Grassland soils have been highlighted as a global soil carbon (C) sink, and have the potential to sequester additional C. Sequestration of C can occur through incorporation of soil organic carbon (SOC) within microaggregates and the silt and clay fractions. The distribution of SOC within macroaggregate fractions gives an insight into both SOC dynamics and its incorporation into the soil. Research to date on soil C has tended to focus on the topsoil (0–30 cm). While many studies have assessed the changes in aggregation and SOC dynamics after land use or management change, this paper assesses aggregation and SOC dynamics in the topsoil and subsoil of twenty-one temperate grassland sites covering four soil types (Haplic Luvisol, Haplic Stagnosol, Haplic Cambisol, Stagnic Cambisol). Results show that there are no differences in SOC between soil types in the surface 0–30 cm, except a decrease in the quantity of microaggregates within macroaggregates in Haplic Stagnosols. In the subsoil, the silt and clay fraction of clay-illuviated soils had a lower percentage of SOC. Soils with clay illuviation and reducing conditions had a decreased proportion of SOC in microaggregates and silt plus clay within small macroaggregates in the subsoil. This could be caused by a combination of (i) reduced incorporation of SOC into smaller fractions, because POM inputs could be limited due to soil saturation limiting root growth, and (ii) reduced mineralisation and subsequent incorporation of POM into microaggregates and silt plus clay within macroaggregates. These results enable elucidation of the mechanisms driving aggregate formation (and thus C sequestration in microaggregates and silt plus clay fractions) in topsoil and subsoil. This study shows that the dynamics of SOC in subsoil horizons is soil-type dependant and that differences between soil types cannot be elucidated when the sampling is limited to 30 cm. This suggests that the IPCC guidelines for SOC measurements should also include the sampling of subsoil horizons in order to get valuable information that allows discerning between soil types.