Organic carbon storage potential in deep agricultural soil layers: Evidence from long-term experiments in northeast Italy

•Organic carbon stock was estimated to 90 cm depth in different soils and crop systems.•Soil layers below tillage showed organic carbon stocks of at least 59 % of the total.•Permanent meadow had the highest soil organic carbon stocks along the full profile.•In croplands, coupling manure input and minimum tillage was the most effective practice.•Sandy soils with initial low organic carbon showed high reactivity to its storage.