Page 1
Texture and sesquioxide effects on water-stable aggregates and organic
matter in some tropical soils
Bernard G. Barthès
a,
⁎
, Ernest Kouakoua
b
, Marie-Christine Larré-Larrouy
a
,
Tantely M. Razafimbelo
c
, Edgar F. de Luca
d
, Anastase Azontonde
e
, Carmen S.V.J. Neves
f
,
Pedro L. de Freitas
g
, Christian L. Feller
c
a
UR SeqBio (Séquestration du carbone et bio-fonctionnement des sols), IRD (Institut de Recherche pour le Développement),
BP 64501, 34394 Montpellier cedex 5, France
b
Cabinet Expertise Ernest Kouakoua, Résidence du Lac, 303 rue P. Cardenal, Logt 13, 34080 Montpellier, France
c
UR SeqBio (Séquestration du carbone et bio-fonctionnement des sols), IRD (Institut de Recherche pour le Développement),
BP 434, 101 Antananarivo, Madagascar
d
Horto Florestal Edmundo Navarro de Andrade, 533 Rua Barão de Piracicamirim, 13416-005 Piracicaba, SP, Brazil
e
LSSEE (Laboratoire des Sciences du Sol, Eaux et Environnement), 01 BP 988, Cotonou 01, Benin
f
Departamento de Agronomia, Universidade Estadual de Londrina, CP 6001, 86051-990 Londrina, PR, Brazil
g
EMBRAPA (Empresa Brasileira de Pesquisa Agropecuária) Solos, 1024 Rua Jardim Botânico, Jardim Botânico, 22460-000 Rio de Janeiro, RJ, Brazil
Received 30 January 2007; received in revised form 3 October 2007; accepted 8 October 2007
Available online 3 December 2007
Abstract
Many tropical soils include sesquioxides, which influence the stability of soil organic matter (OM) and aggregation to an extent that is not fully
characterized. The present study was carried out on a range of 18 topsoil samples (0
–
10 cm) from low-activity clay (LAC) soils from sub-Saharan
Africa and Brazil, and aimed: (i) at characterizing the size distributions of water-stable aggregates and organic constituents, (ii) at studying how
these distributions were affected by texture and sesquioxides, and (iii) how they interacted.
The distributions of stable aggregates were generally dominated by macroaggregates (
N
200
μ
m), and those of organic constituents by fine OM
(
b
20
μ
m). Aggregation was not clearly affected by soil texture, while total soil carbon (Ct) and the amount of carbon (C) as fine OM increased
with soil content in clay plus fine silts (
b
20
μ
m). Stable macroaggregation correlated with Ct and with C amount as fine OM, but each of them
correlated more closely with citrate-bicarbonate-dithionite-extractable aluminium (Al), which was not expected. Stable macroaggregation also
correlated with C amounts as coarse- and medium-sized OM (
N
200 and 20
–
200
μ
m, respectively), but each of them correlated more closely with
oxalate-extracted Al. These results suggested that for the LAC tropical soils under study OM and aggregate stability depended closely on Al-
containing sesquioxides, on Al-substituted crystalline hematite and goethite especially. These sesquioxides also seemed to play a dominant role in
the relations between aggregation and OM. As far as soils rich in sesquioxides are concerned, this confirmed that OM is not the main aggregating
agent, and suggested that physical protection within aggregates is not necessarily the main mechanism for OM stabilization. However, as soil
sesquioxide content cannot be managed easily, the effect of land use on soil OM and aggregation was determinant at the local scale: indeed, for a
given location, stable macroaggregation, Ct and C amount as fine OM generally decreased with land use intensification (i.e. cultivation, tillage,
reduced surface cover).
© 2007 Elsevier B.V. All rights reserved.
Keywords:
Kaolinitic soils; Aggregation; Organic matter; Texture; Sesquioxides; Aluminium
1. Introduction
Soil organic matter (OM) and aggregation are important
determinants of soil fertility and productivity, and are key factors
in the global carbon (C) cycle. Indeed, soil OM is a source of
nutrients, promotes structure and water retention, and is a
Available online at www.sciencedirect.com
Geoderma 143 (2008) 14
–
25
www.elsevier.com/locate/geoderma
⁎
Corresponding author. Tel.: +33 4 99 61 21 36; fax: +33 4 99 61 21 19.
E-mail address:
barthes@mpl.ird.fr
(B.G. Barthès).
0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:
10.1016/j.geoderma.2007.10.003
Page 2
substrate for soil heterotrophs (
Baldock and Skjemstad, 2000
).
Soil aggregation has a major influence on root development, on
water and C cycling, and on soil resistance to erosion (
Kay,
1998
). Moreover, soil OM and aggregation are closely linked
one to another: OM is considered an important aggregate
binding agent, on the one hand, and may be physically protected
against decomposition within stable aggregates, on the other
hand (
Feller and Beare, 1997; Angers and Chenu, 1998
).
However, the stability of soil OM and aggregation as well as
relationships between OM and aggregation are affected by the
presence of sesquioxides. Indeed, the aggregating role of OM
could be less effective in soils that include large amounts of such
inorganic constituents (
Tisdall and Oades, 1982; Six et al.,
2002
). Due to the role of sesquioxides in the stabilization of OM
(
Dalal and Bridge, 1996; Guggenberger and Haider, 2002
), the
importance of aggregation in the stability of OM should also be
re-examined for soils that are rich in sesquioxides. This is the
case for low-activity clay (LAC) soils, which include clay
minerals dominated by 1:1 phyllosilicates, as kaolinite,
associated with crystallized and poorly or non-crystallized iron
(Fe) and aluminium (Al) oxides and hydroxides. They cover
about 60
–
70% of tropical areas (
Segalen, 1994
), where they are
mainly represented by Alfisols, Oxisols and Ultisols according
to the US Soil Taxonomy (
Soil Survey Staff, 1999
) or by
Acrisols, Ferralsols and Nitisols according to the FAO soil
classification (
FAO-ISRIC-ISSS, 1998
). Though they are widely
distributed, LAC tropical soils have received much less attention
than soils from temperate areas, and knowledge on OM and
aggregation in LAC tropical soils still remains fragmentary.
As far as LAC tropical soils are concerned, more information
is thus useful to verify the importance of sesquioxides in soil
OM and aggregate stability, and complementarily, to character-
ize the interrelations between OM and aggregation. The
influence of texture on soil OM is more clearly established.
Several studies, some of them regarding LAC tropical soils,
showed that soil C content increases with clay content (
Feller
and Beare, 1997
). However, the role of texture in stable
aggregation remains questionable (
Le Bissonnais, 1996;
Amézketa, 1999
), and requires additional investigations.
Soil OM may first be considered through the determination
of total soil C content (Ct), as far as soils do not include
carbonates. It can also be characterized by chemical fractiona-
tions, but the importance of the obtained fractions (e.g. fulvic
and humic acids) towards major soil processes such as
aggregation and OM mineralization is not yet well established
(
Feller and Beare, 1997; Baldock, 2002
). Therefore, increasing
attention has progressively been paid to procedures that involve
physical fractionations of soil OM, e.g. particle size fractiona-
tions. Indeed, the characterization of OM associated with soil
primary particles (sands, silts, clay) has proved to be a relevant
approach to separate fractions that have different morphologies,
composition, and dynamics (
Christensen, 1992; Feller and
Beare, 1997
). The characterization of water-stable aggregate
distribution, which may be achieved using simple laboratory
procedures, also provides useful information on soil quality and
behaviour (
Tisdall and Oades, 1982; Amézketa, 1999; Barthès
et al., 2000
).
The objectives of the present work were (i) to characterize
the size distributions of stable aggregates and organic
constituents associated with primary particles, (ii) to study
how these distributions were affected by soil texture and Al- and
Fe-sesquioxides, and (iii) how these distributions were
interrelated, for a range of topsoil layers (0
–
10 cm) from
LAC soils from sub-Saharan Africa and Brazil.
2. Materials and methods
2.1. Sites and sampling
Composite soil samples were collected at 0
–
10 cm depth in 18
plots: five in southern Congo-Brazzaville (two sites in the same
region), four in southern Benin (one site), and nine in Brazil,
including three in Goiás (one site), three in São Paulo (three sites),
and three in Paraná (one site).
Table 1
provides information on
location, climate, soil type, parent material, texture, clay
mineralogy, and land use. The climate is tropical (Congo,
Benin, Goiás) or subtropical (Paraná, São Paulo), ranges in
mean annual rainfall and temperature being 1100
–
1600 mm and
23
–
27 °C, respectively. Soils are mainly Oxisols and Ultisols,
with sandy to clayey texture, and clay mineralogy is dominated by
kaolinite. Further information on sites and soils was given by
Kouakoua et al. (1999)
for Congo, Goiás and Paraná,
Barthès et
al. (2000)
for Benin, and
De Luca (2002)
for São Paulo. Samples
were generally collected using 0.25- to 0.5-L cylinders, air-dried
at room temperature, then gently crushed using a mortar and
pestle and sieved through a 2-mm mesh. Aliquots were finely
ground to pass a 0.2-mm mesh, also using a mortar and pestle.
2.2. Aggregate fractionation
The distribution of water-stable aggregates was determined on
2-mm sieved air-dried soil samples using a test adapted from
Kemper and Rosenau (1986)
. Four grams of soil were rapidly
immersed into deionised water for 30 min then wet-sieved during
6 min, using a motor-driven holder lowering and raising 200-
μ
m
sieves into containers of water. On the one hand, the fraction
N
200
μ
m was dried at 105 °C then weighed. It was successively
sieved into dispersive NaOH solution (0.05 M), dried at 105 °C,
and weighed to determine coarse sand content. Stable macroag-
gregate (
N
200
μ
m) content was calculated as the difference
between fraction
N
200
μ
m and coarse sands. On the other hand,
the fraction
b
200
μ
m was transferred to a glass cylinder, which
was shaken by hand (30 end-over-end tumblings) and left to stand
during the correct settling time for the 0
–
20
μ
m fraction (pipette
method). An aliquot of the fraction
b
20
μ
m was then siphoned
from the upper 10 cm of the suspension, dried at 105 °C, weighed,
and referred to the cylinder volume to determine microaggregate
(
b
20
μ
m) content. Mesoaggregate (20
–
200
μ
m) content was
calculated by difference between total soil and other fractions.
Compared to the macroaggregate fraction (
N
200
μ
m), which
included genuine aggregates only due to coarse sand-correction,
the meso- and microaggregate fractions included aggregates as
well as primary particles of the same size. Measurements were
made on four replicates per sample. It should be noticed that the
15
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 3
aggregate fractions were not extracted thus were not available for
further analyses.
2.3. Particle size fractionation of organic matter
The particle size fractionation of OM requires maximum soil
dispersion with minimum particle alteration. It was carried out on
2-mm sieved air-dried soil samples of 40 g (sandy soils) or 20 g
(other soils), in duplicate, following
Gavinelli et al. (1995)
. Each
sample was pre-soaked overnight at 4 °C in deionised water with
2.5 g L
−
1
sodium metaphosphate. It was then shaken with agate
balls in a rotary shaker for 2 h (sandy soils) or 6 h (others). Indeed,
a long shaking duration is necessary to achieve maximum
dispersion in clayey samples, but in coarse textured samples it
may lead to an alteration of coarse organic particles by sands
(
Feller et al., 1991
). Next the soil suspension was wet-sieved
through 200- and 50-
μ
m sieves, successively. The fractions
remaining in the sieves were washed with water and the washings
added to the 0
–
50
μ
m suspension. The suspension
b
50
μ
m was
ultrasonicated for 10 min with a probe-type ultrasound generating
unit, then it was sieved on a 20-
μ
m screen, and the residual
material obtained from the sieve was washed. The three fractions
N
20
μ
m were dried at 60 °C and weighed. The suspension
b
20
μ
m was transferred to a 1-L glass cylinder, where water was
added to bring the volume to 1 L. Then the cylinder was shaken by
hand (30 end-over-end tumblings) and left to stand during the
correct settling time for the 0
–
2
μ
m fraction (pipette method). The
upper 30 cm of the suspension, which included particles
b
2
μ
m
only, was siphoned off. The process (i.e. water addition, shaking,
settling, and siphoning) was repeated until the supernatant was
clear thus the fraction 0
–
2
μ
m extracted completely (five cycles at
least). The 0
–
2
μ
m suspension was then flocculated with
strontium chloride, centrifuged, filtered, and the pellets were
washed with water, dried at 60 °C and weighed. For some samples
(Benin, Saõ Paulo), the supernatant from centrifugation and the
washing waters from previous sieving operations were grouped,
then an aliquot was collected and filtrated at 0.2
μ
m for the
analysis of soluble organic C. The 2
–
20
μ
m fraction, which
remained at the bottom of the sedimentation cylinder, was also
dried at 60 °C and weighed. All these fractions, separated from
completely dispersed soil samples, were called dispersed
fractions, as opposed to aggregate fractions. In the present
Table 1
Presentation of the studied sites, soils, and plots
Country (state),
city
Latitude,
longitude
Mean
annual
rainfall and
temperature
Soil type
a
(and parent material)
Texture
Clay mineralogy
Land use
No.
K
H
Go
Gi
Congo-Brazzaville,
Loudima
04°00
′
S,
13°30
′
E
1100 mm
Typic Haplorthox
Clayey
+++
0
++
0
Savanna
1
25 °C
Orthic Ferralsol
(schisto-calcareous sediment)
Cassava
2
Natural fallow
3
Congo-Brazzaville,
Mantsoumba
04°10
′
S,
13°30
′
E
1400 mm
Typic Haplorthox
Clayey
+++
0
++
0
Savanna
4
25 °C
Orthic Ferralsol
(schisto-calcareous sediment)
Cassava
5
Benin, Cotonou
06°24
′
N,
02°20
′
E
1200 mm
Typic Tropudult
Sandy clay loam
+++
++
++
0
Maize, no input
6
27 °C
Dystric Nitisol
(sandy clay sediment)
Fertilized maize
7
Maize
–
legume intercrop
8
Rotation of maize and
maize
–
legume intercrop
9
Brazil (Goiás),
Goiânia
16°S,
49°30
′
W
1500 mm
Typic Haplustox
Clayey
+++
++
++
++
Bush savanna
10
23 °C
Orthic Ferralsol
(detritic
–
lateritic sediment)
Maize/bean rotation
11
Bracharia
pasture
12
Brazil (São Paulo),
Pradópolis
21°22
′
S,
48°03
′
W
1600 mm
Typic Hapludox
Clayey
+++
++
0
++
Sugarcane (residue burning)
13
23 °C
Orthic Ferralsol
(basalt)
Brazil (São Paulo),
Matão
21°36
′
S,
48°22
′
W
1500 mm
Typic Hapludult
Sandy clay loam
+++
++
++
0
Sugarcane (residue burning)
14
23 °C
Orthic Acrisol
(sandstone)
Brazil (São Paulo),
Serrana
21°12
′
S,
47°35
′
W
1500 mm
Typic Quartzipsamment
Sandy
+++
+
+
+
Sugarcane (residue burning)
15
23 °C
Ferralic Arenosol
(sandstone)
Brazil (Paraná),
Londrina
23°23
′
S,
51°11
′
W
1600 mm
Typic Haplorthox
Clayey
+++
++
++
++
Forest
16
21 °C
Rhodic Ferralsol
(basalt)
Oat
17
Citrus
orchard with
legume intercropping
18
K: kaolinite; H: hematite; Go: goethite; Gi: gibbsite.
+++: dominant; ++: present; +: traces; 0: absent.
a
Soil Survey Staff (1999)
and
FAO-ISRIC-ISSS (1998)
.
16
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 4
paper, data regarding the dispersed fractions 50
–
200 and 20
–
50
μ
m have been combined (20
–
200
μ
m), as well as those
regarding the fractions 2
–
20 and 0
–
2
μ
m (
b
20
μ
m).
2.4. Analyses of carbon, pH, and iron and
aluminium sesquioxides
Carbon contents of finely ground (
b
0.2 mm) soil aliquots and
dispersed fractions were determined by dry combustion using a
Leco CHN 600 elemental analyzer (St Joseph, MI). In the absence
of carbonates, all C was assumed to be organic. Soluble organic C
was analyzed using a Shimadzu TOC 5000 analyzer (Kyoto,
Japan). The pH in water was determined with a soil:solution ratio
of 1:2.5. On whole soil aliquots,
“
free
”
Fe (Fe
CBD
) and Al (Al
CBD
)
were extracted with citrate-bicarbonate-dithionite (CBD),
“
amor-
phous
”
Fe (Fe
OX
) and Al (Al
OX
) with ammonium oxalate, and all
were analyzed using an Unicam P 900 (Cambridge, UK) atomic
absorption spectrometer (
Rouiller et al., 1994
).
2.5. Statistical analyses
Simple and multiple linear correlation coefficients were
calculated between variables, and their significance was
denoted
NS
for
p
≥
0.05,
⁎
for
p
b
0.05,
⁎⁎
for
p
b
0.01, and
⁎⁎⁎
for
p
b
0.001. Principal component analyses (PCAs) were also
carried out on the dataset. The objective of PCA is to reduce the
number of original variables in a smaller number of represen-
tative and uncorrelated factors, and to detect structure in the
relationships between original variables (
Lebart et al., 1997
).
3. Results
The studied variables are presented in
Table 2
, and the main
correlations between them in
Table 3
.
3.1. Soil texture and contents in Fe- and Al-sesquioxides
The cumulative yield of dispersed fractions ranged from 95
to 101%, and averaged 99%. Clay plus fine silts (
b
20
μ
m)
ranged from 8 to 87%; they correlated negatively with coarse
sands (
N
200
μ
m), and to a lesser extent, with coarse silts plus
fine sands (20
–
200
μ
m). Soil content in Fe
CBD
ranged from 8 to
99 g kg
−
1
, and Al
CBD
, Fe
OX
and Al
OX
from 0.4 to 7.6 g kg
−
1
.
All correlated positively one with another in general, particu-
larly Fe
CBD
and Fe
OX
, and Al
CBD
and Al
OX
. Moreover, all
correlated positively with clay plus fine silts, Al
CBD
especially.
Clay plus fine silts tended to increase and coarse sands to
decrease with land use intensification (i.e. cultivation, tillage,
reduced surface cover).
1
In contrast, the effect of land use on
Fe- and Al-sesquioxides was not clear.
3.2. Soil aggregate distribution; relations with texture and Fe-
and Al-sesquioxides
The cumulative yield of aggregate fractions plus coarse
sands was 100% by definition, the mesoaggregate fraction
being calculated by difference. Aggregate distributions were
generally dominated by macroaggregates (
N
200
μ
m): they
included 42 to 82% of the mass of aggregate fractions (coarse
sands being excluded), and represented a higher soil proportion
than mesoaggregates (20
–
200
μ
m) in 16 out of the 18 samples.
The proportion of microaggregates (
b
20
μ
m) was very low.
Macro- and mesoaggregate fractions did not correlate signifi-
cantly (except on a coarse sand-free basis, due to the low
proportion of microaggregates). The proportion of microag-
gregates did not relate clearly with other aggregate fractions.
Considering all 18 plots, the macroaggregate fraction
correlated negatively with coarse sands and positively with
clay plus fine silts, Al
CBD
and Al
OX
(
Fig. 1
a), and to a lesser
extent, with Fe
CBD
and Fe
OX
. It seemed that macroaggregates
correlated with clay plus fine silts
“
indirectly
”
(relation A), as a
consequence of their close negative correlations with coarse
sands (relation B), which related negatively with clay plus fine
silts (relation C). Indeed, the product of coefficient correlations
assigned to relations B and C was similar to the coefficient
correlation assigned to relation A. Meso- and microaggregate
fractions related less closely with texture and sesquioxides.
3.3. Soil carbon; relations with texture and Fe-
and Al-sesquioxides
For the studied samples, Ct ranged from 5 to 43 g C kg
−
1
.
The cumulative C yield in dispersed fractions ranged from 78 to
101%, and averaged 90%. Rather low recoveries could be
attributed, at least partly, to soluble C. Indeed, when it was
determined, soluble C accounted for 6 to 9% of Ct (data not
shown), which agreed with the data reported by
Christensen
(1992)
. On average, the dispersed fractions
N
200, 20
–
200 and
b
20
μ
m included respectively 37, 15 and 27 g C kg
−
1
fraction
(C concentrations). This corresponded to 1.5, 0.8 and 1.9 times
Ct (C enrichments), represented 1.6, 3.1 and 13.2 g C kg
−
1
soil
(C amounts), and accounted for 8, 17 and 65% of Ct,
respectively. Size distributions of organic constituents were
thus dominated by fine OM (46 to 80% of Ct). Considering all
18 soil samples, Ct correlated positively with C amount in every
dispersed fraction, especially with that of fraction
b
20
μ
m.
Carbon amounts in the dispersed fractions 20
–
200 and
N
200
μ
m correlated closely one with another, but correlated
less closely with that of fraction
b
20
μ
m.
Considering all 18 soil samples, texture related clearly with
the characteristics of several C pools (
Fig. 2
). Clay plus fine
silts correlated positively with Ct and C amount in the dis-
persed fraction
b
20
μ
m (
Fig. 1
b and c), with C concentration
and enrichment in the dispersed fraction
N
200
μ
m, and
negatively with C enrichment in the dispersed fraction
b
20
μ
m. Soil content in Al
CBD
, and in Al
OX
to a lesser
extent, correlated closely with Ct and C amount in the
dispersed fraction
b
20
μ
m (
Fig. 1
b and c), and less closely,
1
The effects of land use on many soil properties are fairly well established
(cf. end of section 4.7) and the present paper did not aim at discussing them
extensively. However, increasing topsoil clay content upon land use
intensification has not been often reported. It might be due to erosion, which
generally increases with land use intensification and leads to the outcropping of
sub-superficial layers that are more clayey than superficial ones in many LAC
tropical soils.
17
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 5
Table 2
Size distributions of aggregate and dispersed fractions, C contents of whole soil and dispersed fractions, and whole soil pH (in water) and contents in citrate-bicarbonate-dithionite- and oxalate-extractable iron and
aluminium (for no., see
Table 1
)
No.
Mass of dispersed fractions
(g kg
−
1
soil)
Mass of aggregate fractions
a
(g kg
−
1
soil)
Total
soil C
content
(g C kg
−
1
soil)
C concentration of dispersed
fractions (g C kg
−
1
fraction)
C amount in dispersed fractions
(g C kg
−
1
soil)
pH
(H
2
O)
Fe
CBD
Al
CBD
Fe
OX
Al
OX
(g kg
−
1
soil)
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
1
38
104
857
669
238
55
35.0
76.2
32.1
27.6
2.9
3.3
23.7
5.0
30.4
6.4
1.3
2.1
2
40
188
775
571
354
48
21.8
34.7
11.8
21.5
1.4
2.2
16.7
6.0
35.6
6.8
1.4
2.2
3
31
201
760
621
309
41
36.4
69.0
27.8
36.6
2.1
5.6
27.8
5.5
32.0
7.6
1.5
3.3
4
93
260
632
746
154
7
42.5
72.0
39.5
35.5
6.7
10.3
22.4
5.0
33.7
6.9
2.9
5.8
5
32
179
771
585
366
17
18.8
51.0
13.5
18.6
1.6
2.4
14.4
4.4
nd
nd
1.2
1.9
6
524
202
226
164
207
18
5.1
1.4
4.5
11.1
0.7
0.9
2.9
5.1
9.0
0.7
0.5
0.4
7
625
216
145
210
132
13
6.5
1.3
6.9
22.8
0.8
1.5
3.3
5.2
7.7
0.7
0.4
0.4
8
611
202
175
238
97
11
11.4
2.1
11.4
30.3
1.3
2.3
5.3
5.0
8.6
0.7
0.4
0.5
9
590
208
192
246
148
17
8.5
1.5
7.7
21.4
0.9
1.6
4.1
5.2
8.4
1.0
0.4
0.4
10
81
449
469
615
313
2
22.6
21.3
8.0
34.0
1.7
3.6
15.9
5.5
35.0
4.8
2.2
nd
11
47
383
571
541
382
10
21.4
25.3
3.9
29.9
1.2
1.5
17.1
5.2
38.9
4.7
1.9
4.9
12
67
456
477
623
276
8
22.0
22.8
3.7
34.9
1.5
1.7
16.6
5.9
29.9
5.4
1.9
3.9
13
78
325
597
661
246
20
20.7
11.1
16.9
21.1
0.9
5.5
12.6
nd
98.8
6.0
1.7
3.1
14
503
300
192
329
143
12
7.4
1.4
4.8
27.2
0.7
1.5
5.2
nd
15.1
2.5
0.5
0.7
15
411
505
83
298
320
15
7.0
1.6
4.4
49.2
0.7
2.2
4.1
nd
11.2
1.5
1.0
0.6
16
47
146
766
752
189
13
30.8
118.1
35.3
22.6
2.6
5.2
17.3
6.8
87.1
5.1
5.3
5.4
17
39
101
868
551
388
23
17.8
71.9
21.7
14.9
0.5
2.2
12.9
5.7
70.6
4.5
5.3
2.7
18
29
122
834
639
309
30
23.0
87.4
18.9
18.9
1.1
2.3
15.7
5.9
72.0
5.1
5.2
2.8
nd: not determined.
a
Aggregate fraction
N
200
μ
m did not include coarse sands; aggregate fractions 20
–
200 and
b
20
μ
m included primary particles of the same size.
18
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/
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(2008)
14
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Page 6
Table 3
Main correlations between the studied variables (fraction C concentration in g kg
−
1
fraction, fraction C amount and other variables in g kg
−
1
soil
b
2 mm)
Dispersed fraction mass
Macroaggregates
Ct
Fraction C concentration
Fraction C amount
Fe
CBD
Al
CBD
Fe
OX
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
N
200
μ
m
20
–
200
μ
m
b
20
μ
m
N
200
μ
m 20
–
200
μ
m
b
20
μ
m
Fraction mass
20
–
200
μ
m
0.070
b
20
μ
m
−
0.906
⁎⁎⁎
−
0.483
⁎
Macroaggregates
−
0.935
⁎⁎⁎
−
0.085
0.852
⁎⁎⁎
Ct
−
0.774
⁎⁎⁎
−
0.200
0.759
⁎⁎⁎
0.883
⁎⁎⁎
Fraction C
concentration
N
200
μ
m
−
0.712
⁎⁎⁎
−
0.541
⁎
0.842
⁎⁎⁎
0.769
⁎⁎⁎
0.771
⁎⁎⁎
20
–
200
μ
m
−
0.505
⁎
−
0.537
⁎
0.663
⁎⁎
0.676
⁎⁎
0.831
⁎⁎⁎
0.840
⁎⁎⁎
b
20
μ
m
−
0.023
0.686
⁎⁎
−
0.262
0.097
0.221
−
0.130
−
0.009
Fraction C amount
N
200
μ
m
−
0.345
−
0.095
0.332
0.550
⁎
0.782
⁎⁎⁎
0.493
⁎
0.737
⁎⁎⁎
0.297
20
–
200
μ
m
−
0.411
−
0.048
0.372
0.627
⁎⁎
0.777
⁎⁎⁎
0.490
⁎
0.793
⁎⁎⁎
0.293
0.856
⁎⁎⁎
b
20
μ
m
−
0.859
⁎⁎⁎
−
0.160
0.821
⁎⁎⁎
0.878
⁎⁎⁎
0.953
⁎⁎⁎
0.729
⁎⁎⁎
0.686
⁎⁎
0.210
0.601
⁎⁎
0.597
⁎⁎
Fe
CBD
−
0.701
⁎⁎
−
0.225
0.711
⁎⁎
0.723
⁎⁎
0.446
0.615
⁎⁎
0.481
−
0.315
0.057
0.364
0.424
Al
CBD
−
0.926
⁎⁎⁎
−
0.098
0.863
⁎⁎⁎
0.928
⁎⁎⁎
0.889
⁎⁎⁎
0.662
⁎⁎
0.622
⁎⁎
0.137
0.516
⁎
0.601
⁎
0.938
⁎⁎⁎
0.575
⁎
Fe
OX
−
0.621
⁎⁎
−
0.310
0.668
⁎⁎
0.641
⁎⁎
0.468
0.801
⁎⁎⁎
0.546
⁎
−
0.207
0.199
0.306
0.423
0.776
⁎⁎⁎
0.460
Al
OX
−
0.769
⁎⁎⁎
0.049
0.638
⁎⁎
0.854
⁎⁎⁎
0.810
⁎⁎⁎
0.672
⁎⁎
0.601
⁎
0.158
0.623
⁎⁎
0.653
⁎⁎
0.759
⁎⁎⁎
0.611
⁎⁎
0.760
⁎⁎⁎
0.632
⁎⁎
⁎
p
b
0.05;
⁎⁎
p
b
0.01;
⁎⁎⁎
p
b
0.001.
19
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14
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with C amounts in the coarser fractions. By contrast, Fe
CBD
and Fe
OX
did not relate clearly with Ct and fraction C
amounts. Multiple linear correlations between C amount in a
given fraction, on the one hand, and two or more variables
regarding texture and/or sesquioxides, on the other hand, were
not closer than the corresponding simple linear correlations or
were not significant.
3.4. Possible determinants of soil aggregation and organic
matter
Considering all 18 soil samples, soil content in stable
macroaggregates correlated positively with Ct and with C
amount in the dispersed fraction
b
20
μ
m (
Fig. 3
), but each of
them correlated more closely with Al
CBD
(
Fig. 1
). To a lesser
extent, macroaggregate content correlated with C amount in the
dispersed fractions
N
200 and 20
–
200
μ
m, but each of them
correlated more closely with Al
OX
. Multiple linear correlations
between macroaggregate content, on the one hand, and Al-
extracts and fraction C amounts, on the other hand, were not
closer than the corresponding simple correlations. Relationships
between meso- or microaggregate fractions and Ct or fraction C
amounts were not significant.
Data thus suggested that Al
CBD
, and Al
OX
to a lesser extent,
were major determinants of soil aggregation and OM content in
the studied soils. However, when considering each location
separately, Al
CBD
and Al
OX
varied generally within narrow
ranges, and the above-mentioned correlations were not
significant in general. By contrast, the effect of land use
became important: land use intensification (i.e. cultivation,
tillage, reduced surface cover) generally caused a decrease in
the proportion of macroaggregates, in C concentration and
amount in every dispersed fraction, thus in Ct. These effects
have already been described for several of the studied sites
(
Kouakoua et al., 1999; Barthès et al., 2000
), and will not be
detailed here.
Fig. 1. Relationships between clay plus fine silts (
b
20
μ
m), citrate-bicarbonate-dithionite-extractable Al (Al
CBD
), oxalate-extractable Al (Al
OX
), on the one hand, and:
a, soil content in water-stable macroaggregates (
N
200
μ
m); b, total soil C content (Ct); and c, C amount in the dispersed fraction
b
20
μ
m (Cgo, Ben, Goi, SPau and Par
represent samples from Congo, Benin, Goiás, São Paulo and Paraná, respectively).
20
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Page 8
3.5. Principal component analysis (PCA)
Three main PCAs were carried out, with active variables that
contributed to factor definition, and supplementary variables
that did not. Considering supplementary variables helped
limiting the over-influence of linked variables (e.g. C amounts
of fractions and their sum Ct). Clay plus fine silts, coarse silts
plus fine sands, Al
CBD
and Fe
CBD
were always considered
active variables. For the first PCA, C amounts in dispersed
fractions were also considered active, while Ct, aggregate
fractions, Al
OX
and Fe
OX
were considered supplementary. For
the second PCA, fraction C concentrations were considered
active, and Ct, fraction C amounts and aggregate fractions
supplementary. Finally, for the third PCA, macro- and
mesoaggregate fractions were considered active, and Ct and
fraction C amounts supplementary.
On the whole, the three PCAs showed similar structure in the
relationships between variables (the first PCA is presented in
Fig. 4
). The first factor included 58
–
62% of the total variability,
the second factor 20
–
22%, and the three first factors together
90
–
92%. Macroaggregate content and Al
CBD
were always very
closely associated, and were associated with C amount
b
20
μ
m
and Ct, confirming their strong interrelations. The coordinates
of these four variables on the first factor ranged from
−
0.77 to
−
0.96. Clay plus fine silts were associated with this group of
variables, but to a lesser extent. Amounts of C in the dispersed
fractions
N
200 and 20
–
200
μ
m were associated one with
another, as well as the corresponding C concentrations. When
Fig. 2. Relationships between clay plus fine silts (
b
20
μ
m) and C concentration,
enrichment and amount in dispersed fractions.
Fig. 3. Relationships between total soil carbon content (Ct), C amount in the
dispersed fraction
b
20
μ
m, and stable macroaggregate (
N
200
μ
m) content (Cgo,
Ben, Goi, SPau and Par represent samples from Congo, Benin, Goiás, São Paulo
and Paraná, respectively).
Fig. 4. Principal component analysis (PCA) with seven active variables, in bold,
and six supplementary variables, in italics (C
N
200, C20
–
200 and C
b
20
represent C amount in the dispersed fractions
N
200, 20
–
200 and
b
20
μ
m,
respectively).
21
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–
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Page 9
Fe
OX
and Al
OX
were included in the analysis, they were
associated with Fe
CBD
and Al
CBD
, respectively.
4. Discussion
4.1. Size distribution of stable aggregates
Stable macroaggregates (
N
200
μ
m) dominated aggregate
distributions in general. A high content in stable macroaggre-
gates has often been reported for clayey LAC tropical soils
(
Oades and Waters, 1991; Albrecht et al., 1998; Six et al., 2002
)
but less frequently for coarse-textured ones (
Dalal and Bridge,
1996; Spaccini et al., 2001
), where it has been considered that
macroaggregation could be less stable (
Albrecht et al., 1998
).
Thus stable aggregation was well developed in the studied soils,
and this probably stands for most LAC tropical soils. Within
aggregate fractions (i.e. on a coarse sand-free basis), the very
close correlation between macro- and mesoaggregates sug-
gested an aggregate hierarchy: macroaggregate disruption
released mesoaggregates mainly, and conversely, macroaggre-
gates resulted mainly from bounding of mesoaggregates.
Aggregate hierarchy has not often been reported in LAC
soils.
Oades and Waters (1991)
reported such a hierarchy for an
Alfisol and a Mollisol (dominated by 2:1 clay mineralogy), but
not for a clayey Oxisol.
Six et al. (2000a)
also found a higher
degree of aggregate hierarchy in 2:1 than in 1:1 clay-dominated
soils. However,
Feller et al. (1996)
reported a tendency to an
aggregate hierarchy for an Ultisol, an Inceptisol and an Oxisol.
4.2. Size distribution of organic constituents
In the present study, the dispersed fractions
N
200, 20
–
200
and
b
20
μ
m included 8, 17, and 65% of Ct in average,
respectively. This was consistent with data from tropical soils
reviewed by
Feller and Beare (1997)
, which indicated that
proportions of Ct in the dispersed fractions
N
50, 2
–
50 and
b
2
μ
m generally ranged from 10 to 30%, 20 to 40%, and 35 to
70%, respectively. The present study also showed that Ct
correlated positively with C amount in every dispersed fraction
(in g C kg
−
1
soil), with fine fraction C especially. Thus changes
in Ct resulted from changes in every size class, but depended
more closely on those in the fine fraction. Studying 21 grassland
topsoils from Saskatchewan to Texas,
Amelung et al. (1998)
similarly reported positive correlations between Ct and C
amount in every dispersed fraction, in the fine one especially,
and found that the dispersed fraction
b
20
μ
m included 58 to
99% of Ct. For a clayey Oxisol under bush savanna and pasture
in Brazil,
Roscoe et al. (2001)
also found positive correlations
between Ct and fraction C amounts. Moreover, the present
study showed that C amounts in the dispersed fractions 20
–
200
and
N
200
μ
m correlated closely one with another but correlated
poorly with C amount in the fine fraction. Similar relationships
were found between fraction C concentrations (in g C kg
−
1
fraction). This suggested that organic compartments
N
200 and
20
–
200
μ
m had similar dynamics, which differed from those of
organic fraction
b
20
μ
m. Indeed, several works supported the
evidence that organic constituents
N
20
μ
m are plant debris
mainly, the coarser ones being less decomposed and having a
more recognizable morphology than the finer ones, whereas
organic constituents
b
20
μ
m are more humified and have a
slower turnover (
Feller and Beare, 1997; Baldock and
Skjemstad, 2000; Guggenberger and Haider, 2002
).
4.3. Influence of texture on soil aggregation
Macro- and mesoaggregate fractions correlated positively
with clay plus fine silts. However, this seemed to result largely
from negative correlations between all three variables and
coarse sands: an increase in coarse sands caused simultaneous
decreases in clay plus fine silts and aggregate fractions, and
there was no evidence of direct influence of clay plus fine silts
on aggregate fractions, macroaggregates especially. This
contrasted with the rather general agreement that clay content
positively affects aggregate stability, as reported in reviews by
Le Bissonnais (1996) and Amézketa (1999)
. However, these
authors mentioned contradictory results, especially when
considering wide ranges of soils, because significant correla-
tions between clay and aggregation have often been established
within narrow ranges of soils.
Dalal and Bridge (1996)
did not
consider that clays were important agents of aggregate
stabilization in Oxisols and Ultisols, and
Six et al. (2002)
did
not find any correlation between clay content and aggregate
stability over a wide range of soils from tropical and temperate
areas.
4.4. Influence of texture on soil organic matter
The present study confirmed the positive relation between Ct
and clay plus fine silts, which was reported for LAC tropical
soils by
Feller and Beare (1997)
. When plotting Ct against clay
plus fine silts (in %), slope coefficient was very close to that
calculated by these authors (0.036 vs. 0.037). Indeed, soil
texture is considered a key variable affecting Ct, as a result of
electrostatic binding between negatively charged clay surfaces
and organic colloids via cation bridges (
Ingram and Fernandes,
2001; Paustian et al., 2003
). Due to sorption of fine organic
constituents to fine mineral particles, C amount in the fine
fraction increases with the proportion of fine primary particles,
as confirmed by
Feller et al. (1996)
over a range of tropical
soils. As fine fraction C includes the highest proportion of Ct, Ct
also increases with clay plus fine silts. Thus clayey soils contain
more C, but the present study showed that C enrichment
affected their coarse fraction mainly, whereas in coarse-textured
soils C enrichment rather affected the fine fraction.
Christensen
(1992)
also reported in his review that C enrichment in the fine
fraction was inversely related to its proportion in the whole soil.
Dalal and Bridge (1996)
similarly noticed preferential C
enrichment of the clay fraction in coarse-textured soils.
Studying grassland topsoils from Canada to Texas,
Amelung
et al. (1998)
observed that C concentrations in the dispersed
fractions 250
–
2000 and 20
–
250
μ
m were inversely related to
their respective proportions in the whole soil, and suggested that
it was largely explained by a dilution effect. The influence of
texture on C concentration in coarse fractions could also be
22
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 10
attributed to encrustation of plant debris by fine mineral
particles, which are attached to mucilages produced by
microorganisms that feed on debris (
Tisdall and Oades, 1982;
Angers and Chenu, 1998
). This helped explain the positive
correlation between clay plus fine silts and C concentration and
enrichment in the dispersed fraction
N
200
μ
m. Additionally, the
present study showed that the relationship between the mass of
the fraction and the C amount it contains was less clear for the
dispersed fractions 20
–
200 and
N
200
μ
m than for that
b
20
μ
m.
This suggested that soil OM dynamics were more affected by
fine than by coarse primary particles. This was confirmed by
Amelung et al. (1998)
for a range of North American
grasslands, where C amount in the dispersed fraction
b
20
μ
m
correlated positively with clay plus fine silts, whereas C
amounts in coarser fractions did not relate to texture.
4.5. Influence of sesquioxides on soil aggregation
Soil content in stable macroaggregates correlated closely
with Al
CBD,
Al
OX
, and to a lesser extent, with Fe
CBD
and Fe
OX
.
Many authors mentioned the importance of sesquioxides in
aggregate stability, in LAC soils especially (
Kemper and Koch,
1966; Oades and Waters, 1991; Le Bissonnais, 1996; Six et al.,
2000b
). The role of these compounds has been attributed to their
flocculation capacity, to their binding effect of clay particles to
organic molecules, and to their possible precipitation as gels on
clay surfaces, but this was reported to affect mesoaggregation
mainly (
Amézketa, 1999
). Electrostatic bindings between clays
and oxides have also been mentioned, but due to their limited
range of action, it has been hypothesized that these interactions
result in macroaggregates that are not stable enough to resist fast
wetting (
Six et al., 2002
).
While the predominant role of free Fe oxides in macroag-
gregate stability has been reported by several authors, for
Oxisols especially (
Kemper and Koch, 1966; Dalal and Bridge,
1996
), the importance of Al-containing crystalline sesqui-
oxides, as observed in the present study, has not been strongly
supported by the literature. The effectiveness of gibbsite in
maintaining soil aggregation has been reported to be higher
than that of Fe-oxides (
El-Swaify and Emerson, 1975
).
However, correlation between Al
CBD
and macroaggregation
could not be attributed to gibbsite because: (i) gibbsite is little
attacked by CBD thus releases little Al
CBD
(
Jackon et al.,
1986
), and (ii) Al
CBD
was maximum in Congolese samples,
which did not include gibbsite. Aluminium-substituted crys-
talline hematite and goethite are dissolved by CBD (
Jackson
et al., 1986
), and were probably the main sources of non-
oxalate extractable Al
CBD
in the studied samples. As macro-
aggregate content correlated moderately with Fe
CBD
, the
aggregating role of crystalline Fe-sesquioxides seemed to
result principally from the degree of Al-for-Fe substitution in
their structure. Indeed, several authors reported that the specific
surface area of crystalline Fe-sesquioxides increases with Al-
substitution, mainly due to decreasing crystallite size (
Schwert-
mann and Taylor, 1989; Ruan and Gilkes, 1995
). As
aggregation relates to specific surface area (
Goldberg et al.,
1988; Le Bissonnais, 1996; Amézketa, 1999
), the increase in
stable macroaggregates with Al
CBD
might thus result from the
increase in specific surface area with Al-substitution on
crystalline Fe-sesquioxides.
4.6. Influence of sesquioxides on soil organic matter
The present study also showed close relationships between
organic constituents and Al-containing sesquioxides, coarse
fractions (
N
20
μ
m) relating with non-crystalline compounds
preferentially (Al
OX
), and fine fractions (
b
20
μ
m), much more
closely, with both crystalline and non-crystalline ones (Al
CBD
).
The affinity of OM to Al- and Fe-sesquioxides, either crystalline
or non-crystalline, has been reported in the literature, regarding
fine organic fractions as well as incompletely humified residues,
which can be stabilized by sorption, by entrapment, or by
complexation (
Guggenberger and Haider, 2002
).
The role of amorphous Al and Fe in soil OM protection
against biodegradation, through the formation of organo-
mineral complexes, has been highlighted by numerous authors
(
Boudot et al., 1986; Baldock and Skjemstad; 2000; Guggen-
berger and Haider, 2002
).
Baldock and Skjemstad (2000)
reported that Al-complexes resulted in a greater stabilization of
OM than Fe-complexes, as observed in the present study: C
amount in every dispersed fraction correlated more closely with
Al
OX
than with Fe
OX
.
Moreover, it is likely that the main sources of non-oxalate
extractable Al
CBD
in the studied samples were Al-substituted
crystalline Fe-sesquioxides, as mentioned in the previous
section. As also mentioned, the increase in Al-for-Fe substitu-
tion causes an increase in specific surface area, and this in turn
causes an increase in OM stabilization (
Baldock and Skjemstad;
2000; Kaiser and Guggenberger, 2003
). This helped to explain
the correlations between Al
CBD
and Ct or fraction C amounts.
Considering that micropores are preferential sites for OM
sorption (
Kaiser and Guggenberger, 2003
), the particular
affinity of organic particles
b
20
μ
m to non-oxalate extractable
Al
CBD
might be due to the small crystallite size of Al-
substituted Fe-sesquioxides, the micropores of which are
accessible to small particles only. Studying the mineral
association of OM in the surface horizons of an Alfisol and
an Oxisol,
Shang and Tiessen (1998)
also observed the
enrichment of the fine fractions in Al
CBD
, while Al
OX
was
particularly concentrated in the light fractions (i.e. including
coarse organic particles mainly).
4.7. Determinants of soil aggregation and organic matter
content
Many authors observed positive correlations between stable
macroaggregation and OM, which were attributed to the
aggregating role of OM and to physical protection of OM
within aggregates (
Feller and Beare, 1997; Angers and Chenu,
1998; Six et al., 2002
). Several works reported that some
organic fractions, such as polysaccharides or particulate OM,
were particularly involved in these relationships (
Tisdall and
Oades, 1982; Kouakoua et al., 1999; Baldock, 2002
). This was
not supported by the present results, macroaggregation being
23
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 11
more closely correlated with Ct than with any organic size
fraction.
Moreover, considering all 18 soil samples, stable macro-
aggregation was more closely linked with Al
CBD
than with other
parameters, organic ones especially.
Tisdall and Oades (1982)
already underlined that organic materials were not necessarily
the main aggregating agents, especially in soils that included
more than 10% sesquioxides.
Oades and Waters (1991)
also
suggested that oxides were dominant stabilizing agents in the
Oxisol they studied.
Six et al. (2002)
similarly hypothesized that
stable macroaggregation was less dependent on OM in highly
weathered tropical soils than in temperate ones, because the
formers had a greater potential to form physicochemical
macroaggregates, in the presence of oxides especially. More-
over, C amount in any dispersed fraction was more closely
linked with Al
CBD
or Al
OX
than with macroaggregation or
texture. This suggested that the stability of OM was more
dependent on its association with Al-containing sesquioxides
than on physical protection within stable aggregates or sorption
to clay particles. It could thus be assumed that Al-containing
sesquioxides were the main determinants of stable macroag-
gregation and OM content in the LAC tropical soils under study.
However, when each location was considered separately, the
influence of land use increased markedly. The negative
consequences of land use intensification (i.e. cultivation, tillage,
reduced surface cover) on many soil properties are fairly well
established. As far as tropical soils are concerned, they have
been reported for topsoil aggregation (
Resck et al., 2000;
Spaccini et al., 2001
) and for total and fraction C (
Feller and
Beare, 1997; Solomon et al., 2002
;
Vågen et al., 2005
). This
underlines that at the local scale, due to rather homogeneous
mineralogical properties, land use remains an important
determinant of soil OM content and aggregation.
5. Conclusion
Over the range of LAC tropical soils that were studied,
aggregate distributions were dominated by stable macroaggre-
gates (
N
200
μ
m), and C distributions in primary particles by
fine fraction C (
b
20
μ
m). Macroaggregation was not clearly
affected by texture, whereas total C and fine fraction C
increased with clay plus fine silts. The results also suggested
that Al-containing sesquioxides were major determinants of
macroaggregate and OM stability. Indeed, macroaggregate
content correlated positively with Ct and with C in every
fraction, with fine fraction C especially, but it correlated more
closely with CBD-extractable Al. This suggested that Al-
containing sesquioxides had a more important aggregating role
than OM in LAC tropical soils. Moreover, C amounts in fine
(
b
20
μ
m) and coarse (
N
20
μ
m) fractions related more closely
with CBD- and oxalate-extractable Al, respectively, than with
stable macroaggregate content or texture. Thus the stability of
soil OM was more influenced by its association with Al-
containing sesquioxides than by physical protection within
stable macroaggregates or sorption to clay particles. It could
also be assumed that Al-containing sesquioxides played a
dominant role in the relations between OM and aggregation.
However, considering each location separately, soil content in
sesquioxides varied within narrow ranges, and as it cannot be
managed easily, the importance of land use on OM and
aggregation remained fundamental.
Acknowledgements
The authors thank Vincent Eschenbrenner for his help
regarding the clay mineralogy of the Brazilian samples, and two
anonymous referees for their helpful comments.
References
Albrecht, A., Angers, D.A., Beare, M.H., Blanchart, E., 1998. Déterminants
organiques et biologiques de l'agrégation : implications pour la recapitalisa-
tion de la fertilité physique des sols tropicaux. Cah. Agricultures 7,
357
–
363.
Amelung, W., Zech, W., Zhang, X., Follett, R.F., Tiessen, H., Knox, E., Flach,
K.W., 1998. Carbon, nitrogen, and sulfur pools in particle-size fractions as
influenced by climate. Soil Sci. Soc. Am. J. 62, 172
–
181.
Amézketa, E., 1999. Soil aggregate stability: a review. J. Sustain. Agr. 14,
83
–
151.
Angers, D.A., Chenu, C., 1998. Dynamics of soil aggregation and C
sequestration. In: Lal, R., Kimble, J.M., Follett, R.F., Stewart, B.A. (Eds.),
Soil Processes and the Carbon Cycle. Advances in Soil Science. CRC Press,
Boca Raton, FL, pp. 199
–
206.
Baldock, J.A., 2002. Interactions of organic materials and microorganisms with
minerals in the stabilization of soil structure. In: Huang, P.M., Bollag, J.M.,
Senesi, N. (Eds.), Interactions Between Soil Particles and Microorganisms,
Impact on the Terrestrial Ecosystem. Wiley and Sons, Chichester, UK,
pp. 85
–
131.
Baldock, J.A., Skjemstad, J.O., 2000. Role of the soil matrix and minerals in
protecting natural organic materials against biological attack. Org.
Geochem. 31, 697
–
710.
Barthès, B., Azontonde, A., Boli, B.Z., Prat, C., Roose, E., 2000. Field-scale
run-off and erosion in relation to topsoil aggregate stability in three tropical
regions (Benin, Cameroon, Mexico). Eur. J. Soil Sci. 51, 485
–
496.
Boudot, J.P., Bel Hadj, B.A., Chone, T., 1986. Carbon mineralization in
Andosols and aluminium-rich highland soils. Soil Biol. Biochem. 18,
457
–
461.
Christensen, B.T., 1992. Physical Fractionation of Soil and Organic Matter in
Primary Particle Size and Density Separates. In: Stewart, B.A. (Ed.), Adv.
Soil Sci., vol. 20. Springer, New-York, pp. 1
–
90.
Dalal, R.C., Bridge, B.J., 1996. Aggregation and organic matter storage in sub-
humid and semi-arid soils. In: Carter, M.R., Stewart, B.A. (Eds.), Structure
and Organic Matter Storage in Agricultural Soils. Advances in Soil Science.
Lewis Publishers, Boca Raton, FL, pp. 263
–
307.
De Luca, E.F., 2002. Matéria orgânica e atributos do solo em sistemas de
colheita com e sem queima da cana-de-açúcar. Ph.D. Thesis, Univ. São
Paulo, Brazil.
El-Swaify, S.A., Emerson, W.W., 1975. Changes in the physical properties of
soil clays due to precipitated aluminum and iron hydroxides: swelling and
aggregate stability after drying. Soil Sci. Soc. Am. Proc. 39, 1056
–
1063.
FAO-ISRIC-ISSS (Food and Agriculture Organization of the United Nations,
International Soil Reference and Information Centre, International Society for
Soil Science), 1998. World Reference Base for Soil Resources. FAO, Rome.
Feller, C., Beare, M.H., 1997. Physical control of soil organic matter dynamics
in the tropics. Geoderma 79, 69
–
116.
Feller, C., Burtin, G., Gérard, B., Balesdent, J., 1991. Utilisation des résines
sodiques et des ultrasons dans le fractionnement granulométrique de la
matière organique des sols. Intérêt et limites. Sci. Sol 29, 77
–
94.
Feller, C., Albrecht, A., Tessier, D., 1996. Aggregation and organic matter storage
in kaolinitic and smectitic tropical soils. In: Carter, M.R., Stewart, B.A. (Eds.),
Structure and Organic Matter Storage in Agricultural Soils. Advances in Soil
Science. Lewis Publishers, Boca Raton, FL, pp. 309
–
359.
24
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
Page 12
Gavinelli, E., Feller, C., Larré-Larrouy, M.C., Bacye, B., Djegui, N., Nzila, J. de D.,
1995. A routine method to study soil organic matter by particle-size
fractionation: examples for tropical soils. Commun. Soil Sci. Plant Anal. 26,
1749
–
1760.
Goldberg, S., Suarez, D.L., Glaubig, R.A., 1988. Factors affecting clay
dispersion and aggregate stability of arid-zone soils. Soil Sci. 146, 317
–
325.
Guggenberger, G., Haider, K.M., 2002. Effect of mineral colloids on
biogeochemical cycling of C, N, P, and S in soil. In: Huang, P.M., Bollag,
J.M., Senesi, N. (Eds.), Interactions Between Soil Particles and Micro-
organisms, Impact on the Terrestrial Ecosystem. Wiley and Sons,
Chichester, UK, pp. 267
–
322.
Ingram, J.S.I., Fernandes, E.C.M., 2001. Managing carbon sequestration in
soils: concepts and terminology. Agr. Ecosys. Environ. 87, 111
–
117.
Jackson, M.L., Lim, C.H., Zelany, L.W., 1986. Oxides, hydroxides, and
aluminosilicates, In: Klute, A. (Ed.), Methods of Soil Analysis: Part 1,
Physical and Mineralogical Methods, 2nd Edn. Am. Soc. of Agronomy,
Madison, WI, pp. 101
–
150.
Kaiser, K., Guggenberger, G., 2003. Mineral surfaces and soil organic matter.
Eur. J. Soil Sci. 54, 219
–
236.
Kay, B.D., 1998. Soil structure and organic carbon: a review. In: Lal, R.,
Kimble, J.M., Follett, R.F., Stewart, B.A. (Eds.), Soil Processes and the
Carbon Cycle. CRC Press, Boca Raton, FL, pp. 169
–
197.
Kemper, W.D., Koch, E.J., 1966. Aggregate stability of soils from western
United States and Canada. Colorado Agricult. Exp. Stn. Bull. 1355, 1
–
52.
Kemper, W.D., Rosenau, R.C., 1986. Aggregate stability and size distribution, In:
Klute, A. (Ed.), Methods of Soil Analysis: Part 1, Physical and Mineralogical
Methods, 2nd Edn. Am. Soc. of Agronomy, Madison, WI, pp. 425
–
442.
Kouakoua, E., Larré-Larrouy, M.C., Barthès, B., de Freitas, P.L., Neves, C.,
Sala, G.H., Feller, C., 1999. Relations entre stabilité de l'agrégation et
matière organique totale et soluble à l'eau chaude dans des sols ferrallitiques
argileux (Congo, Brésil). Can J. Soil Sci. 79, 561
–
569.
Lebart, L., Morineau, A., Piron, M., 1997. Statistique Exploratoire Multi-
dimensionnelle. Dunod, Paris.
Le Bissonnais, Y., 1996. Soil characteristics and aggregate stability. In: Agassi,
M. (Ed.), Soil Erosion, Conservation, and Rehabilitation. Dekker, New
York, pp. 41
–
60.
Oades, J.M., Waters, A.G., 1991. Aggregate hierarchy in soils. Aust. J. Soil Res.
29, 815
–
828.
Paustian, K., Conant, R., Ogle, S., Paul, E., Six, J., 2003. Environmental and
management drivers of soil organic carbon stock changes. In: Smith, C.A.S.
(Ed.), Soil Organic Carbon and Agriculture: Developing Indicators for
Policy Analyses. Proc. of an OECD Expert Meeting. Agriculture and Agri-
Food Canada, Ottawa, and Organisation for Economic Co-operation and
Development, Paris, pp. 21
–
34.
Resck, D.V.S., Vasconcellos, C.A., Vilela, L., Macedo, M.C.M., 2000. Impact of
conversion of Brazilian Cerrados to cropland and pastureland on soil carbon pool
and dynamics. In: Lal, R., Kimble, J.M., Stewart, B.A. (Eds.), Global Climate
Change and Tropical Ecosystems. CRC Press, Boca Raton, FL, pp. 169
–
196.
Roscoe, R., Buurman, P., Velthorst, E.J., Vasconcellos, C.A., 2001. Soil organic
matter dynamics in density and particle size fractions as revealed by the
13
C/
12
C isotopic ratio in a Cerrado's oxisol. Geoderma 104, 185
–
202.
Rouiller, J., Souchier, B., Bruckert, S., Feller, C., Toutain, F., Védy, J.C., 1994.
Méthodes d'analyses des sols. In: Bonneau, M., Souchier, B. (Eds.),
Pédologie. 2. Constituants et Propriétés du Sol. Masson, Paris, pp. 619
–
652.
Ruan, H.D., Gilkes, R.J., 1995. Dehydroxylation of aluminous goethite: unit cell
dimensions, crystal size and surface area. Clays Clay Min. 43, 196
–
211.
Schwertmann, U., Taylor, R.M., 1989. Iron oxides, In: Dixon, J.B., Weed, S.B.
(Eds.), Minerals in Soils Environments, 2nd Edn. Am. Soc. of Agronomy
and Soil Sci Soc. of America, Madison, WI, pp. 379
–
438.
Segalen, P., 1994. Les Sols Ferrallitiques et leur Répartition Géographique.
Tome 1. Introduction Générale. Les Sols Ferrallitiques: leur Identification et
Environnement Immédiat. ORSTOM Editions, Paris.
Shang, C., Tiessen, H., 1998. Organic matter stabilization in two semiarid tropical
soils: size, density, and magnetic separations. Soil Sci. Soc. Am. J. 62,
1247
–
1257.
Six, J., Paustian, K., Elliott, E.T., Combrink, C., 2000a. Soil structure and
organic matter: I. Distribution of aggregate-size classes and aggregate-
associated carbon. Soil Sci. Soc. Am. J. 64, 681
–
689.
Six, J., Elliott, E.T., Paustian, K., 2000b. Soil structure and soil organic matter:
II. A normalized stability index and the effect of mineralogy. Soil Sci. Soc.
Am. J. 64, 1042
–
1049.
Six, J., Feller, C., Denef, K., Ogle, S.M., de Moraes Sá, J.C., Albrecht, A., 2002.
Soil organic matter, biota and aggregation in temperate and tropical soils
—
effect of no-tillage. Agronomie 22, 755
–
775.
Soil Survey Staff, 1999. Soil Taxonomy. A Basic System of Soil Classification
for Making and Interpreting Soil Surveys, 2nd Edn. United States
Department of Agriculture, Natural Resources Conservation Service,
Washington, DC.
Solomon, D., Fritzsche, F., Lehmann, J., Tekalign, M., Zech, W., 2002. Soil
organic matter dynamics in the subhumid agroecosystems of the Ethiopian
highlands: evidence from natural
13
C abundance and particle-size
fractionation. Soil Sci. Soc. Am. J. 66, 969
–
978.
Spaccini, R., Zena, A., Igwe, C.A., Mbagwu, J.S.C., Piccolo, A., 2001.
Carbohydrates in water-stable aggregates and particle size fractions of
forested and cultivated soils in two contrasting tropical ecosystems.
Biogeochemistry 53, 1
–
22.
Tisdall, J.M., Oades, J.M., 1982. Organic matter and water-stable aggregation in
soils. J. Soil Sci. 33, 141
–
163.
Vågen, T.G., Lal, R., Singh, B.R., 2005. Soil carbon sequestration in sub-
Saharan Africa: a review. Land Degrad. Dev. 16, 53
–
71.
25
B.G. Barthès et al. / Geoderma 143 (2008) 14
–
25
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