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

Abstract

Organo-mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle-size fractions < 6.3 mm 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-bicarbonatesoluble short-range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of $ 200 m 2 g À1 (goethite) and $ 800 m 2 g À1 (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 m 2 Fe oxide for the Dystric Cambisol and 1.1 mg C per m 2 Fe oxide þ 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.

Can Fe isotope fractionations trace the pedogenetic mechanisms involved in podzolization?

“…This evolution, synchronized with an increase in OC in the B horizon, marks the beginning of podzolization from 270 to 335 years as demonstrated by the strong correlation between elementary OC and Fe DCB fluxes in the B horizons (R 2 = 0.99, n = 3, correlation significant at 1% confidence level). This correlation was already observed in other Podzols (e.g., Eusterhues et al, 2005). Once the required pH has been reached, podzolization starts within 50 years.…”

Disturbance Legacy on Soil Carbon Stocks and Stability within a Coastal Temperate Forest of Southwestern British Columbia, Canada

“…In addition, Grand and Lavkulich (2012) showed that Fe and Al can form organometallic complexes with SOC. Yet, as this mechanism enables SOC stabilization, the translocation of Al and Fe in the mineral layers of Podzolic soils (dominant in coastal temperate forests of British Columbia) may enhance SOC storage (Eusterhues et al, 2005).…”

Effect of metal oxide on surface area and pore size of water-dispersible colloids from three German silt loam topsoils

“…The organic matter is stabilized by physical inclusion and chemical reaction with metal oxide surfaces (Kaiser and Guggenberger, 2000;Wagai and Mayer, 2007;Kaiser and Guggenberger 2003;Eusterhues et al 2005). Various forms of iron oxides can be found in soils (Cornell and Schwertmann, 2003).…”