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Distribution of Organic Carbon in the Humic Fractions ofDiagnostic Horizons from Brazilian SoilsAdemir Fontana a; Marcos Gervasio Pereira b; Lúcia Helena Cunha dos Anjos b;Vinicius de Melo Benites ca Graduate Course of Agronomy-Soil Science, Universidade Federal Rural do Rio deJaneiro (UFRRJ), Seropédica, Brazilb Soils Department, UFRRJ, Seropédica, Brazilc Embrapa Soils, Rio de Janeiro, Brazil

Online Publication Date: 01 April 2008To cite this Article: Fontana, Ademir, Pereira, Marcos Gervasio, Anjos, Lúcia

Helena Cunha dos and Benites, Vinicius de Melo (2008) 'Distribution of Organic Carbon in the Humic Fractions ofDiagnostic Horizons from Brazilian Soils', Communications in Soil Science and Plant Analysis, 39:7, 951 - 971To link to this article: DOI: 10.1080/00103620801925323URL: http://dx.doi.org/10.1080/00103620801925323

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Distribution of Organic Carbon in theHumic Fractions of Diagnostic Horizons

from Brazilian Soils

Ademir Fontana,1 Marcos Gervasio Pereira,2 Lucia HelenaCunha dos Anjos,2 and Vinicius de Melo Benites3

1Graduate Course of Agronomy–Soil Science, Universidade Federal

Rural do Rio de Janeiro (UFRRJ), Seropedica, Brazil2Soils Department, UFRRJ, Seropedica, Brazil

3Embrapa Soils, Rio de Janeiro, Brazil

Abstract: This study was developed on 52 soil profiles, 61 surface diagnostic horizons,

and 26 subsurface diagnostic horizons classified according to the Soil Taxonomy and

Brazilian Soil Classification System (SiBCS) as Mollisols (Chernossolos), Spodosols

(Espodossolos), Entisols (Gleissolos), Oxisols (Latossolos), and Histosols (Organosso-

los). The objective was to quantify the carbon (C) in organic matter fractions and to

correlate it with soil chemical attributes. Soil organic matter was fractionated into

fulvic acids (C-FAF), humic acids (C-HAF), and humin (C-HUM), and the ratios

C-HAF/C-FAF and AE (alkaline extract)/C-HUM were calculated. Humin was the

predominant fraction in Mollisols and Oxisols, which showed values of AE/C-HUM

and C-HAF/C-FAF lower than 1.0. The humin fraction was also predominant in

surface horizons of Spodosols and Entisols, whereas a higher content of C-FAF and

C-FAH was observed in the subsurface horizons, with values higher than 1.5 for the

AE/C-HUM ratio. C-HAF was predominant in the Histosols, and C-HAF/C-FAF

ratio values were higher than 2.0. The highest correlation values with soil attributes

were observed for C-HAF, C-HUM, and total organic C with pH, sum of bases, and

cation exchange capacity. The differences in humic substances distribution was a

Received 31 October 2006, Accepted 5 May 2007

Address correspondence to Ademir Fontana, Graduate Course of Agronomy–Soil

Science, Universidade Federal Rural do Rio de Janeiro (UFRRJ), Seropedica, Brazil.

E-mail: [email protected]

Communications in Soil Science and Plant Analysis, 39: 951–971, 2008

Copyright # Taylor & Francis Group, LLC

ISSN 0010-3624 print/1532-2416 online

DOI: 10.1080/00103620801925323

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useful parameter to characterize soil orders in the Brazilian soil classification system

and to understand pedogenic processes.

Keywords: Chemical fractioning, humic substances, pedogenesis, soil organic matter

INTRODUCTION

Increasing attention has been given to soil organic matter in pedological studies

and characterization of tropical soils. Originally, the Brazilian soil classification

only used total organic carbon (C) content as a differential attribute to distinguish

mineral from organic soil materials and to identify diagnostic surface horizons

(Embrapa 1988). The Brazilian Soil Classification System (SiBCS) (Embrapa

2006) introduced new differential attributes associated with soil organic matter

(SOM) such as decomposition level and fiber content, among others. This way,

the SiBCS approximated soil taxonomy systems in which qualitative and quan-

titative variables related to SOM were used as diagnostic attributes, especially for

Spodosols, Mollisols, and Histosols classes (Soil Survey Staff 1999).

Soil organic matter, by means of humic substances, is involved in

important soil genesis processes such as (a) favoring weathering (Ehrlich

1990), (b) reducing iron oxides’ crystallinity (Pereira and Anjos 1999;

Cornell and Schwertmann 1996), (c) promoting modifications in hematite/goethite ratio (Kampf and Schwertmann 1983), and (d) having an effect

intensity of on various pedogenetic processes (Duchaufour 1977; Buol,

Hole, and McCracken 1980; Fanning and Fanning 1989).

Organic compounds are referred to as main agents influencing pedoge-

netic processes that originate in Mollic, Spodic, Histic, and Gley diagnostic

soil horizons (Anjos, Pereira, and Ramos 1999). Valladares et al. (2003),

studying Histosols from different Brazilian states, emphasized the usage of

attributes derived from SOM fractioning, such as the fulvic acid (C-FAF)

and humic acid (C-HAF) fractions, and the alkaline extract (AE)/humin

(C-HUM) ratio to separate Histosols in fifth and sixth (family and series)

categorical levels, demonstrating that these fractions could be used for

individualization of soil classes at a low taxonomic level.

The objective of this study was to quantify the C in organic-matter

fractions and to correlate it with soil chemical attributes of selected diagnostic

horizons from Brazilian soils.

MATERIAL AND METHODS

Fifty-two soil profiles from different Brazilian states were sampled, described,

and characterized according to routine soil survey procedures. Soil classes

were distributed as following: 13 Mollisols (Chernossolos), 5 Spodosols

(Espodossolos), 10 Entisols (Gleissolos), 16 Oxisols (Latossolos), and 8

A. Fontana et al.952

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Histosols (Organossolos). A total of 87 diagnostic horizons were selected,

including 61 surface horizons/epipedons and 26 subsurface horizons (Table 1).

Profile Descriptions and Characterizations

The soil profiles were described according to procedures in the Manual for

Soil Description and Collecting (Santos et al. 2005) and classified

according to Soil Taxonomy (Soil Survey Staff 1999) and Embrapa (2006).

Soil chemical and physical properties were analyzed according to Embrapa

(1997). They included soil pH; contents of calcium (Ca2þ), magnesium

(Mg2þ), sodium (Naþ), potassium (Kþ), aluminum (Al3þ), and H þ Al;

total organic C (TOC); sand, silt, and clay fractions; and sum of bases

(S value) and cation exchange capacity, CEC (T value).

Humic Substance Chemical Fractioning

The humic substances were separated using differential solubility techniques,

applying the humic fractions concept established by Humic Substances Inter-

national Society (Swift 1996) with modifications according to Benites,

Madari, and Machado (2003). Soil samples containing about 1.0 g of C for

mineral horizons or 0.5 g of C for histic horizons and 20 mL of 0.1 mol

L21 sodium hydroxide (NaOH) were used in the extraction of C-HAF and

C-FAF, with a contact time of 24 h. The separation between the alkaline

extract (AE ¼ C-FAF þ C-FAH) and the residue was performed by

Table 1. Surface and subsurface diagnostic horizons identified according to Soil

Taxonomy (USA) and equivalents in SiBCS (Brazil)

Soil Taxonomya SiBCSbSample

numbers

Surface horizons/epipedons

Anthropic A antropico 1

Mollic A chermozemico 13

Umbric A humico 5

Ochric A moderado 21

Ochric weak A fraco 1

Histic H hıstico 17

Subsurface horizons

Spodic B espodiso 5

Oxic B latossolico 16

Gley B and C glei 5

aSoil taxonomy (Soil Survey Staff 1999).bSiBCS (Embrapa 2006).

Organic Carbon in Humic Fractions 953

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centrifugation at 5000 g (gravity) for 30 min. Residue washings were carried

out with the same solution, and extracts previously obtained were added,

resulting in a final volume of approximately 40 mL. The residue was

collected, dried, and reserved for determination of C as humin compounds

(HUM). The alkaline extract (AE) had the pH lowered to 1.0 þ 0.1 with 20%

sulfuric acid (H2SO4) and was decanted for 18 h. The precipitate, HAF, was

separated from the soluble fraction (FAF) by means of filtering, and the

volumes of both fraction extracts were adjusted to 50 mL with distilled water.

The determination of organic C in FAF and HAF extracts was performed

according to the method of Yeomans and Bremner (1988) using 5 mL of

extract, 1 mL of 0.042 mol L21 potassium dichromate (K2Cr2O7), and 5 mL of

concentrated H2SO4; 0.012 mol L21 ferrous ammonium sulfate [Fe(NH4)2

(SO4)2. 6H2O] was used for titration. Organic C in the HUM fraction was

extracted by adding 5 mL of 0.167 mol L21 potassium dichromate (K2Cr2O7)

and 10 mL of concentrated H2SO4 and quantified by titration with

0.25 mol L21 ferrous ammonium sulfate [Fe(NH4)2(SO4)2. 6H2O], using

ferroin (0.025 mol L21) as indicator.

From the content of organic C in the fulvic acid fraction (C-FAF), humic

acid fraction (C-HAF), and humin fraction (C-HUM), the percentage of each

fraction in relation to TOC (% TOC), the C-HAF/C-FAF ratio, and the

AE/C-HUM ratio was calculated.

Statistical Analyses

Statistical techniques and the Pearson correlation method were used for the com-

parison of organic C contents in humic fractions and TOC in each soil class and

surface and subsurface horizons. To compare the soil classes as related to soil

organic C attributes, the principal components analysis (PCA) was performed

using the XL Stat program. All attributes were standardized, taking the

average as equal to 0 and variance equal to 1.0 (Morrison 1976). The

following attributes were selected by correlation analysis: TOC, %FAF,

%HAF, %HUM, C-HAF/C-FAF ratio, AE/C-HUM ratio, and total clay content.

RESULTS AND DISCUSSION

Principal Component Analysis Applied to Groups of Soils

PCA of diagnostic horizon variables were initially performed considering the

two first factors with accumulated variance of 74.20%, following another

evaluation for soil orders, resulting in an accumulated variance of 74.46%.

Because the diagnostic horizons’ data distribution showed some horizons

with a behavior distinct from the others, a study was conducted to establish

standards based on contribution of selected variables. The selected diagnostic


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horizons Mollic, Umbric, Ochric, Spodic, and Histic presented accumulated

variance percentage of 71.96%. Considering the dispersion of values corre-

sponding to scores of factors in axis F1 and F2, PCA distinguished groups

of diagnostic horizons (Figure 1) mainly influenced by total organic C

content. For instance, Histic horizons were emphasized in X axis (positive)

and horizons with low TOC contents were emphasized in Y axis (negative),

thus confirming the importance of this attribute for separating organic and

mineral soil materials in SiBCS, in addition to validating PCA as a tool for

distinguishing soil classes.

In a second PCA evaluation, the diagnostic horizons and soil orders (Figures

1 and 2) were placed in six groups as follows: (a) group mainly influenced by high

AE/C-HUM ratio, (b) with participation of high %HAF, (c) more influenced by

high TOC contents, (d) with expressive participation of %HUM, (e) with expres-

sive participation of clay content; and (f) mainly influenced by characteristics

such as presence of high water table during part of the year (poor drainage). In

the last group, variation on quality and quantity of total organic C, differences

in soil mineral matrix (granulometry), and their combination may be influencing

SOM dynamics and humic substances distribution, hence influencing

significantly the correlation of variables with the dispersion axis.

Diagnostic Mollic, Umbric, Ochric, Spodic, and Histic horizons were

selected, based on Figure 1 patterns and the high influence of variables

studied, to identify the relation with humic substance contents and ratio.

PCA pointed and ordered the variables of highest influence within each diag-

nostic horizon (Figure 3), allowing better separation of diagnostic horizons

than in previous analysis.

Figure 1. Dispersion of scores of factors 1 and 2 from PCA and groups of diagnostic

horizons of the Brazilian soils.


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Organic Carbon Content and Humic Substances

Well-Drained Mineral Soils (Mollisols and Oxisols)

Total organic C values ranged from 4.4 to 44.8 g kg21 (average of 19.2 g

kg21), and the highest contents were observed in Mollisols, with most

horizons presenting values of more than 30 g kg21. This expresses the high

Figure 2. Dispersion of scores of factors 1 and 2 from PCA and groups of soil orders.

Figure 3. Dispersion of scores of factors 1 and 2 from PCA and groups of selected

diagnostic horizons of the Brazilian soils.


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stability of organic matter of Mollisols, due to calcium ions that favor

formation of calcium humates (Kononova 1966; Theng 1979; Duchaufour,

1983). The lowest TOC contents were observed in oxic subsurface

horizons, with intermediate values in the surface horizons of Oxisols

(Table 2).

The organic C in the humic substances showed highest average contents

for C-HUM, and it was the predominant SOM fraction in all horizons. The

values ranged from 3.0 to 36.8 g kg21 (average of 11.8 g kg21), with an

average C-HUM contribution of 61.0% of total organic C (Table 2). The

C-HUM in well-drained mineral soils was the most relevant humic substances

fraction, possibly due to the high stability of this faction and strong interaction

with soil mineral matrix, thus delaying SOM mineralization and enabling its

accumulation.

The C-HAF/C-FAF ratio presented a large variation of values (Table 2),

ranging from 0.08 to 3.50 (average of 0.83), with 71.1% of values less than

1.0, demonstrating the predominance of C-FAF rather than C-HAF, especially

in the Oxisols. The dispersion of C-HAF/C-FAF ratio is also noted in

Figure 4, where a negative correlation of the C-HAF/C-FAF ratio with clay

content is verified.

Zhang, Thompson, and Sandor (1988), working with soils derived from

loess in Iowa State, observed in Mollisols a C-HAF/C-FAF ratio of 1.68

and 1.95 for uncultivated and cultivated soils respectively. A different

behavior was shown by Alfisols, where the values were 2.01 and 0.73 for

uncultivated and cultivated soils. Martin-Neto, Rosell, and Sposito (1998),

in uncultivated Mollisols from the Argentina Pampa region, found much

higher values of the C-HAF/C-FAF ratio, between 2.5 and 4.6. According

to Dabin (1980–1981), Ortega (1982), and Canellas et al. (2000), in

weathered soils, values less than 1.0 for this ratio are associated with

intense mineralization of organic residues. Possibly the climate and parent

materials also play an important rule in the C-HAF/C-FAF ratio, even for

the same soil orders, explaining the differences found for the Brazilian soils.

On the other hand, the AE/C-HUM ratio showed small variation, with

values ranging from 0.17 to 1.07 (average of 0.51), and 93.3% of values

were less than 1.0, demonstrating the predominance of C-HUM in well-

drained mineral soils. The lowest values (less than 0.35) were observed for

the Mollisols, which is in agreement with findings of Melo (2002) and

Benites, Ker, and Mendonca (2000). Studies on SOM fractioning from

Brazilian Oxisols (Benites 1998; Benites, Ker, and Mendonca 2000; Lima

2001; Melo 2002; Cunha et al. 2003) show a trend of values less than 1.0,

both for C-HAF/C-FAF and AE/C-HUM ratios.

On Mollisols, Pearson correlation for organic C in humic fractions and

TOC with soil chemical properties and clay content showed that only

C-HAF stood out, presenting significant and negative correlation with S

value (r ¼ 2 0.67��) and clay content (r ¼ 20.68��). This result is related

to formation in Mollisols of high-stability compounds of C-HAF with Ca2þ


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Table 2. Organic carbon in humic substancesa from Brazilian Mollisols and Oxisols

Profile Horizon

C-FAF

(g kg21)

C-HAF

(g kg21)

C-HUM

(g kg21)

C-SUM

(g kg21)

TOC

(g kg21)

FAF

(% TOC)

HAF

(% TOC)

HUM

(% TOC)

C-HAF/C-FAF

AE/C-

HUM

Mollisols (n ¼13)

CMS1b A 2.1 2.5 21.0 25.6 30.7 7 8 68 1.19 0.22

CMS2 A 2.9 3.9 19.6 26.4 30.1 10 13 65 1.34 0.35

CMS3 A 1.6 5.6 36.8 44.0 44.8 4 13 82 3.50 0.20

CMS4 A 3.9 1.5 31.9 37.3 44.8 9 3 71 0.38 0.17

CMS5 A 3.0 3.9 36.4 43.3 44.8 7 9 81 1.30 0.19

CMS6 A 2.6 4.1 31.9 38.6 43.5 6 9 73 1.58 0.21

CMS7 A 1.9 3.8 21.0 26.7 30.2 6 13 70 2.00 0.27

CMS8 A 2.3 2.6 19.9 24.8 30.7 7 8 65 1.13 0.25

CMS9 A 1.4 4.5 19.9 25.8 36.9 4 12 54 3.21 0.30

CMS10 A1 2.5 2.2 26.7 31.4 44.8 6 5 60 0.88 0.18

CPR Ap 1.5 0.4 7.1 9.0 23.2 6 2 31 0.27 0.27

CRJ Ap 0.8 0.9 9.4 11.1 22.4 4 4 42 1.13 0.18

CRN A1 2.5 1.6 12.3 16.4 15.6 16 10 79 0.64 0.33

Oxisols (n ¼ 16)

LAM1 A 5.1 15.0 26.6 46.7 44.8 11 33 59 2.94 0.76

LAM1 Bw1 1.1 0.3 4.4 5.8 4.8 23 6 92 0.27 0.32

LAM2 A 3.1 2.7 9.0 14.8 17.3 18 16 52 0.87 0.64

LAM2 Bw1 2.4 0.6 3.6 6.6 7.0 34 9 51 0.25 0.83

LBA A 6.0 2.5 9.5 18.0 18.8 32 13 51 0.42 0.89

LBA Bw2 1.5 0.4 3.5 5.4 5.2 29 8 67 0.27 0.54

LGO A 5.3 2.7 13.5 21.5 24.2 22 11 56 0.51 0.59

LGO Bw2 1.2 0.2 3.1 4.5 4.4 27 5 70 0.17 0.45

A.

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LMG1 A 6.3 4.8 11.1 22.2 22.3 28 22 50 0.76 1.00

LMG1 Bw2 3.6 2.6 7.2 13.4 13.8 26 19 52 0.72 0.86

LMG2 Ap 3.6 2.5 8.8 14.9 15.0 24 17 59 0.69 0.69

LMG2 Bw1 1.6 0.6 3.7 5.9 6.0 27 10 62 0.38 0.59

LMG3 A1 2.6 2.4 6.9 11.9 12.5 21 19 55 0.92 0.72

LMG3 Bw1 2.2 0.6 5.5 8.3 8.0 28 8 69 0.27 0.51

LMG4 A 1.8 1.8 9.0 12.6 18.5 10 10 49 1.00 0.40

LMG4 Bw1 1.5 0.3 5.1 6.9 8.1 19 4 63 0.20 0.35

LMG5 A 2.1 1.3 7.9 11.3 15.5 14 8 51 0.62 0.43

LMG5 Bw1 1.9 0.7 5.3 7.9 9.3 20 8 57 0.37 0.49

LMS Ap 3.5 1.0 7.0 11.5 11.2 31 9 63 0.29 0.64

LMS Bw1 1.3 0.1 4.1 5.5 4.5 29 2 91 0.08 0.34

LPA Ap 4.6 2.5 9.0 16.1 16.2 28 15 56 0.54 0.79

LPA Bw1 2.7 0.5 3.0 6.2 7.0 39 7 43 0.19 1.07

LPR A 3.2 3.5 10.2 16.9 16.6 19 21 61 1.09 0.66

LPR Bw2 3.1 2.7 7.0 12.8 12.4 25 22 56 0.87 0.83

LRJ1 A 1.3 1.4 6.6 9.3 15.9 8 9 42 1.08 0.41

LRJ1 Bw1 1.1 0.3 3.7 5.1 5.7 19 5 65 0.27 0.38

LRJ2 Ap 3.2 2.9 8.5 14.6 15.1 21 19 56 0.91 0.72

LRJ2 Bw2 2.0 0.4 3.3 5.7 6.4 31 6 52 0.20 0.73

LRS A1 5.5 3.2 8.5 17.2 16.5 33 19 52 0.58 1.02

LRS BW1 1.7 0.2 3.5 5.4 4.4 39 5 80 0.12 0.54

LSC Ap 4.1 2.5 12.2 18.8 25.7 16 10 47 0.61 0.54

LSC Bw1 1.8 0.4 6.9 9.1 9.3 19 4 74 0.22 0.32

aC-FAF ¼ organic carbon in the fulvic acids; C-HAF ¼ organic carbon in the humic acids; C-HUM ¼ organic carbon in the humin fraction; AE/C-HUM ¼ ratio between C-FAF þ C-HAF/C-HUM; TOC ¼ total soil organic carbon.

bFirst letter refers to soil order (C ¼ Chernossolo/Mollisols, L ¼ Latossolo/Oxisols); the second and third refer to the Brazilian state where

samples were collected: AM ¼ Amazonas, BA ¼ Bahia, GO ¼ Goias, MG ¼ Minas Gerais, MS ¼ Mato Grosso do Sul, PA ¼ Para,

PR ¼ Parana, RJ ¼ Rio de Janeiro, RS ¼ Rio Grande do Sul, RN ¼ Rio Grande do Norte, SC ¼ Santa Catarina.

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(calcium humates) (Theng 1979), making the complete extraction of this

fraction during SOM fractioning less efficient. In the Oxisols, significant

values of Pearson correlation of organic matter with soil chemical properties

were observed (Table 3). For surface horizons, C-FAF presented a significant

and positive correlation with Hþ (r ¼ 0.61��), CEC, and clay content.

However, the highest correlation values were observed between C-HAF and

the S value (r ¼ 0.78��) and CEC (r ¼ 0.82��); C-HUM with S value

(r ¼ 0.83��) and CEC (r ¼ 0.89��); and TOC with S value (r ¼ 0.84��)

and CEC (r ¼ 0.92��). For the Oxisols subsurface horizons, the highest

values were for correlation of C-HUM with Hþ and CEC (r ¼ 0.82�� and

r ¼ 0.81��, respectively), and the lowest were for correlation of C-FAF

with these properties.

Mendonca and Rowell (1996) studied Oxisols from Brazilian Cerrado

region with predominance of kaolinite and gibbsite minerals in the clay

fraction, and they observed a significant contribution from humic substances

in CEC. It was noted that even with clay content increasing with depth,

CEC remained basically unchanged.

Mineral and Poorly or Imperfectly Drained Soils (Spodosols and

Entisols)

Because of the occurrence of surface histic horizons in some Spodosols and

Entisols sampled, the TOC contents presented a wide variation, with values

ranging from 2.4 g kg21 to 317.0 g kg21, the highest values in the histic

horizons. For the mineral horizons, values of TOC were between 2.4 and

Figure 4. Dispersion diagram of the C-HAF/C-FAF ratio and clay content for the

soil orders in Mollisols (n ¼ 13) and Oxisols (n ¼ 32).


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76.5 g kg21. The narrowest variation amplitude, with contents ranging from

7.1 to 19.2 g kg21, was observed for spodic B horizons (Table 4). Vejre

et al. (2003) observed in soils from Denmark that surface horizons of

Spodosols presented a large TOC variation, in opposition to the spodic

horizons. Such behavior was also verified by Benites (2002) in upland soils

under rupestrian vegetation in Serra do Espinhaco and Serra da Mantiqueira,

on Brazilian southeast region. Gomes et al. (1998) and Gomes (2005) found a

similar distribution in Spodosols from Sao Paulo and Rio de Janeiro sandy

coastal areas.

Fractioning of SOM in soil profiles classified as Spodosols and Entisols

showed highest average values for C-HUM (16.9 g kg21), especially in

surface horizons, with average values for HUM fraction of 35.5%. In the

spodic and gley horizons, C-FAF and C-HAF were predominant in relation

to C-HUM. Carbon content in fulvic acid and humic acid fractions varied in

dominance for the different Spodosols and Entisols soil profiles (Table 4).

According to Santos (1984), the alternating seasonal flooding and drying

that occurs on Spodosols and Entisols landscape allows for intense organic-

matter oxidation in the surface horizons. This favors formation of HUM

Table 3. Pearson correlation for organic carbon in humic fractions and TOC with

chemical properties and clay content in Oxisols and Histosols

Properties C-FAF C-HAF C-HUM C-SUM TOC

Oxisols—surface horizons (n ¼ 16)

pH — 0.59�� 0.60�� 0.56� 0.58��

S — 0.78�� 0.83�� 0.78�� 0.84��

Hþ 0.61�� — — — 0.42�

CEC 0.45� 0.82�� 0.89�� 0.87�� 0.92��

Clay 0.49� — — — —

Oxisols—subsurface horizons (n ¼ 16)

pH — — — — —

S — — — — —

Hþ 0.65�� 0.79�� 0.82�� 0.87�� 0.82��

CEC 0.43� 0.58�� 0.81�� 0.74�� 0.67��

Clay — — — — —

Histosols (n ¼17)

pH — 20.54� 20.54� 20.58�� 20.81��

S 0.50� — 0.41� — —

Hþ — 0.47� — — 0.46�

CEC — 0.54� — 0.46� 0.64��

��Significant at 0.1%; ��significant at 1%; �significant at 5%.

Notes. C-FAF ¼ organic carbon in fulvic acids; C-HAF ¼ organic carbon in humic

acids; C-HUM ¼ organic carbon in humin fraction; C-SUM ¼ sum of organic carbon

in all fractions; TOC ¼ total soil organic carbon; pH ¼ pH water (1:2.5); S ¼ sum of

base value; Hþ ¼ hydrogen; CEC ¼ cation exchange capacity.


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Table 4. Organic carbon distribution in humic substancesa from Brazilian Spodosols and Entisols

Profile Horizon

C-FAF

(g kg21)

C-HAF

(g kg21)

C-HUM

(g kg21)

C-SUM

(g kg21)

TOC

(g kg21)

FAF

(% TOC)

HAF

(% TOC)

HUM

(% TOC)

C-FAH/C-FAF

AE/C-

HUM

Spodosols (n ¼ 5)

EPR1b A 1.8 3.3 3.7 8.8 8.8 20 38 42 1.83 1.38

EPR1 Bh2 4.1 1.4 1.5 7.0 7.1 58 20 21 0.34 3.67

EPR2 A 0.8 1.5 0.3 2.6 2.4 33 63 13 1.88 7.67

EPR2 Bh2 4.3 5.9 0.7 10.9 15.3 28 39 5 1.37 14.57

ERJ Ho1 14.3 49.6 145.2 209.1 317.0 5 16 46 3.47 0.44

ERJ Bhj 6.9 0.4 1.4 8.7 12.6 55 3 11 0.06 5.21

ERO Ap 2.6 10.3 10.1 23.0 76.5 3 13 13 3.96 1.28

ERO Bhs1 2.9 2.0 3.3 8.2 19.2 15 10 17 0.69 1.48

ESP A 1.7 4.8 7.0 13.5 16.4 10 29 43 2.82 0.93

ESP Bhsj2 2.4 10.0 0.5 12.9 17.1 14 58 3 4.17 24.8

Entisols (n ¼ 10)

GBA A1 4.7 12.5 31.0 48.2 63.5 7 20 49 2.66 0.55

GBA Cg 1.5 3.3 1.7 6.5 5.4 28 61 31 2.20 2.82

GMS A 2.1 1.5 10.0 13.6 17.9 12 8 56 0.71 0.36

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GPA Ag 2.6 2.1 9.3 14.0 14.3 18 15 65 0.81 0.51

GPA Bg 3.2 3.4 13.2 19.8 24.0 13 14 55 1.06 0.50

GPR Ap 6.0 12.0 19.9 37.9 38.5 16 31 52 2.00 0.90

GRJ1 A 8.1 26.4 24.1 58.6 73.0 11 36 33 3.26 1.43

GRJ2 H 8.9 30.2 47.6 86.7 121.6 7 25 39 3.39 0.82

GRJ3 A 4.9 11.8 25.3 42.0 52.5 9 22 48 2.41 0.66

GRJ4 Ap 2.3 2.7 5.2 10.2 12.5 18 22 42 1.17 0.96

GRJ4 Cg1 0.8 0.7 2.6 4.1 3.6 22 19 72 0.88 0.58

GRS Hdp 10.9 33.2 44.3 88.4 109.8 10 30 40 3.05 1.00

GRS Cg 3.8 11.6 6.0 21.4 25.1 15 46 24 3.05 2.57

GSP Ap 4.2 39.3 6.6 50.1 51.0 8 77 13 9.36 6.59

GSP Cg1 0.2 1.1 2.3 3.6 4.2 5 26 55 5.50 0.57


bFirst letter refers to soil order (E ¼ Espodossolos/Spodosols. G ¼ Gleissolos/Entisols); the second and third refer to the Brazilian state were

samples were collected: BA ¼ Bahia, MS ¼ Mato Grosso do Sul, PR ¼ Parana, RJ ¼ Rio de Janeiro, RO ¼ Rondonia, RS ¼ Rio Grande do Sul,

SP ¼Sao Paulo.

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because of the slow transformation of organic matter into more stable

organic compounds through the loss of OCH3 groups from lignin. For

spodic horizons, the fulvic and humic acids, because of their higher reactivity

and molecular weight, form more soluble complexes, thus interfering more

intensely in the medium and facilitating lixiviation of ions and eluviation

of clay, favoring to the pedogenetic process of podzolization (Deb 1949;

Benites et al. 2001; Buol, Hole, and McCracken 1980; Anjos, Pereira, and

Ramos 1999).

The C-HAF/C-FAF ratio was higher than 1.0 for most soil horizons, and

a wide variation between values was observed (Table 4). For surface horizons

of Spodosols and Entisols, values ranged from 0.71 to 9.36, and for subsurface

horizons, from 0.06 to 5.50. Similar variation was observed by Gomes et al.

(1998), studying soils formed in sandy coastal areas, north of Rio de

Janeiro State. Bonifacio et al. (2006), studying genesis of Spodosols and

Histosols, found predominance of C-FAF over C-HAF, in agreement with

Benites’ (1998) results for Spodosols, though Benites (2002) reported dissim-

ilar behavior in upland soils under rupestrian vegetation in Serra do Espinhaco

and Serra da Mantiqueira, with predominance of C-HAF over C-FAF. The

differences might be related to intensity of soil-forming factors such as

climate, relief (altitude), and vegetation. According to Oliveira (1999), FAF

was the most active component in the podzolization pedogenetic process.

The same was reported by Benites et al. (2001), studying upland soils of

Serra do Brigadeiro State Park (State of Minas Gerais).

For Spodosols, the predominance of humic acids was inversely related to

soil pH, because an increase of HAF content occurred along with a pH

decrease. Also, at a lower pH, the humic acids tend to precipitate because

of their reduced solubility, as indicated by Figure 5. According to Schnitzer

(1986), humic acids form insoluble complexes in pH values less than 6.5

when associated to colloids, thus enabling immobilization and accumulation

of this fraction in acid soils.

The AE/C-HUM ratio also presented a wide variation. The values

ranged from 0.36 to 7.67 in surface horizons and from 0.58 to 24.8 for sub-

surface horizons (Table 4). Especially for spodic and gley horizons, the

values of AE/C-HUM ratio were generally higher than 1.5, indicating

movement of alkaline-soluble compounds with increasing soil depth.

Similar values of AE/C-HUM ratio were observed by Schaefer et al.

(2002) and Benites (1998; 2002), studying Spodosols, and by Melo (2002)

in Entisols, indicating that these relations might provide information about

the genesis of these soils, evidencing layers or horizons with depletion or

accumulation of soil organic C.

On Spodosols, Pearson correlation coefficients for organic C in humic

fractions and TOC with soil chemical properties and clay content showed

high significant and negative correlations between C-HAF and C-SUM with

pH (r ¼ 20.98�� and r ¼ 20.96��) and positive correlation with CEC

(r ¼ 0.80� and r ¼ 0.86�). The surface horizons of Entisols only presented


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correlations between C-HUM and CEC (r ¼ 0.64�). When gley C horizons

were selected, correlations between C-FAF, C-HUM, C-SUM, and TOC

with CTC (r ¼ 0.84� to r ¼ 0.99��) and between C-HUM with clay

content were also verified (r ¼ 0.93��).

The dispersion of C-AE/C-HUM ratio with total sand content (Figure 6)

showed a direct relation between these properties, indicating possible

Figure 5. Dispersion diagram of the HAF percentage with soil pH values for the soil

orders Spodosols–Entisols (n ¼ 25) and Histosols (n ¼ 17).

Figure 6. Dispersion diagram of the AE/C-HUM ratio with the total sand content for

the soil orders Spodosols (n ¼ 10) and Entisols (n ¼ 25).


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influence of granulometry on distribution of humic substances in relation

to TOC.

Organic and Very Poorly Drained Soils (Histosols)

The Histosols profiles had a wide variation of TOC, ranging from 114.3 to

568.8 g kg21 (average of 351.3 g kg21). This variation may result from

the heterogeneity of organic deposits in different environments and

climatic conditions (Table 5). Couto and Resende (1985), studying SOM

of soils with gley and histic horizons, in the southeastern region of Bahia

State, found an average value of 369 g kg21 of TOC from a collection of

59 soil horizons.

The fractioning of soil organic matter of Histosols showed that C-HAF

presented the highest average values (94.5 g kg21), with a small dominance

in relation to C-HUM, which had an average of 90.3 g kg21 (Table 5). The

average percentage of HAF was 30.1%, and HUM was of 25.5%. Benites

(2002), studying upland soils under rupestrian vegetation in Serra do

Espinhaco and Serra da Mantiqueira, also observed predominance of humic

acids to the detriment of other SOM fractions. Valladares et al. (2007),

studying Histosols from different states of Brazil, and Conceicao et al.

(1999), in soils from coastal lowlands of Rio de Janeiro State, found dissimilar

behavior where C-HUM was predominant.

The C-HAF/C-FAF ratio in Histosols varied from 1.91 to 16.95, present-

ing the highest average values (6.14) among all soil orders studied (Table 5).

The humic acid fraction accumulation in the Histosols is associated with

humic substances synthetic route, dominant at constant water-lodging con-

ditions, instead of the solubility of HAF as occurs in mineral soils. As

showed in Figure 5, increase of pH resulted in no significant change of

HAF values. Similar behavior was observed by Benites (2002) and Valladares

et al. (2007) in Histosols. The AE/C-HUM ratio also presented wide variation,

with values ranging from 0.41 to 7.24 and an average of 1.64 (Table 5). Most

horizons (76%) had values less than 2.0, in agreement with Valladares et al.

(2007).

For Histosols, C-FAF and C-HUM presented significant and positive

correlation with S value. The C-HAF, C-HUM, and TOC showed corre-

lation with CEC, with emphasis for TOC (r ¼ 0.64�), evidencing the

importance of TOC for increasing soil CEC. The C-HAF, C-HUM,

C-SUM, and TOC had significant and negative correlation with pH

(r ¼ 20.54� to r ¼ 20.81��). C-HAF and TOC also presented significant

and positive correlation with Hþ (r ¼ 0.47� and 0.46�), demonstrating the

contribution of SOM fractions in soil acidity and pH values (Table 3).

These results are relevant to the management of Histosols, mainly consider-

ing that agricultural practices such as drainage and intensive liming result in

fast decrease of TOC.


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Table 5. Organic carbon distribution in humic substancesa from Brazilian Histosols

Profile Horizon

C-FAF

(g kg21)

C-HAF

(g kg21)

C-HUM

(g kg21)

C-SUM

(g kg21)

TOC

(g kg21)

FAF

(% TOC)

HAF

(% TOC)

HUM

(% TOC) C-FAH/C-FAF AE/C-HUM

OALb Hdopj 17.6 101.3 112.7 231.6 522.3 3 19 22 5.76 1.06

OAL Hdopj1 16.8 189.5 151.1 357.4 529.3 3 36 29 11.28 1.37

ODF Hd1 14.4 92.8 26.6 133.8 145.8 10 64 18 6.44 4.03

ODF Hd2 12.1 57.2 25.3 94.6 167.6 7 34 15 4.73 2.74

OMG Hdo1 11.7 22.4 71.9 106.0 114.3 10 20 63 1.91 0.47

OMS Hdo1 14.8 57.2 26.0 98.0 182.4 8 31 14 3.86 2.77

OMS 2Hd 6.9 75.3 38.5 120.7 459.8 2 16 8 10.91 2.14

ORJ Hdp 21.8 90.1 88.8 200.7 349.5 6 26 25 4.13 1.26

ORJ Hdo1 29.2 91.5 106.2 226.9 375.0 8 24 28 3.13 1.14

ORJ Hdo2 36.1 92.8 81.1 210.0 391.3 9 24 21 2.57 1.59

ORS Hdpj 15.3 126.3 113.8 255.4 470.0 3 27 24 8.25 1.24

ORS Hdj 14.2 93.4 113.8 221.4 568.8 2 16 20 6.58 0.95

ORS Hdoj 7.2 122.0 201.8 331.0 556.4 1 22 36 16.94 0.64

OSC H1 32.7 78.0 95.1 205.8 277.2 12 28 34 2.39 1.16

OSC H2 26.0 108.5 224.5 359.0 445.7 6 24 50 4.17 0.60

OSP Hp1 23.3 104.6 40.6 168.5 231.0 10 45 18 4.49 3.15

OSP Hp2 15.1 104.4 16.5 136.0 186.2 8 56 9 6.91 7.24


bFirst letter refers to soil order (O ¼ Organossolos/Histosols); the second and third refer to the Brazilian state where samples were collected:

AL ¼ Alagoas, DF ¼ Distrito Federal, MG ¼ Minas Gerais, MS ¼ Mato Grosso do Sul, RJ ¼ Rio de Janeiro, RS ¼ Rio Grande do Sul,

SC ¼ Santa Catarina, SP ¼ Sao Paulo.

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CONCLUSIONS

Mollisols and Oxisols presented HUM as dominant humic substance fraction,

with AE/C-HUM and C-HAF/C-FAF ratio values less than 1.0. In Spodosols

and Entisols, HUM was predominant in surface horizons, whereas in subsur-

face horizons, FAF and HAF fractions had highest participation and AE/C-

HUM ratio values were more than 1.5. For Histosols, C-HAF was predominant

and C-HAF/C-FAF ratio values were greater than 2.0.

Correlation coefficient values of humic substances and TOC with soil

chemical attributes and granulometry presented distinct patterns between

soil orders. The best correlations of C-HAF, C-HUM, and TOC were

observed with pH, S value, and CEC.

The differences in humic substance distribution of diagnostic horizons, by

means of chemical fractioning, was a useful parameter to characterize soil

orders in the Brazilian Soil Classification System. Variations in values of

SOM fractions and their ratios, between soil orders and within the soil

profiles, are also relevant to understand pedogenic processes.

ACKNOWLEDGMENTS

We are grateful to Conselho Nacional de Desenvol Vimento Cientıfico e

Tecnologico (CNPq), Graduate Course in Agronomy–Soil Science (CPGA-

CS/UFRRJ), and Embrapa Solos for the financial and technical support for

this project.

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