Soil organic matter (SOM) density fractionation of andic horizons from the south of Italy

The soil function in the C sequestration is widely acknowledged and processes of SOM stabilization are partially due to interaction of SOM with the mineral fraction. The mineral phases mostly involved in SOM stabilization have pH dependent charge. In Andosols, the variable charged surfaces (ITM-imogolite type materials) are the main minerals present. Their role in the SOM stabilization is important and has been quantified in recent investigations. The aim of this work is to evaluate, in selected Andosols, the distribution of SOM in the different pools: free (FPOM), occluded (OPOM) and mineral associated (MOM). A physical fractionation, by the use of heavy liquids at variable densities (LST fastfloat), was carried out on two volcanic soil horizons, different for andic properties and organic C content. A method calibration was firstly performed, to define the i) density value to separate the FPOM from the other OM pools and ii) time and intensity of ultrasonication, aimed to destroy the soil aggregates occluding organic matter but preserving the mineral stabilized fraction. FPOM was separated using liquids at 1.6 g ml-1 density, while OPOM was recovered after soil sonication at 600 J ml-1. MOM was then treated with increasing density liquids (from 1.8 to 2.6 g ml-1) in order to separate the mineral components on the base of their densities. The mineralogical composition of the separated density fractions was defined by XRD, FT-IR spectroscopy and solid state NMR and the associated C content measured by Elemental Analyzer.