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| Titel |
Composition of the humin fraction of Terra Preta de Índios soils by NMR and multivariate curve resolution |
| VerfasserIn |
E. H. Novotny, M. H. B. Hayes, G. Song, E. R. de Azevedo, T. J. Bonagamba |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028072
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| Zusammenfassung |
| Most of the soils in the Amazon Region are acid, with low cation-exchange capacity (CEC)
values, low fertility, and low production potential. There exists a class soils in this
environment that have an archeo-anthropedogenic horizon of pre-Columbian origin, called
“Terra Preta de Índios” (TPI), also known as Amazonian Dark Earths. These soils are
characterized by higher fertility and more stable organic matter (OM) than the surrounding
soils.
The high fertility of TPI, and especially their sustainability, is attributed to the high levels
of OM and to their physical-chemical properties. Up to 35-45% of the organic C in TPI is in
the form of pyrogenic carbon (black carbon), compared to 14% in surrounding soils lacking
an archeo-anthropedogenic horizon.
Pyrogenic carbon derived from the partial carbonization of ligno-cellulosic materials, is
composed of hydrogen-deficient condensed aromatic structures and, as the result of chemical
and biochemical transformations, has a high charge density from carboxylic groups linked
mainly to the aromatic core.
Humin is the most recalcitrant and least understood fraction of soil OM. By definition,
humin is the fraction that is not soluble in traditional aqueous alkaline soil extractants. It
represents more than 50% of the soil organic carbon (OC) in mineral soils and more than
70% of the OC of lithified sediments, and is therefore the most important pool of
the soil OM. However, for a study of humin by NMR it is necessary to isolate it
and to achieve a degree of fractionation/purification. In this regard, an appropriate
procedure1
has been applied to two soil samples from TPI and the material obtained was studied by solid
state 13C NMR. The data analyses were carried out by Multivariate Curve Resolution
(MCR).
Results and Discussion
MCR resolved the spectral matrix into three components, and these can be attributed to
wood, humin, and oxidised charcoal materials
The spectrum for the wood component agrees very well with that of eucalyptus
wood. On the other hand, the humin spectrum shows an intense signal in the alkyl
region (0-40 ppm) with a prominent signal for crystalline polymethylene at 33 ppm.
These signals, in association with the carboxyl signal (173 ppm), indicate fatty acid
origins for these components. Additionally, some altered aromatic structures (a
broad and featureless signal centred at 130 ppm) and cellulosic material (110 and 70
ppm) were also evident. The broad signal at 54 ppm was probably from N-alkyl
compounds from proteinaceous material since the corresponding O-aryl signal
from the alternative methoxyl contribution was lacking. Finally, the oxidised char
component was characterised by the broad and featureless signal at 128 ppm and
the carboxyl at 172 ppm. The shoulder at 167 ppm gave evidence for carboxyl
directly attached to the aromatic backbone. This char component also had minor
contributions from aliphatic compounds, such as alkyl, N-alkyl, methoxyl, and
carbohydrates.
With regard to the estimated concentrations of each of these components, the coarse char
samples and the residual humin (before and after DMSO/H2SO4 extraction) presented a
higher content of unaltered wood (cellulose and lignin) material.
The humin extracted by DMSO/H2SO4 was very rich in long chain aliphatic compounds
(alkyl groups: 0-40 ppm). This lipidic fraction was also extracted by the alkali urea treatment
and by soil dispersion using ultrasounic vibration. Greater amounts of oxidised char were
also isolated following the ultrasonic treatment. This component remained in the residual
humin, and it was also abundant in the coarse char fraction, indicating that the solvent
systems adopted were unable to solubilise these materials. The extractable humin probably
corresponds to fatty acids associated with the clay fraction and the un-extractable fraction
was probably composed of particulate char (partially, and oxidised) and vegetable
tissues.
In conclusion, the extractable humin in these soils is mainly composed of hydrophobic
compounds strongly associated with the mineral fraction. The DMSO/H2SO4 solvent
is very efficient for solubilising these lipidic compounds. On the other hand, the
disruption of the clay aggregates, by ultrasonication, allows additional oxidised
char to be released and solubilised by the DMSO/acid and by the alkaline urea
systems.
Acknowledgements. The authors are grateful for support from Science Foundation
Ireland, from Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de
Janeiro – Brazil, and from Fundação de Amparo à Pesquisa do Estado de São Paulo – Brazil. |
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