Impact of iron oxides and soil organic matter on the surface physicochemical properties and aggregation of Terra Rossa and Calcocambisol subsoil horizons from Istria (Croatia)

We investigated the influence of Fe oxides and soil organic matter (SOM) on the surface physicochemical properties of mineral particles involved in the formation of nanostructured and micron-sized mineral aggregates of two Terra Rossa and one Calcocambisol soils developed on limestone. We determined the mineral composition and particle-size distribution (PSD) by X-ray diffraction (XRD) and laser diffraction (LD), respectively. The morphologies of the mineral particles were investigated using field-emission scanning electron microscopy (FE-SEM), while soil organic matter (SOM) was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The electrophoretic mobility (EPM), specific surface area (SSA) and cation exchange capacity (CEC) of the samples were also examined. We analysed surface physicochemical properties on soil prior and after selective removal of SOM and Fe oxides. The soils were characterized by their relatively simple bulk and clay mineral compositions, distinctly different contents of SOM and diverse Fe oxides. The EPM data showed that the removal of the SOM did not significantly affect the negative charge of particles, but the removal of Fe oxides shifted the EPM to more negative values, indicating an increase in permanently negatively charged clay surfaces. The removal of SOM did not affect the SSA of the Terra Rossa soils, but doubled the SSA in the Calcocambisol. The SSA of the Terra Rossas were exceptionally high, up to 130–145 m2 g−1, probably because of nano-sized authigenic (pedogenic) kaolinite. The presence of larger clay mineral particles in the Calcocambisol samples resulted in lower SSA (32–76 m2 g−1). The LD analyses showed that the removal of the SOM did not influence the PSD of the Terra Rossa soils, while the removal of the Fe oxides, mainly nanosized hematite, governed the disintegration of the micron-sized soil aggregates and induced a significant increase of the submicron-sized mineral fraction. In contrast, the removal of the SOM significantly influenced the PSD of the Calcocambisol, causing the disintegration of the native, micron-sized aggregates and an increase in the clay fraction. This study demonstrated the important role of the SOM in the aggregation processes for the Calcocambisol and the Fe oxides, especially hematite, in the Terra Rossa subsoil horizons. It was also shown that the surface properties of the Terra Rossa, particularly EPM and SSA, were determined by the presence of a nano-sized mineral fraction that mostly contains authigenic kaolinite.