Elsevier

Geoderma

Volume 138, Issues 1–2, 15 February 2007, Pages 170-176
Geoderma

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

https://doi.org/10.1016/j.geoderma.2006.11.005Get rights and content

Abstract

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.

Introduction

Hydrogeochemical processes occurring in superficial soil layers of humid tropical areas and the diversified animal community that controls forest litter decomposition and carbon and mineral elements turnover affect markedly soil organic matter (SOM) dynamics. In this ecosystem, earthworms are a major component of soil fauna, often representing more than 50% of soil animal biomass (Fragoso and Lavelle, 1992). These animals have an important ecological role in influencing soil structural organization, by opening channels and depositing materials in the surface and deep layers of the soil profile (Lavelle, 1988), SOM decomposition, mineralization and humification, and nutrient cycling (Stout and Goh, 1980, Lavelle and Martin, 1992).

The Negro river is an important tributary to the Amazon river, and is located in the northwestern part of Brazil at the border areas with Colombia and Venezuela. Most of its basin, especially in the upper part, is constituted by an extented flat surface (165000 km2) largely covered by hydromorphic, Spodosol-like sandy soils (Dubroeucq and Volkoff, 1998). The formation of these tropical water-saturated soils and their ecological function in the Amazon basin is little known because of difficulties in their access and in opening pedological pits due to the thickness of the sandy layer and the presence of water and a hard cemented and often thick Bh horizon (Dubroeucq and Blancaneaux, 1987).

In these conditions the forest cover is strongly dependent on soil biological activity and hydrogeochemical processes. In particular, vegetation residues accumulate in large amount on the soil surface where they are subjected to intense microbial and faunal processes (Kindel and Garay, 2002), and SOM dynamics is largely affected by the existing hydromorphic conditions. The limited availability of oxygen delays the decomposition process of SOM, and favors the formation of water-soluble organic constituents that can be easily transferred to the drainage water. Drainage groundwater from Spodosols is known to be the primary contributor to the black waters in the Negro river basin in Brazilian Amazonas. The black color is due to dispersed and/or dissolved organic materials having chemical and physical properties similar to Spodosol humic substances (Leenheer, 1980). Organic matter also affects the dynamics of soil mineral elements, especially Al, through the podzolization process (Farmer et al., 1983).

Since these soils are extensivey weathered, and their clay fraction is mostly constituted of residual quartz grains, gibbsite and kaolinite, organic matter represents the main source of nutrients, and largely contributes to cation exchange capacity, soil pH buffering and stabilization of soil structure (Zech et al., 1997), thus playing a major role in the ecosystem productivity of these soils. Information available on organic matter composition and dynamics in tropical soils is limited (Zech et al., 1997), especially for tropical Spodosols (Andreux et al., 1987, Bravard and Righi, 1991). However, some studies (Turenne, 1974, Volkoff and Cerri, 1988, Rivero et al., 1998) have shown the utility of chemical and spectroscopic analysis for characterising the humic fractions of organic matter in these soils (Zech et al., 1997, Rosa et al., 2005).

The objectives of this study were to evaluate: (a) the morphology and physical and chemical properties of three layers of three selected hydromorphic sandy soils collected in the upper Negro river basin; and (b) the composition, structure, and functionalities of humic acids (HAs) isolated from these soil layers by using a combination of analytical chemical methods and spectroscopic techniques.

Section snippets

Sampling sites and soils

The sampling area is located in the northwest region (01°07′48″ N and 66°48′29″ W) of Amazonas State, Brazil, and is characterized by an equatorial perhumid climate with annual rainfall of 2900 mm and no relevant dry period. The average annual temperature is about 25 °C, with negligible variations. The dominant pedological covers of the region are Spodosol-like soils in the widespread flat areas, and well-drained Oxisols in the less extended hilly areas (Dubroeucq and Volkoff, 1998). Generally,

Morphological properties of soil layers

The soil surface exhibits a peculiar microrelief formed by rounded, conical shaped earthworm casts (EC) from 5- to 33-cm tall and in number of 1.3 to 5.5/m2. The presence of giant earthworms (Glossoscolecidae) and their activity in these hydromorphic sandy soils were systematically verified in a soil survey carried out along a transect of more than 200 km in this region (Mafra et al., in press). These animals ingest most organic residues in the soil surface and mix them with the mineral

Conclusions: Organic matter dynamics in hydromorphic soils

Despite the existence of small differences in their morphology, probably due to the mineralogical composition of the clay-size fraction and the depth of the saprolite layer beneath the spodic horizon, the three Spodosols examined show similar chemical and HA properties that point to similar organic matter dynamics. In particular, the similar morphology, chemical properties, and HA composition, structure and functionalities of ECs and adjacent A horizons indicate that organic matter dynamics in

Acknowlegments

This study was supported by PRONEX-1997 from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil. It is also part of the ALFA Program held at University of Bari, Italy, which was financed by European Commission. We wish to thank Mr. D. Mondelli for assistance in the humic acids analysis.

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