Humic acids from hydromorphic soils of the upper Negro river basin, Amazonas: Chemical and spectroscopic characterisation

The objective of this study was to characterise the composition, structure and functionalities of humic acids (HAs) isolated from hydromorphic sandy soils of the upper Negro river basin, Amazonas, Brazil, in order to obtain information on organic matter dynamics in these tropical forest ecosystems. For this purpose, three soil layers, i.e., the superficial earthworm casts, the A horizon and the Bh horizon, were sampled from each of three sites covered by extended campinarana forests, and analysed for their morphology, particle size distribution, and chemical properties. The HAs were isolated from the soil layers and characterized for their elemental and functional group composition and by visible, Fourier transform infrared and fluorescence spectroscopies. Forest litter covered permanently the soil surface that exhibited a peculiar microrelief formed by earthworm casts. The soil profile featured an A horizon rich in decomposing organic matter, followed by a sandy albic E horizon, and by a dark spodic Bh horizon enriched in well-humified organic matter. The soil layers were markedly sandy with very small clay content, acidic, and rich in organic C and N contents that decreased with increasing soil depth. With depth the cation exchange capacity and Al3+ content also decreased markedly in one soil, but increased in the other two soils. The HAs from the superficial soil layers, i.e., the earthworm casts and A horizons, apparently consisted of simple molecular components of wide heterogeneity, prevalent aliphatic character, rich of N-containing and polysaccharide-like structural units, poor in carboxylic groups, and of low humification degree. These features suggested a rapid decay of litter organic matter in the superficial soil layers, which was mostly controlled by intense biological activity. Differently, the HAs isolated from Bh horizons were impoverished of aliphatic, N-containing and polysaccharide-like components, enriched in carboxylic groups and of greater humification degree. These results could be associated to the transfer of humified organic matter originating either from the surface black waters or formed in the surface layers downward the soil profile, which might be related to the hydromorphic conditions predominating in these soil environments.