Organic matter accumulating in Aeh and Bh horizons of a Podzol — chemical characterization in primary organo-mineral associations

Abstract

The chemical structure of soil organic matter from the eluvial (Aeh) and (illuvial) Bh horizon of a Podzol was studied in primary organo-mineral associations by 13C CPMAS NMR spectroscopy and acid hydrolysis. In Podzols, organic matter is leached from the forest floor and Aeh horizons into the Bh horizon, where it is intimately associated with the mineral phase. In the Aeh horizon the majority of the residual organic matter was present in methylene structures, contributing 42% to the organic matter associated with the clay fraction. In the Aeh horizon decreasing particle size was typically accompanied by increasing ratios alkyl C-to- O–alkyl C, suggesting increasing decomposition of polysaccharides for the residual organic matter. The illuvial Bh horizon, containing high proportions of iron- and aluminum-oxides and hydroxides, was higher in aromatic carbon, while polysaccharides were similar and methylene carbon were smaller than in the Aeh horizon. In the particle size separates proportions of polysaccharides were constant, resulting in decreasing ratios alkyl C/ O–alkyl C. Proportions of hydrolyzable amino acids were larger in the Aeh horizon (52% of the total N) than in the Bh horizon (21%), both for bulk soils and size separates. Our results suggested that in Bh horizons of Podzols, aromatic structures and also presumably labile structures like polysaccharides can be stabilized by organo-mineral associations.