Soil organic matter chemistry in allophanic soils: a pyrolysis-GC/MS study of a Costa Rican Andosol catena

Soil organic matter (SOM) in allophanic soils is supposed to accumulate due to protection caused by binding to allophane, aluminium and iron. We investigated a catena of allophanic and non-allophanic soils in Costa Rica to determine the effect of such binding mechanisms on SOM chemistry. These soils contain no contribution of black carbon. Molecular characterization of litter, extractable and dispersed organic matter was done by Curie-point pyrolysis-GC/MS. The molecular chemistry of the organic fractions indicates a strong decomposition of plant-derived organic matter and a strong contribution of microbial sugars and N-compounds to SOM. Both the decomposition of plant-derived SOM - including that of relatively recalcitrant compounds - and the relative contribution of microbial SOM were greater in allophanic samples than in non-allophanic ones. This suggests that chemical protection does not act on primary OM, although it may influence the accumulation of secondary OM in these soils. The effect of allophane on SOM contents in such perhumid soils is probably through incorporation of decomposition products and microbial SOM in very fine aggregates that - in a perhumid environment - remain saturated with water during much of the year. Greater concentrations of aliphatics are found in allophanic residues, but there is no evidence of any specific mineral-organic binding. The results do not support the existing theory of chemical protection of plant-derived components through binding to allophane, iron and aluminium.