Despite their low carbon content, most subsoil horizons contribute to more than half of the global soil carbon stocks. Often, carbon in deep soil horizons is characterised by a high proportion of carbon compounds with mean residence times of more than thousand years. The reasons for these long residence times are only partly understood. Stabilised carbon in deep soil horizons is vertically as well as horizontally stratified, due to the heterogeneous spatial and temporal distribution of carbon inputs by rhizodeposition, DOC and bioturbation. The special conditions prevailing in deep soil horizons may greatly influence biogeochemical cycles, making their study a challenging issue. We invite contributions reporting recent research advances on subsoil organic matter stocks, composition, and turnover from all disciplines of soil research, including soil chemistry, physics, (micro-)biology and pedology.