Relative importance of sorption versus aggregation for organic matter storage in subsoil horizons of two contrasting soils

Soil organic matter (OM) stabilization by the mineral phase can take place through sorption and aggregation. In this study we examined both of these processes, (i) organic carbon (OC) sorption onto clay-sized particles and (ii) OC occlusion in silt-size aggregates, with the objective of evaluating their relative importance in OM storage and stabilization in soil. We studied two loamy soil profiles (Haplic Luvisol and Plinthic Cambisol) currently under agricultural use down to a depth of 2 m. Our approach was based on two parallel fractionation methods using different dispersion intensities; these methods isolated a free clay fraction (non-occluded) and a clay fraction occluded within water-stable silt-size aggregates. The two clay fractions were analysed for their C content and C-14 activity. The proportion of sorbed OC was estimated as OC loss after hydrofluoric acid (HF) demineralization. Our results showed an important contribution to SOM stabilization by occlusion of OC into silt-size aggregates with depth through both soil profiles. In the Haplic Luvisol, OC associated with clay and located in silt-size aggregates accounted for 34-64% of the total soil OC, whereas in the Plinthic Cambisol this occluded material represented 34-40% of total OC. In the Haplic Luvisol, more OC was located in silt-size aggregates than was sorbed onto clay-size minerals, suggesting that silt-size aggregation plays a dominant role in OC storage in this soil. In the Plinthic Cambisol, the abundance of sorbed OC increased with depth and contributed more to the stored C than that associated with silt-size aggregates. Radiocarbon dating of both clay fractions (either occluded within silt-size aggregates or not) suggests, in the case of the Plinthic Cambisol, a preferential stabilization of OC within silt-size aggregates.