadly, on the role of subsoils in terrestrial ecosystems. We invite experimental, observational, and modeling studies exploring subsoil processes and structural dynamics, including interaction between biological drivers (e.g. roots, soil fauna and microorganisms) and hydrological, carbon and nutrient cycling processes across agricultural, grassland, forest and other terrestrial ecosystems.). Particular attention will be given to studies that highlight the importance of subsoils and the structural pathways connecting them to the broader soil–plant–atmosphere system. We also welcome, imaging studies, synthesis work and reviews addressing deep soil horizons and their roles in nutrient cycling, climate change responses, and ecosystem resilience.