Organic matter in volcanic soils

Volcanic soils accumulate soil organic matter (SOM) usually through the formation of organo-mineral complexes. In terms of residence time, SOM quality depends on whether it is free or associated to the mineral fraction forming metal (non-allophanic andosols) or allophane complexes (allophanic andosols). This work deals with carbon sequestration mechanisms in volcanic ash soils from the Canary Islands. Physical and chemical soil properties were analysed and SOM studied by analytical pyrolysis (Py-GC/MS) in bulk and in two density fractions. Our findings points to different mechanisms for SOM protection 1) In allophanic soils is mainly associated to noncrystalline organo-mineral complexes, is rich in polysaccharides and N-compounds probably from chitin. 2) In nonallophanic soils a selective preservation mechanism is observed, SOM is formed of relatively unaltered plant components related to a protective effect of amorphous minerals favouring organo-metal complexes/coatings.