Properties of black soil humic acids from high altitude rocky complexes in Brazil

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).


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).


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.


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).


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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Properties of black soil humic acids from high altitude rocky complexes in Brazil

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