The large C stocks present in the subsoil could be an important factor in the feedback of SOM dynamics to climate change. Additionally, the subsoil has gained interest as potential hotspot for enhanced C sequestration as stabilized SOM. Therefore, subsoil SOM dynamics have become an important research topic. However, issues addressed remain scattered, and natural variability in structure and dynamics of subsoil SOM between soil types or ecosystems is often overlooked. The general focus of this session will be to increase our understanding of how properties and dynamics of subsoil SOM are affected by soil type, ecosystem and soil management. Specific issues of interest are: Spatial heterogeneity of the soil profile and its role in C sequestration and stabilisation in subsoil (e.g. flow paths for DOC and water , root channels, rhizosphere, input of fresh organic matter, aggregate structure, Submicron-scale, O2 availability, redox potential, pH…) Differences in residence time of SOM compounds between topsoil and subsoil horizons (e.g. Bulk soil organic matter vs compounds such as proteins, lignin, sugars…) and how are these differences related to soil type, management or ecosystem? Vulnerability of subsoil carbon to climate change (e.g CO2, temperature, drought effects) and to environmental change (e.g. N addition). Can we identify subsoil biophysical conditions that lead to vulnerable “hotspots”? Incorporation of subsoil C dynamics in SOM models: current approaches and knowledge gaps.