COMPOSITION OF ORGANIC N IN TEMPERATE HUMID REGION SOILS (NW SPAIN)

The composition of the organic N fraction of the surface horizons of 16 widely differing temperature humid region soils, and of deeper horizons of four of these soils, was determined by a stepwise acid hydrolysis procedure. On average one-third of the organic N was very recalcitrant and accumulated in the unhydrolysable residue. Small proportions of the other two-thirds were successively extracted in the following four hydrolysis stages. More than half of the organic N was shared equally by alpha-aminoacid and hydrolysable unidentified fractions, 6% was NH4+-N, 5% amide-N and 3% hexosamine-N. The results also indicated the existence of a labile organic N fraction, which was larger in soils with high organic C and N contents, and a recalcitrant fraction that increases with decreasing organic N. The relationship between extractable Al content and organic N was explained by an indirect mechanism which takes into account the biostability of organo-aluminium compounds.