Organo‐mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores

Abstract

SummaryOrgano‐mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle‐size fractions < 6.3 μm of two soils to examine the importance of Fe oxides, short‐range order Al silicates and the surface areas of minerals and micropores on the formation of organo‐mineral associations. In the subsoil Fe oxides were most strongly statistically correlated with the mineral‐bound organic carbon. We therefore assume that they are the most important substrates for the formation of organo‐mineral associations. There is no indication that this is caused by physical protection of organic matter in their micropores (< 2 nm). In the Haplic Podzol, dithionite–citrate–bicarbonate‐soluble short‐range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of ∼ 200 m2 g−1 (goethite) and ∼ 800 m2 g−1 (ferrihydrite), corresponding to crystal diameters of only a few nm. We assume that the resulting large amount of oxide‐specific reactive surface sites (conditionally charged hydroxyl groups) is responsible for their dominant role as sorbents. With maximum C loadings of 1.3 mg C per m2 Fe oxide for the Dystric Cambisol and 1.1 mg C per m2 Fe oxide + short‐range order Al silicates for the Haplic Podzol the subsoils of both soils seem to have reached saturation with respect to organic matter sorption. In contrast to subsoil horizons, organo‐mineral associations from topsoils contain much larger amounts of organic matter. Here a larger C loading on Fe oxides or a greater importance of other sorbents in addition to the oxides must be assumed.