Characterization of fulvic and humic acids from an oxisol-spodosol toposequence of Amazonia, Brazil

Abstract

Fulvic and humic acids (FA and HA) extracted from the A 1 and Bh horizons of an Oxisol-Spodosol of Amazonia were characterized by elemental analysis, nitrogen fractionation, and visible and IR spectroscopies. In Oxisols, FA are very aliphatic and have large contents of nitrogen, which is present essentially in labile forms (ammonia and amino acids). The corresponding HA are poorly aromatic, and are also rich in labile nitrogen forms. Going down the sequence to Spodosols, FA become less aliphatic and nitrogen rich. The degree of aromaticity of HA increases from Oxisols to Spodosols and total nitrogen tends to decrease; however, the proportion of non-hydrolysable nitrogen increases. These changes may be related to differences in clay content of the soils, vegetative cover and migration of organic compounds.