Properties of black soil humic acids from high altitude rocky
complexes in Brazil
Vinicius de Melo Benitesa,*, Eduardo de Sa Mendoncab, Carlos Ernesto G.R. Schaeferb,Etelvino Henrique Novotnyc, Efraim Lazaro Reisd, Joao Carlos Kerb
aEmbrapa Solos, Rua Jardim Botanico, Jardim Botanico 1024, 22560-000 Rio de Janeiro, RJ, BrazilbDepartamento de Solos, Universidade Federal de Vicosa, 36571-000 Vicosa, MG, Brazil
cEmbrapa Milho e Sorgo, Rodovia MG-424, km 65, Cx.P. 151, 35701-970, Sete Lagoas, MG, BrazildDepartamento de Quımica, Universidade Federal de Vicosa, 36571-000 Vicosa, MG, Brazil
Received 5 February 2004; received in revised form 23 November 2004; accepted 29 November 2004
Available online 4 January 2005
Abstract
Forty two samples of black soils were collected from superficial horizons of the High Altitude Rocky Complex (HARC) that
is situated on top of the most prominent mountain ranges of eastern Brazil. Our objective was to characterize the humic acids
extracted from these pedoenvironments and evaluate the effect of natural paleo-fires on thermogravimetric properties, elemental
composition, visible and infrared absorption of humic acids (HA). The high thermostability, low H:C ratio, and abundance in
carboxylic groups, suggest that the HARC black soil HA present larger biostability and reactivity. The molecular characteristics
indicated that HARC black soil HAs have a great contribution of transformed charred vegetable residues on its origin. This
persistence and reactivity of black soil HA explain the capacity of these soils to maintain their cation exchange capacity, since
the soil clay fraction is composed predominantly by low activity minerals.
D 2004 Published by Elsevier B.V.
Keywords: Humic acids; Tropical soils; Charcoal; Black carbon; FTIR
1. Introduction
The Brazilian landscape is characterized by vast
areas of plains and high plateaus, having few areas
higher than 2000 m. Two notable exceptions are the
Mountain ranges of Espinhaco and Mantiqueira, in the
Atlantic Forest border, forming the most prominent
watersheds of eastern Brazil. These two chains are
formed by contrasting lithologies. Quartzites are the
dominant rocks in the Espinhaco massif, whereas
igneous rocks (granites and gneisses) are typic of the
Mantiqueira. In both areas rain forest and high altitude
grass fields occur, the latter being dominant in the
highest parts of these mountains on shallow soils.
Floristically, these two mountain areas have been
0016-7061/$ - see front matter D 2004 Published by Elsevier B.V.
doi:10.1016/j.geoderma.2004.11.020
* Corresponding author.
E-mail address: [email protected] (V.M. Benites).
Geoderma 127 (2005) 104–113
www.elsevier.com/locate/geoderma
considered very characteristic, being now jointly
classified as HARC (after Joly, 1970).
These highland formations occur along the great
watersheds of Brazil, supplying water for a myriad of
small and large urban centers, sheltering fragments of
Atlantic forest, thus representing a hot-spot for
conservation and biodiversity. Their peculiar shallow
soils have some similarities with the high altitude areas
of Andes (Boom et al., 2001), being considered relicts
of ancient ecosystems, formerly of wider distribution.
The HARC is characterized by a unique flora,
constituted by escleromorphic types (Joly, 1970), well
adapted to both nutritional and water limitations,
presenting morphological and physiological adapta-
tions. Fire-tolerant plants, such as Velloziaceae, are
commonly found (Safford, 1999), suggesting a long-
term evolution under natural fire pressure.
The HARC is a mosaic of vegetations with two
alternating strata: a herbaceous and a sub-arboreal,
besides swampy and rocky outcrops. HARC areas
possess a very high endemicity, resulting from their
geographical isolation. The disjunctive distribution of
these complexes suggests a former wider continuous
distribution, during times of favorable conditions.
The great number of endemic species has led to the
creation of a number of conservation units throughout.
Soils are generally shallow, sandy, oligotrophic,
possessing high Al saturation (Benites et al., 2001;
Dias et al., 2002; Schaefer et al., 2002). The relatively
high content and the strong melanized aspect of the
organic matter cause a black color to develop. Fire,
both anthropogenic and natural, is widespread in these
highland areas, and common pre-historic fire events
are implied by the great amount of dispersed charcoal
fragments deep into the soil. Most plants are, also, fire
tolerant, or even fire dependent (Rizzini, 1979). In this
respect, Silva and Vidal-Torrado (1999) have corre-
lated the presence of finely divided charcoal frag-
ments in soils of the Serra da Mantiqueira to the
paleo-fires during exceptionally dry phases in the
Quaternary.
However, the close relationship between natural
fire and HARC vegetation is still a matter of
controversy, though the presence of fire-tolerant trees
and xeromorphic adaptations to fire are widespread in
these environments (Rizzini, 1979).
In this work we aimed to characterize the HA
extracted from several soil samples from HARC both
Serra da Mantiqueira and Serra do Espinhaco,
evaluating the effect of fires and soil charcoal
contents in the HA synthesis and in its characteristics
in comparison with HA from adjacent non-black
soils.
2. Material and methods
2.1. Site characterization
Forty two samples from superficial horizons of
HARC soils with black color and as reference, twelve
samples from adjacent non-black soils and two from
litter were collected in Serra do Espinhaco and Serra
da Mantiqueira. Samples were collected in nine
conservation units, such as national and state parks,
environmental protection areas and private reserves of
nature preservation (Table 1). Sampling was accom-
plished along both latitudinal and altitudinal gradients,
in areas of Caatinga, Cerrado and Atlantic Rain Forest
biomes, in which HARC is locally present at the
highest positions.
2.2. Soil characterization
All soils are sandy, oligotrophic and acid, with high
levels of exchangeable aluminum and organic matter
(Table 2). Shallow Lithosols (entisols) are the
dominant soils, but Histosols, Spodosols and Incepti-
sols are also present. Finely divided charcoal frag-
ments are observed deep in the soil. Details on the
chemical and physical characteristics of these soils
can be found in Benites et al. (2003).
2.3. HA extraction and purification
HAwere extracted and purified from all samples as
indicated by International Humic Substances Society
(Swift, 1996). HA were extracted with 0.1 M NaOH
under N2 atmosphere. After shaking for 24 h, the
material was centrifuged at 10,000�g for 30 min. The
solution was collected and the pH was immediately
adjusted to 2.0 with 6 M HCl.
After 18 h the fulvic acid fraction was siphoned and
discarded. The remaining material was centrifuged at
5000�g for 10 min and the supernatant was discarded.
The precipitate was redissolved in 200 mL of 0.1 M
V.M. Benites et al. / Geoderma 127 (2005) 104–113 105
NaOH under N2 atmosphere and centrifuged at
10,000�g for 30 min. The solution was collected and
pH was immediately adjusted to 2 with 6 M HCl. The
acidified solution was centrifuged at 5000�g for 10
min. Precipitate HA were twice treated with 0.5%
HF+HCl solution for 24 h and centrifuged at 5000�g.
The purified samples were washedwith 200mL of 0.01
M HCl, centrifuged at 5000�g and transferred for 100
mL cellophane bags, dialyzed and lyophilized. Stand-
ard sample of HA of peat from IHSS (no. IS 103 H) was
used as reference in all analyses.
2.4. Physical and chemical analyses of HA
Thermodecomposition curves of HAwere obtained
by a TGA-50 SHIMADZU thermogravimetric ana-
lyzer using 3.3F0.1 mg samples over static air. The
initial weight was stabilized at 30 8C and heating
curve was obtained from 5 8C min�1 to 105 8C, with a
holding time of 10 min, following by heating at 5 8C
min�1 up to 650 8C. The thermodecomposition curve
was acquired by microcomputer using the TA-50 WSI
program (Shimadzu, 1989). The weight loss at 105 8C
Table 2
Characteristics of black soils surface horizons where humic acids were extracted
Soil
sample
Sand
(%)
Silt
(%)
Clay
(%)
BDa
(g cm�3)
TOCb
(mg g�1)
pH Ca2+ Mg2+
(cmolckg�1)
Al3+
(cmolckg�1)
CECc
(mg g�1)
FeDCBd
(mg g�1)
Feoxe
(mg g�1)
FeO/Df
(mg g�1)
Igneous rock
black soil
(n=26)
69F18 16F6 14F4 0.76F0.12 115F52 4.64F0.43 0.2F0.1 0.2F0.1 8.9F5.9 22.1F6.9 16.1F14.1 5.6F4.4 0.45F0.20
Quartzite
black soil
(n=16)
82F16 9F5 10F6 0.89F0.17 78F42 4.04F0.42 0.2F0.3 0.3F0.3 5.0F4.7 18.2F5.9 3.9F10.0 1.2F1.6 0.74F0.43
MeansFstandard deviations.a Bulk density.b Total organic carbon.c Cation exchange capacity.d Ditionite–citrate–carbonate extractable iron.e Oxalate extractable iron.
Table 1
Descriptive characteristics of High Altitude Rocky Complexes areas where the soils was sampled
Conservation unit Coordinate Altitude a.s.l. (m) Regional biome Climate (Kfppen) Lithology
National Park of Chapada Diamantina 12831VS 900 to 1100 Caatinga Aw Quartzite
41833VW
State Park of Diamantina 18814VS 1000 to 1300 Cerrado Cwa Quartzite
43836VW
National Park of Serra do Cipo 19830VS 900 to 1200 Cerrado Cwa Quartzite
43844VW
State Park of Itacolomi 20817VS 1000 to 1400 Cerrado Cwa Quartzite
43830VW
National Park of Caparao 20825VS 2300 to 2700 Atlantic Rain Forest Cwb Granite
41857VW
State Park of Serra do Brigadeiro 20840VS 1400 to 1700 Atlantic Rain Forest Cwb Migmatite
42831VW
Particular Reserve of Mitra do Bispo 22810VS 1700 to 2000 Atlantic Rain Forest Cwb Granite
44823VW
State Park of Ibitipoca 21833VS 1400 to 1700 Atlantic Rain Forest Cwb Quartzite
43854VW
National Park of Itatiaia 22829VS 1800 to 2300 Atlantic Rain Forest Cwb Sienite
44833VW
V.M. Benites et al. / Geoderma 127 (2005) 104–113106
was considered as sample moisture. At the end of the
burning, the residue was considered as the ash
content. The weight loss between 105 and 350 8C
and between 350 and 650 8C was determined. The
quotient of this two burn events was calculated and
was defined as a thermogravimetric index (TGI).
The elemental composition of the HA was deter-
mined in two replicates using a Perkin Elmer 2400
CHN analyzer. The C, H and N values were corrected
for dry ash free weight, using the amount of moisture
and ashes obtained by the thermogravimetric analysis.
The amount of O was determined by difference from
the corrected data. The H:C, C:N and O:C atomic
ratios were then calculated.
The absorbance spectra in the visible (380–700 nm)
were obtained in 100 mg AH L�1 solutions in 0.1 M
NaHCO3, at a 200 nm min�1 rate, in 5 nm intervals.
The absorptivity spectra were calculated using the
Lambert-Beer equation. We also calculated the E4:E6
ratio (Chen et al., 1977).
The infrared absorbance spectra were obtained
with a Perkin-Elmer FTIR Spectrum 1000 spectrom-
eter in the range between 4000 and 400 cm�1 using
KBr pellets (2 mg of humic acid+200 mg of KBr).
The spectra were corrected considering a baseline
from 1850 to 3670 wave numbers and normalized
dividing the spectra by its integral area.
3. Results
3.1. Thermogravimetric analysis
The HA presented an overall low ash content in the
range of 0.7% to 4.4% (Table 3). The thermodegra-
dation curve of HA showed two well-defined burning
events (Fig. 1). According to Shurygina et al. (1971),
the first loss by ignition peak is caused by the losses
of aliphatic structures, oxygenated functional groups
and peptides; the second loss is related to the
destruction of aromatic nucleus. The relationship
between the two peaks represents the resistance of
the humic substance to thermal degradation, which
was defined as a thermogravimetric index (TGI).
The greater TGI of the HA extracted from black
soils compared with references non-black soils, litter
HA and IHSS reference HA (Table 3) indicated the
possible occurrence of polycyclic aromatic nucleus,
that have greater thermal resistance, in its structure.
Some HA samples extracted from soils on quartzites,
presented thermogravimetric curves basically formed
by a single peak between 350 and 650 8C, resulting in
high TGI and showing very high thermal resistance.
3.2. Elemental analysis
The elementary analysis of the HA extracted from
these black soils shows in general a higher C and lower
H contents compared with references samples (Table
3). It results in a low H:C ratio indicating the presence
of high aromaticity and/or aromatic ring condensation
in the structure of HA form. This ratio was especially
low for HA from black soils over quartzite.
Despite the higher standard deviation of the C:N
ratio for the HA from black soils over quartzite, this
group together with its reference litter HA presented
greater values for this ratio (Table 3), while the O:C
ratio of HA from black soils over quartzite was a little
smaller than the other HA samples.
In general the position of non-black soil HAs in the
Van Krevelen diagram is concentrated near the lignin
zone (Fig. 2). In contrast, the black soil HAs were
plotted near the charcoal zone especially those from
the quartzite black soil. It is a consequence of a lower
H:C and O:C atomic ratios of black soil HAs
compared to the non-black soil HA samples.
3.3. Visible spectra analysis
Like is typical for HA, the visible spectra did not
show any structure with the light absorption decreas-
ing with increasing wavelength. However, the absorb-
ance spectra at the visible range show that the HA
from black soils presented higher absorptivity than the
references HA and samples obtained from quartzite
substrate higher than over igneous one (Table 3).
These data may be related to the larger amount of
conjugated aromatic rings, typical of the polycyclic
aromatic nucleus, which is efficient in light absorption
in the visible range (Kononova, 1966; Orlov, 1985).
The HA E4:E6 ratios were variable with smaller
values associated with black soils contrasting with
larger values from non-black soils and litter HA. The
HA from soils over quartzite presented the smallest
values in comparison with igneous rock substrate
(Table 3).
V.M. Benites et al. / Geoderma 127 (2005) 104–113 107
Table 3
Thermogravimetric, elemental and colorimetric characteristics of humic acids extracted from soils on HARC
HA sample Ash
(%)
TGIa C
(% dry ash
free basis)
H
(% dry ash
free basis)
O
(% dry ash
free basis)
N
(% dry ash
free basis)
C:N
atomic
ratio
H:C
atomic
ratio
O:C
atomic
ratio
E4:E6 EHAb (L g�1 cm�1)
Igneous rock
black soil
(n=26)
2.1F0.7 2.80F0.33 58.45F1.81 3.86F0.40 33.06F1.59 4.62F0.80 15.18F2.69 0.80F0.10 0.43F0.03 5.2F1.1 6.4F1.6
Igneous rock
non-black soil
(n=3)
4.1F0.4 2.04F0.41 57.70F1.29 4.36F0.01 33.51F1.48 4.41F0.19 15.25F0.33 0.90F0.02 0.44F0.03 5.4F1.2 4.3F1.8
Quartzite
black soil
(n=16)
1.3F0.5 3.51F0.78 60.86F2.09 3.46F0.51 32.23F1.19 3.44F0.86 22.24F7.25 0.69F0.12 0.40F0.03 4.5F0.6 8.0F2.0
Quartzite
non-black soil
(n=9)
1.7F0.4 2.64F0.22 58.34F1.03 4.13F0.31 33.56F0.82 3.98F0.54 17.45F2.90 0.85F0.08 0.43F0.02 4.8F0.9 5.6F1.8
Litter over
igneous rock
1.2 2.04 55.48 4.57 35.64 4.31 15.03 0.99 0.48 8.2 3.3
Litter over quartzite 1.2 2.86 58.62 4.15 34.15 3.08 22.21 0.85 0.44 6.3 4.7
IHSSc 1.9 2.74 56.73 3.82 35.79 3.65 18.13 0.81 0.47
MeansFstandard deviations.a TGI—thermogravimetric index.b EHA—absorptivity at 465 nm of HA solution.c IHSS—peat humic acid standard from IHSS (no. IS 103H).
V.M
.Benites
etal./Geoderm
a127(2005)104–113
108
3.4. FTIR analysis
The FTIR spectra of all samples showed an intense
broad band at 3300 cm�1 generally attributed to O–H
and secondarily to N–H stretching of various func-
tional groups (Fig. 3). A broad band at 3100 cm�1 due
to C–H stretching of aromatic rings could also be
observed, its band normally shows a low intensity and
is not usually evident in soil HA FTIR spectra.
Absorption bands at 2922 cm�1 and 2852 cm�1 is
ascribed to aliphatic C–H stretching. Broad bands at
2500 cm�1 is overtone from carboxylic groups
stretching (2�1246 cm�1) and at 2000 cm�1 is
overtone from C–O polysaccharides stretching mode
(2�1060 cm�1). Strong absorption bands were
observed at 1720 cm�1 and 1246 cm�1 (Fig. 4). The
former is attributed to CMO stretching of COOH and
others carbonyl groups: e.g., ketones, and the latter to
C–O stretching and O–H deformation of COOH and
C–O stretching of phenol. The ionized carboxyl group
absorptions occurred at 1627 and 1380 cm�1. In this
region occurs also the absorptions from deformation
of aliphatic C–H (1454 cm�1) and the region between
1620 and 1600 cm�1 can also be attributed to
aromatic CMC vibrations, H-bonded CMO of con-
jugated ketones and water deformation.
The HA samples from igneous rock substrate (soil
and litter) and from non-black soils presented charac-
teristic absorption bands from secondary amides such
as those occurring in proteins and polypeptides. These
bands are at 1655 cm�1 due to carbonyl stretching
mode and is referred to Amide I band and at 1540 cm�1
(Amide II band) assignable as the N–H deformation
mode. Moreover these samples and also the litter HA
from quartzite presented more intense band attributable
to polysaccharides (1060 cm�1) in relation to the HA
from black soils over quartzite while these samples
presented a shoulder at 1560 cm�1 characteristic from
skeletal stretching mode of the CMC bonds of aromatic
ring.
4. Discussion
The TGI, the H:C atomic ratio and the absorptivity
on the visible range presented very high, positive
correlation (Fig. 5). All of them indicate the aromaticity
and condensation degree of the polycyclic aromatic
nucleus. The lower E4:E6 and H:C ratios and higher
TGI and absorptivity obtained for HA samples from
black soils over quartzite (Table 3) could be associated
with its higher aromatic characteristic and lower
proteins and polysaccharides contents as detected by
FTIR (Fig. 4). Although these HA presented very low
H:C ratio, indicating high aromaticity and aromatic
ring condensation, it presented also higher O:C ratio
than charcoal (Fig. 2) and high carboxylic/carboxylate
contents as showed by FTIR analysis (Fig. 4) indicating
that, besides recalcitrant, these HA are very reactive
and it could contribute significantly with the low CEC
0,000A
B-0,002
-0,004
-0,006
-0,008
-0,010
-0,012
-0,014
-0,016100 200 300
dW
/dT
C
temperature C400 500 600
Fig. 1. First derivative of humic acid thermograms showing a
bimodal shape with a peak of side chain thermodegradation (A) and
a peak of nuclei thermodegradation (B).
0.50
0.60
0.70
0.80
0.90
1.00
0.30 0.35 0.40 0.45 0.50atomic ratio O:C
ato
mic
rat
io H
:C
IHSS peat HA Quartzite black soil HAIgneous Rock black soil HA Non black soil HALitter HA
Lignite Lignin Peat
HumicAcids
Fig. 2. Detail of a Van Krevelen diagram for humic acids extracted
from black and non-black soils in HARC (IHSS and litter HA like
reference).
V.M. Benites et al. / Geoderma 127 (2005) 104–113 109
1454Aliphatic CH
1380COO-
1627COO-
Ketones 1246
COOH
1237Phenol
1720COOH; Ketones
1060Polysaccharides
1560Aromatic
1655Amide I; Quinone; Ketones
1540Amide II
LQ
Q
NBQ
I
NBI
LI
Wavenumber (cm-1)900100011001200130014001500160017001800
Fig. 4. Details of FTIR spectra (900–1800 cm�1 region) of some selected humic acids extracted from HARC black soils.
NBI
QP
olys
acch
arid
esA
lcoh
ol; E
ther
s
Alip
h. C
H; C
OO
-
Am
ide
II
COO-
AromaticAmide IKetonesCOOH
PhenolCOOHKetones
PolysaccharidesOvertone
COOHOvertone
AromaticCH
AliphaticCH OH; NH
Wavenumber (cm-1)
5001000150020002500300035004000
Fig. 3. FTIR spectra of some selected humic acids extracted from HARC black soils.
V.M. Benites et al. / Geoderma 127 (2005) 104–113110
of these soils whose mineral fraction is composed
mainly by sand and clays of low activity.
The relatively higher carboxylic in relation to
carboxylate contents of HA from black soils over
quartzite contrasting with igneous counterparts (Fig. 4)
can be due to lower ash contents of the former (Table 3).
The FTIR spectra confirm the aromatic nature and
abundance of carboxylic functional groups of the HA
extracted from HARC black soils (Fig. 4). The
spectra features of samples with higher condensation
degree (black soils over quartzite) are similar to
spectra obtained in hydrolyzed HA extracted from
leonardite (Stevenson and Goh, 1971) and HA
produced from charcoal hydrolyses (Haumaier and
Zech, 1995). This suggests that the studied HA from
black soils are basically composed by condensed
aromatic nucleus, consistent with results obtained by
thermogravimetry.
EHA= 0.06 + 0.38 TGI
r2 = -0.88***
Thermogravimetric index2 3 4 5 6 2 3 4 5 6
H:C = 1.25 - 0.16 TGIr2 = -0.84***
Thermogravimetric index
H:C
ato
mic
rat
io
EH
A
EH
A
0.4
0.5
0.6
0.7
0.8
0.9
1.0
EHA= 2.82 - 2.12 H:Cr2 = -0.92***
H:C atomic ratio0.4 0.6 0.8 1.0
*** significant at p < 0.001
Fig. 5. Correlation graphics between the absorptivity in 465 nm (q465), thermogravimetric index (TGI) and H:C atomic ratio of HARC black
soil humic acid samples (n=42).
44
38
32
26
20
14
82.0 2.5
IHSS peat humic acidHARC igneous rockHARC quartzite
3.0 3.5 4.0
Thermogravimetric index
C:N
ato
mic
rat
io
4.5 5.0 5.5 6.0
Fig. 6. Graphic between thermogravimetric index and C:N atomic ratio of HARC black soil HA samples from igneous rock (migmatite, sienite
and granite) and quartzite (n=42).
V.M. Benites et al. / Geoderma 127 (2005) 104–113 111
The lack of characteristic bands from secondary
amides at ~1650 cm�1 (amide I band) and at ~1540
cm�1 (amide II band) for HA extracted from litter and
black soils over quartzite indicate the low content of
proteins and polypeptide residues that is associated
with the high C:N ratio. In these quartzitic environ-
ments, leaching is intensive, due to the sandy texture,
allowing the selective loss of more soluble compounds,
usually richer in nitrogen, moreover have the difference
in vegetal residues input, being, generally, low forest/
shrubby vegetation for igneous rock soils and shallow
soils on quartzite. The contrast between the quartzite
and igneous rock soil HA characteristics is evident
comparing its C:N atomic ratio and the TGI (Fig. 6).
The high resistance to thermodegradation and high
humification degree of HA from black soils suggest
that they may have a common pyrogenic origin, having
similarities with their original compounds, the char-
coal. In addition, a selective loss of compounds with
greater solubility and lesser stability due to intense
leaching in these soils have to be also considered. Most
probably, both pathways are simultaneously occurring.
The chemical characteristics of HA from HARC black
soils are similar to HA extracted from other black soils
(Kumada, 1983; Skjemstad et al., 1996; Golchin et al.,
1997; Schmid et al., 2002). Kumada (1983) further
suggests that fragments from charcoal added to the soil
may be a source of carbon to the humic substances
genesis. With time, the black carbon may be partially
oxidized, and carboxylic groups with low pKa may be
produced in the sides of the aromatic ring, increasing
the charge and the reactivity of the organic matter in
charred materials from soils subjected to burning
(Glaser et al., 2001). Therefore, the HA stability in
HARC black soils results from its inherited structure of
charred plant residues, whereas its reactivity results
from the pedogenic alterations of these residues.
The charcoal transformation is probably mediated
by micro- and macroorganisms, which besides the
physical action, produce extracellular enzymes capable
to attack polyaromatic hydrocarbons. Microorganisms
capable to alter coal causing its depolymerization and
formation of soluble humic substances can be found in
tropical soils (Crawford and Gupta, 1993). Due to the
great amount of finely distributed carbonized residues
and the presence of black carbon HA in the soil,
occurrence of microorganisms capable to degrade such
structures in HARC black soils is expected.
5. Conclusions
Despite of a broad range of climate conditions and
latitude the HARC Black Soils HA show some
similarities that suggest the same humic acids
formation mechanism occurred in all of the HARC
studied areas.
The high aromaticity and functional group content
of black soil HA are probably due to the contribution
of biotransformed charred vegetable residues, tending
to present larger persistence and reactivity, which is
very important in these low activity soils.
Acknowledgements
We are grateful to IEF-MG and IBAMA for the
soil sampling permission on the conservation areas.
We also thank Dr. Alexandre Pimenta for the
permission to carry out the thermogravimetric analysis
in the Wood Energy Laboratory (DEF-UFV). This
research was supported by the FAPEMIG.
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