Contrasting composition of free and mineral-bound organic matter in top- and subsoil horizons of Andosols

Andosols are characterised by high organic matter (OM) content throughout the soil profile, which is mainly due to the stabilisation of soil organic matter (SOM) by mineral interactions. The aim of the study was to examine whether there were differences in the chemical composition of mineral-associated SOM and free OM in the top A horizon and in the subsoil (horizons below the A11 horizon). Our experimental approach included the replicated sampling of a fulvic and an umbic Andosol under pine and laurel forest located on the island of Tenerife with a Mediterranean sub-humid climate. We determined the extent of the organo-mineral interactions by comparing the sizes of the light (free) and heavy (dense) soil fractions obtained by physical separation through flotation in a liquid with a density of 1.9 g cm^–3. We determined the elemental and isotopic composition of both fractions and analysed their chemical composition by analytical pyrolysis. The elemental and isotopic composition showed similar values with depth despite the different vegetation and climatic conditions prevailing at the two sites. Carbon (C) stabilised by mineral interactions increased with depth and represented 80–90% of the total C in the lowest horizons. The heavy fractions mainly released N-containing compounds upon analytical pyrolysis, whereas lignin-derived and alkyl compounds were the principal pyrolysis products released from the light fractions of the top- and subsoil horizons. Principal component analysis showed that the chemical composition of OM stabilised by mineral interaction differs in the different horizons of the soil profile. In the A horizons, the chemical composition of this OM was similar to those of the light fractions, i.e. litter input. There was a gradual change in the bulk molecular composition from a higher contribution of plant-derived molecules in the light and heavy fractions of the A horizon to more microbial-derived molecules as well as black C-derived molecules at depth. We conclude that transport processes in addition to decomposition and possibly in situ ageing affect the chemical composition of mineral-associated OM in subsoils.