Conformational Analysis of Humic Acids from Amazonian Dark Earth Soils observed by AFM

F L Leite,L Martin-Neto,D K Deda,T J F Da Cunha,M L Simões,W T L Da Silva

doi:10.5923/j.scit.20120204.02

Abstract

Conformat ional analysis of humic acid (HA), extracted fro m an A mazon anthropogenic soil and an adjacent soil which did not have anthropogenic A horizon, was done by atomic force microscopy (AFM), through the deposition of humic acid layers on muscovite mica strips using drop-casting deposition technique. This short commun ication presents, for the first time, AFM images of the sub-micron level structure of humic acid o f the A mazonian Dark Earths, compared to natural pedogenic Amazonian soil. The adsorbed anthropogenic HA form ring-shaped aggregates with diameters on the scale of several tens of nanometers, possibly showing evidences of a supramo lecular formation. The formation of these structures was not verified for HA fro m pedogenic soil. In this case, it was observed particles with g lobular shape and a homogenous distribution of them on the mica surface. Structural characteristics of each sample were observed by nuclear resonance magnetic spectroscopy (NMR). The spectra analysis indicated that pedogenic HA are richer in aliphatic groups, as metho xylic and in polysaccharides structures, while the h igher concentrations of aro matic carbon, including also pheno- lic carbon, were observed in the anthropogenic HA, justifying its higher hydrophobic character.

Highlights

Soil Organic Matter (SOM ) play an important role on many aspects of the nature of soil and environmental processes
Hu mic Substances (HS) are the major o rganic constituents of soils and aquatic environments and are generated by the microbiological and chemical degradation, and transformation of organic matter (OM), resulting in chemical structures which are more stable than the starting material[1,2,3,4]
The globular and cone-shaped features were assigned to islands of humic acid (HA) adsorbed on mica surface, and we re identified t wo genera l types of structures, which can be summarized as: part icles or spheroids and network of particles

Summary

Introduction

Soil Organic Matter (SOM ) play an important role on many aspects of the nature of soil and environmental processes. Most of the soils in the Amazon Basin are acidic, with low cation-exchange capacity, fert ility and production potential In this environment, where soil fert ility is a limiting factor for sustainable agricultural develop ment, occurs the “Terra Preta de Índio” soil[5], known as Amazonian Dark Earth, Anthropogenic Dark Earth, Indian Black Earths, or Archaeological Dark Earths. These soils, that have an archeo-anthropedogenic horizon, wh ich is a surface horizon with variable depth in soil profiles, exhibit elevated OM contents and either ceramic p ieces or lithic art ifacts[6]. The orig in o f th is OM is thought to be

Objectives

Methods

Results

Conclusion