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Geoderma 132 (2

Morphological changes in Camponotus punctulatus

(Mayr) anthills of different ages

Norma B. Gorositoa,*, Pierre Curmib,1, Vincent Hallairec,2,

Patricia J. Folgarait a, Patrick M. Lavelled,3

aCentro de Estudios e Investigaciones (CEI), Universidad Nacional de Quilmes, Roque Saenz Pena 180,

B1876BXD Bernal, Buenos Aires, ArgentinabEtablissement National d’Enseignement Superieur Agronomique de Dijon (ENESAD), Equipe Milieu Physique et Environnement,

BP87999, 26 Bd du Dr Petitjean, 21079 Dijon Cedex, FrancecInstitut National de la Recherche Agronomique (INRA), Unite Sol et Agronomie, 65 rue St Brieuc, 35042 Rennes Cedex, France

dLaboratoire d’Ecologie de Sols Tropicaux (LEST)-Institut de Recherche pour le Developement (IRD),

32 rue Henri de Varagnat, F-93143 Bondy Cedex, France

Received 14 May 2004; received in revised form 16 May 2005; accepted 17 May 2005

Available online 23 June 2005

Abstract

In north-eastern Argentina, Camponotus punctulatus builds large numbers of big and coherent anthills after abandon-

ment of rice cultivation. These anthills easily reach 1 m in height and 2 m in diameter, and a density of 1800 nests ha�1.

We studied the internal morphology of C. punctulatus aged anthills of 4, 6 and 15 years, respectively, by describing and

quantifying, meso- and macroporosity of undisturbed soil samples using image analysis. Four different parts were

distinguished on these cone-shaped anthills: the loose granular cortex, the summit, the core of the anthill and the base

of the anthill at ground level. The percentage of macroporosity in anthills significantly differed between the parts of the

anthill, but changed little with age except for the 15 year old anthill that had increased percentages of mesopores and

macropores with rounded and irregular shapes. Walls of the chambers and galleries were highly compacted, highlighting

intense ant activity in the anthills, although it was localised mainly in the upper central part. After 6 years the anthills

became surrounded by a discontinuous peripheral depression, which likely affects water drainage and infiltration. In 15

0016-7061/$ - s

doi:10.1016/j.ge

* Correspondi

E-mail addr

pfolgarait@unq.1 Fax: +33 3 82 Fax: +33 2 23 Fax: +33 1 4

006) 249–260

ee front matter D 2005 Elsevier B.V. All rights reserved.

oderma.2005.05.010

ng author. Fax: +54 11 4365 7182.

esses: ngorosito@unq.edu.ar (N.B. Gorosito), p.curmi@enesad.fr (P. Curmi), hallaire@roazhon.inra.fr (V. Hallaire),

edu.ar (P.J. Folgarait), patrick.lavelle@bondy.ird.fr (P.M. Lavelle).

0 77 25 00.

3 48 54 30.

8 02 59 70.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260250

year old anthills the lateral depressions became a continuous ditch where water accumulates giving place to a constant

wetted zone inside the anthill. Our results support the previous consideration of C. punctulatus as an ecosystem engineer,

although in this case related to the changes produced on the soil surrounding the anthills which may affect the survival and

distribution of other soil organisms.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Anthills; Anthills’ age; Camponotus punctulatus; Image analysis; Porosity

1. Introduction

Ants, throughout their activity, are acknowledged

to contribute to the structure and functioning of soils

(Baxter and Hole, 1967; Cox et al., 1992; Wang et al.,

1995; Folgarait, 1998). Ants influence soil properties

through the construction of their nests by changing

physical characteristics, such as porosity and infiltra-

tion (Wang et al., 1995). In addition, ant nests are

associated with high levels of nutrients and organic

matter (Lobry de Bruyn and Conacher, 1990; Folgar-

ait, 1998; Kristiansen and Amelung, 2001; Cammer-

aat et al., 2002).

From a pedological point of view ants can build

two types of nests (Paton et al., 1995). Type I, less

conspicuous in the landscape, are crater-shaped,

small in diameter and height; soil material is simply

deposited on the surface and the nests are highly

susceptible to erosion. Type II are larger, coherent

epigeic nests, often cemented, sometimes covered by

vegetation. These are very persistent through time

that may significantly affect the spatial heterogeneity

of the soil surface.

Camponotus punctulatus (Mayr) is a native ant

species widely distributed in Argentina from the

northern border (also present in Brazil and Uruguay)

to the latitude 448S (Bonetto et al., 1961; Kusnezov,

1951). This ant species may build small nests below

ground under the grass tussocks or large conspicuous

anthills (Kusnezov, 1951). In Northeast Argentina we

have found that the disturbances produced by agricul-

tural practices lead C. punctulatus to construct nests

of type II (Folgarait, 1998). This is particularly obvi-

ous in the fallow periods (4 to 15 years) following

paddy rice cultivation where densities of type II nests

may reach 1800 anthills ha�1 depending on the age of

abandonment of the rice fields (Folgarait et al., 2002).

In natural grasslands, we have also observed the

presence of anthills of type II, but the density was

much lower at 23 anthills ha�1. These cone-shaped

anthills, covered by vegetation, are extremely hard

when dry, and they can reach ~1 m in height and

~2 m in diameter (Folgarait et al., 2000). C. punctu-

latus does not present a direct hazard to the crop since

this ant is not herbivorous, feeding mostly on honey-

dew and other insects (Gorosito et al., 1997). Indeed,

C. punctulatus activity promotes specific plant com-

munities on the anthills, and anthill soil has a greater

fertility than the surrounding soil (Folgarait et al.,

2002). Activities within the anthill of C. punctulatus

also favour the development of bacteria, measured as

dehydrogenase activity (Gonzalez Polo et al., 2004),

and other mesofauna such as mites, and speed up the

mineralization of phosphorus inside the anthill (Paris

et al., 2001). Farmers, however, find these anthills

inconvenient and expensive to remove when returning

the area to cropping. Horses and livestock get injured

when stepping into the ditches that form around the

anthills and no tillage can be done if these structures

remain intact.

A research programme has been developed to

understand the reasons for anthill proliferation in

abandoned rice fields, their consequences, and to

combat the inconvenience they cause to the farmers

when returning the fields to cropping (Folgarait et

al., 2003). As part of this programme we have

studied the internal organization of the anthills of

C. punctulatus and its dynamics. We tested the

hypothesis that there were differences in the internal

morphology of anthills of different ages. A macro-

morphological and micromorphological characterisa-

tion of anthills allowed us to identify the main

fabrics and their genesis, using a qualitative ap-

proach and a quantitative estimation of porosity by

image analysis. This study was conducted on anthills

of different ages (4, 6 and 15 year old anthills)

representing a chronosequence of different fallow

periods used by farmers.

Table 1

Pore classification according to their size and shape for both scales

of resolution

Area (Am2) Shape classes

Size

classes

R E I

Ieb5 5b Ieb10 IeN10

Mesopores Areab1 d 103 1 R1 E1 I1

1 d 103b

areab1 d 1052 R2 E2 I2

AreaN1 d 105 3 R3 E3 I3

Macropores Areab1 d 106 4 R4 E4 I4

1 d 106b

areab1 d 1075 R5 E5 I5

AreaN1 d 107 6 R6 E6 I6

The shape was defined according to the Elongation Index (Ie). R:

rounded pores, E: elongated pores, I: irregular pores.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260 251

2. Materials and methods

2.1. Study site

Field work was carried out at the Aguaceritos farm,

~60 km north of Mercedes, Corrientes Province, in

Argentina (298S, 588W). The climate is subtropical,

without a definite dry season. Mean annual precipita-

tion is 1270 mm and the mean annual temperature is

20.1 8C (Fernandez et al., 1993). Soils are alfisols

(Soil Survey Staff, 1992), which have developed on a

several meters thick clayey saprolite from calcareous

sandstones. After rice cultivation, large compact clods

occurred in the ploughed layer and a distinct plough

pan was created by mechanical tillage at 15 cm depth

(see Lesturgez, 2000 for soil details). After 2 years of

fallow, the whole ploughed horizon is homogeneously

compacted due to cattle trampling and cannot be

distinguished from the plough pan.

2.2. Sampling

2.2.1. Anthills selection for general description

After the last rice harvest the land enters the fallow

period. During this period C. punctulatus, which were

not present in the rice field, re-colonised the aban-

doned rice field apparently from the natural grassland.

All the anthills of a given plot are therefore mostly

of the same age starting after the last harvest. In

January 2000, we chose one representative C. punc-

tulatus anthill in three rice fields abandoned (fallow

period) for 4, 6, and 15 years, respectively. The

dimensions of the chosen anthill were almost the

average size observed in rice plots (Folgarait et al.,

2000).

We used a mechanical shovel for cutting the anthills

vertically down to ca. 2 m depth in order to observe

their internal organization. Diameters of galleries were

measured at different depths on the anthill sections.

2.2.2. Micromorphological characterisation

Twelve undisturbed blocks of soil were taken in

order to have a representative sampling of the differ-

ent parts found inside anthills of different ages: in

each of three different anthills aged 4, 6 and 16 years,

respectively, 4 blocks (16 cm height�9 cm width�5

cm depth) were taken from the different parts regis-

tered within each anthill.

In order to avoid structural modifications, all the

blocks were impregnated with a polyester resin after

replacing water by acetone (Ringrose-Voase, 1996). A

fluorescent dye was also added to allow us to see the

macropore and mesopore space when illuminated with

UV light. After impregnation, three vertical sections

of 16�9 cm were prepared for each block, and they

were exposed to UV light to quantify the soil porosity

by computerised image analysis (Hallaire and Curmi,

1994).

Image acquisition was carried out at two levels of

resolutions (100 Am/pixel and 1.85 Am/pixel) on each

soil section. The first resolution, allowed us to see the

macropores whereas the second one was used to

quantify mesopores. Three images (768�576 pixels)

per section (i.e. nine per block) were taken for the

coarse resolution. Ten images (1798�1438 pixels)

were taken at random for the second scale. Addition-

ally, we took six images to evaluate the compaction

around the galleries (1798�1438 pixels). All images

were processed with the software Optimas 6.51 (Opti-

mas 6.51 Media Cybernetics).

Pores were classified into morphological classes

taking into account the area of their section and

their shape (Hallaire et al., 1997). Six size classes

were established according to their area. Pore shape

was determined using the Elongation Index, Ie (pe-

rimeter2/area) (Coster and Chermant, 1985), and three

classes were distinguished: rounded (R), elongated (E)

and irregular pores (I). Therefore, 18 morphological

classes resulted from the combination of different

shapes and sizes (Table 1).

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260252

2.3. Statistical analysis

In the absence of normality and homoscedascity of

data, non-parametric analyses were performed (Stat-

view software, SAS Institute, 1992–1998). To com-

pare more than two variables we used the Kruskall–

Wallis test with a posteriori contrasts adjusted with

S

C

Old ploughed ho

Cortex

0.30 m

0 m

0.35 m

S

C

B

Old ploughed hor

Sub-vert

0.30 m

0 m

0.70 mCortex

S

C

B

Cortex

Old ploughed ho0.30 m

0 m

0.60 m

Wet

0 m

Fig. 1. Scheme of the internal organization of the 4, 6 and 15 year old anth

former ploughed horizon. Soil samples where taken where each letter is l

Bonferroni to keep an alpha level=0.05 and for

paired comparisons we chose Mann–Whitney test

(Daniel, 1990). For all the comparisons related with

the different parts (summit, core, base) within each

anthill, the number of images was 9 and 10 for the

first and second scale of observation, respectively.

Compaction around the galleries was first compared

4 yrs

6 yrs

rizon (P)

izon (P)

15 yrs

ical gallery

Continuous Peripheral depression

Discontinuous

Peripheral depression

Sub-vertical galleryrizon (P)

ted zone

1.50 m

ills. S: summit; C: core; B: base of the epigeic part of the anthill; P:

ocated.

a

6.4 mm

b

6.4 mm

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260 253

within the different parts found inside each anthill,

then among the anthills, and finally we compared the

porosity around the galleries with the mesoporosity

found in the whole anthill (the parts altogether). We

considered a level of significance of pb0.05 for all

comparisons.

e

6.4 mm

d6.4 mm

6.4 mm

f

c

6.4 mm

Fig. 2. Different types of pores and structures found inside the

anthills. All images were 768�576 pixels in size. (a) Rounded

pore, (b) elongated pore, (c) interconnected pores, (d) irregular pore

filled with angular microaggregates, (e) traces of ant mandibles, (f

gallery wall compaction.

3. Results

3.1. Morphological description

A vertical cut of the anthills allowed us to identify

four different parts based on the abundance and mor-

phology of the macropores (Fig. 1): the coherent

summit of the anthill (S), the core (C) and the base

at the ground level of the anthill (B), and finally below

ground the remnants of the former ploughed horizon

(P). A loose cortex of fine granular and irregular

aggregates covered the outside upper part of the ant-

hill. We did not make a micromorphological analysis

of this area due to its lack of cohesion.

Observation of each area inside the anthills led us

to distinguish three types of macropores: rounded,

elongated and irregular (Fig. 2). The rounded pores

(Fig. 2a) corresponded to chambers usually used for

brood caring, while the elongated ones represented

connections among chambers and galleries (Fig.

2b,c) (used for carrying soil particles, brood or

food supplies). Finally, the irregular pores resulted

from the filling of chambers and galleries with

angular microaggregates displaced by ants (Fig.

2d). In some sections we have also observed cham-

bers or galleries filled with rounded earthworms

faecal pellets and oval termites ones. Using a field

magnifier, we observed prints of ant mandibles on

the walls of the galleries and chambers (Fig. 2e),

and also soil compaction around chambers and gal-

leries (Fig. 2f).

The 4 year old anthill, 0.35 m in height and 0.90 m

in diameter (Vol.: 0.09 m3) (Fig. 1), had the thickest

cortex (~10 cm) of fine aggregates (Fig. 1). The

summit was the most friable as shown by the loose

assemblage of small aggregates (Fig. 3). The mean

diameter of the galleries was 1.03 cm (n =16). The 4

year old anthill did not yet have a base as observed in

the older anthills and the core was very hard when dry.

Galleries and chambers were the most abundant in the

)

core (Fig. 3). Remnants of the former ploughed hori-

zon were found under the anthill at the same level as

the ploughed horizon outside the anthill. The

ploughed horizon was the most compacted under the

anthill.

The 6 year old anthill was 0.60 m in height and

1.50 m in diameter (Vol.: 0.17 m3) (Fig. 1) and the

surface cortex was thinner than in the younger ant-

hill. At this stage, the mean diameter of galleries was

0.87 cm (n =34). The 15 year old anthill was 0.70 m

in height and 1.50 m in diameter (Vol.: 0.54 m3) and

had the thinnest cortex (2–3 cm). The hardness of the

anthill was higher in comparison to the younger

ones. At this stage, mean diameter of galleries was

0.76 cm (n =21). The size of the core changed with

age, with a clear enlargement in the 15 year old

anthill (Fig. 1). The base part was present in the 6

year old anthill, and it was more compacted in

comparison to the summit and the core. Finally, the

compaction at the base of the anthill was most

important in the 15 year old anthill. Around the

base of the 6 year old anthill, a discontinuous pe-

6 yrs4 yrs

S

15 yrs

P

C

B

1 cm

Fig. 3. Internal organization of macropores according to each area found inside the anthills. S: summit; C: core; B: base of the epigeic part of the

anthill; P: former ploughed horizon. Size of the binary images were 768�576 pixels.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260254

ripheral depression continued by a large subvertical

gallery was observed. This peripheral depression was

continuous and deeper in the 15 year old anthill (Fig.

Table 2

Percentage of meso- and macroporosity (median and interquartile) for ant

4 years

S C B P

Mesoporosity Median (%) 8.6 9.7 3.7

Interquartile (%) 7.3 6.7 4.0

n 10 10 10

Comparisons a a b

Macroporosity Median (%) 26.8 27.2 0.3

Interquartile (%) 7.9 14.3 0.2

n 9 9 9

Comparisons A A B

Statistical comparisons were among parts within each anthill. Different let

base at the ground level of the anthill, and P: old ploughed horizon.a The n values from 15 year old anthills are different due to a bigger C

1). Remnants of the former ploughed horizon pre-

sented signs of ant activities under the anthills of 6

and 15 years old (Fig. 3).

hills of different ages

6 years 15 years

S C B P S C B P

10.5 9.7 9.3 5.5 7.9 10.1 7.1 9.3

8.6 3.9 6.3 5.3 2.7 4.4 5.8 5.6

10 10 10 10 10 20a 10 10

a a a a ab a b ab

23.8 14.2 12.2 14.5 32.3 30.3 7.7 4.3

12.7 11.2 7.2 10.5 5.4 9.0 3.6 4.6

9 9 9 9 9 13a 9 9

A AB B B A A BC C

ters means significant differences ( p b0.05). S: summit, C: core, B:

area.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260 255

3.2. Micromorphological characterisation

3.2.1. Four year old anthill

The proportion of mesopores, ranged around 8–9%

and was not different between the summit and the core

(Table 2). The summit however, had a greater propor-

tion of irregular pores of the class I3, while the core

had a higher proportion of the class E3, and of round-

ed pores irrespectively of the size (Fig. 4).

4 yrs

0

2

4

6

8

10

12

S B P

6 yrs

0

2

4

6

8

10

12

15 yrs

0

2

4

6

8

10

12

Ave

rage

Per

cent

age

of M

esop

oros

ity (

%)

I3

E3

R3

I2

E2

R2

I1

E1

R1

C

S B PC

S B PC

Fig. 4. Average percentage (%) distribution of morphological types

of mesopores according to the summit (S), core (C), base at the

ground level (B) and the old ploughed horizon (P). R: rounded

pores, E: elongated pores, I: irregular pores, see also Table 1 for size

references.

The ploughed horizon had a significantly lower

mesoporosity in comparison to the epigeic parts of

the anthill. Mesopores occupied 3.7% of the volume

and rounded pores were dominant in all size classes

(Table 2 and Fig. 4).

We did not register statistical differences in the

respective percentages of macroporosity recorded in

the summit and the core. The ploughed part had a

lower porosity in comparison to the epigeic parts of

the anthill (Table 2).

We observed some differences in the distribution of

the morphological pore types among the different

parts of the anthill. From the summit up to the core,

pore size classes 4 and 5 decreased in all shape

classes, while size class 6 remained almost constant,

with some differences in R6 and I6 between the

summit and the core (Fig. 5).

3.2.2. Six year old anthill

We did not find significant differences in mesopore

percentages between the parts of the anthill (Table 2).

The distribution of the morphological classes was not

different either. On average, all the morphological

classes were similarly distributed. The major differ-

ences were observed in classes I3, and E3, which were

present in a higher proportion in the summit, and in

the core, respectively (Fig. 4). The percentage of

mesoporosity in the ploughed horizon was similar to

the one registered in the anthill. The most important

morphological pore types were the classes I3 and E3

(Table 2 and Fig. 4).

Macroporosity exhibited significant differences

among the parts of the anthill. The summit presented

a greater macroporosity in comparison to the base.

However there were no differences between the sum-

mit and the core, in spite of a general trend for a

greater porosity in the summit. We did not register

either differences in macroporosity between the core

and the base of the anthill (Table 2). The ploughed

horizon had a lower porosity in comparison to the

summit, but we did not find differences with the other

parts (Table 2).

Some differences were observed in the distribution

of the morphological pore types, as the summit had

greater proportions of the classes I6, and E6 pores than

the core and the base, while the core had the greatest

proportion of the class R6. The base and ploughed

horizon had similar pore distributions (Fig. 5).

4 yrs

0

5

10

15

20

25

30

35

S

6 yrs

0

5

10

15

20

25

30

35

15 yrs

0

5

10

15

20

25

30

35I6

E6

R6

I5

E5

R5

I4

E4

R4

Ave

rage

Per

cent

age

of M

acro

poro

sity

(%

)

C B P

S C B P

S C B P

Fig. 5. Average percentage (%) distribution of morphological types

of macropores according to the summit (S), core (C), base at the

ground level, and the old ploughed horizon (P). R: rounded pores,

E: elongated pores, I: irregular pores, see also Table 1 for size

references.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260256

3.2.3. Fifteen year old anthill

The mesoporosity significantly differed among the

parts of the anthill with a lower porosity at the base in

comparison with the core (Table 2). There were also

differences in the proportion of the class I3, being

more abundant in the core. The rounded pores were

equally distributed among the parts of the anthill. The

class E3 was higher in the summit and the base part

presented the lowest proportion of the classes I3 and

E3 (Fig. 4).

The percentage of the mesoporosity in the

ploughed horizon was not significantly different

from that observed in the anthill (Table 2). This

horizon had a high proportion of pores of the class

E3, while irregular pores were present with a similar

proportion as measured at the base and the proportion

of rounded pores was similar to the rest of the anthill

(Fig. 4).

We saw significant differences in the percentage of

macropores between the areas of the anthill. The base

presented the lowest macroporosity in comparison with

the summit and with the core (Table 2). The ploughed

horizon had a significant smaller percentage of macro-

pores in comparison with the summit, and the core, but

not with the base (Table 2 and Fig. 5).

The summit and the core of the anthill showed a

high proportion of class R6 of macropores. The sum-

mit also had a great proportion of the classes E6, and

I6. The base of the anthill had an even distribution of

pores, similar to that observed in the ploughed horizon

(Fig. 5).

3.3. Changes in macro and mesoporosity among

anthills

Average percentages of total mesoporosity, consid-

ering the parts of each anthill as a whole, were 9.1%

(Interquartile: IQ=7.5%) in the 4 years old, 9.7%

(IQ=5.5%) in the 6 years old, and 8.6% (IQ=6.8%)

in the 15 years old, which were not significantly

different among each other. Mesoporosity observed

in the summit and the core did not change with time.

The distribution of morphological pore types

showed a similar pattern of pores for all the anthills

especially so for the summit and the core (Fig. 4).

The only difference was the lower proportion of the

class I3 found in the summit at the 15 year old

anthill.

On average the percentage of total macroporosity

in each anthill was 28.9% (IQ=11.3%) in the 4 years

old, 17.5% (IQ=11.3%) in the 6 years old, and

29.5% (IQ=23.4%) in the 15 years old but these

differences were not significant. Macroporosity was

only significantly different in the core part of ant-

hills, with greater porosity in the 15 year old anthill

in comparison to the 6 years old, but not in com-

parison to the 4 year old anthill (Table 2). Irregular

pores of the class I6 increased from 4 to 6 years and

further decreased at 15 years, especially clear at the

summit of the anthill.

Table 3

Statistical comparisons between the percentage of mesoporosity

around the galleries (G) and the percentage of mesoporosity in

the matrix of each anthill (epigeic part of the anthill)

Age Area Median (%) Interquartile (%) N p

4 years G 5.9 3.6 12 p =0.067

M 9.1 7.5 20

6 years G 4.8 3.4 18 p =0.001

M 9.7 5.4 30

15 years G 5.8 2.9 12 p =0.013

M 8.6 4.2 20

Level of significance p b0.05.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260 257

Significant changes were observed, however, for

macropores in the ploughed horizon. Below the 4 year

old anthill the ploughed horizon did have almost no

macropores and had a low percentage of mesopores.

After 15 years macroporosity was still low, and the

mesoporosity achieved values similar to that ones

found at the base of the anthill (Figs. 4 and 5).

3.4. Comparison between the porosity around the

galleries and the mesoporosity of the anthills

Mesoporosity around the galleries showed similar

patterns among the parts within the anthills (4 years

old, p =0.83, n =6; 6 years old, p =0.45, n =6; 15 years

old, p =0.87, n =6) and among the anthills of different

ages ( p =0.31, n =18). However, when comparing

mesoporosity around the galleries with the mesopor-

osity present in the epigeic part of the anthill (matrix),

the mesoporosity around the galleries was significantly

smaller in all cases, although this trend was not sig-

nificantly different in the 4 year old anthill (Table 3).

4. Discussion

4.1. External changes to the anthills

The thickness of the loose cortex, that covers the

anthills, is directly associated to the ant building activ-

ity, decreasing with the age of the anthill and disappear-

ing when dead. Once compacted and incorporated into

the anthill, elements from the loose cortex become

highly coherent giving to the anthill a persistence that

is a characteristic of anthills of type II described by

Paton et al. (1995). Additionally, the cortex covering

the anthills likely plays an important role in absorbing

the impact of the rain drops and in improving the water

infiltration inside the anthills (Lobry de Bruyn and

Conacher, 1994).

After 6 years, we have observed the development of

peripheral depressions of several centimetres depth

around some parts of the anthills, and after 15 years,

the depressions became a continuous ditch around the

anthill. The presence of these ditches in older anthills

had been formerly interpreted as a consequence of their

own weight since an anthill of 7 years old exerts a

pressure of 2.22 kPa (Folgarait, unpublished data).

However, after analysing the structure of the anthills

we alternatively propose that as the remnants of the

former ploughed horizon are observed below the anthill

at the same level than outside, no sinking effect of the

anthill due to its own weight seems to have occurred.

On the other hand, the peripheral depression around the

anthills could be the result of a combination of the

specific physico-hydric behaviour of the soil at the

periphery of the anthill and the effects of livestock.

Water runoff along the anthill induces a wetter zone

around them which had been softened by ant activities.

Therefore ditches may be due to cattle trampling in this

peripheral wetter and softer zone.

4.2. Internal organization of the anthills

In anthills aged 4 years, the summit and the core

parts of the anthill were completely developed, but the

compacted base was still absent. We assume that this

is a consequence of the relative youth of the anthill,

which did not have enough time for differentiating the

areas described. However, in the other two anthills the

base was clearly developed. The porosity in this area

was smaller than the one registered in the summit for

the 6 year old anthill and also smaller than that found

in the core of the 15 year old anthill.

At the base of the 6 and 15 year old anthills, we

observed few active galleries, which were normally

found in the upper part of the anthill (summit and

core) at the three ages. This pattern of gallery distribu-

tion was also found in Lasius neoniger (Wang et al.,

1995). We state that the formation of the base through

time is the result of filling galleries and chambers by C.

punctulatus ants. In this way, galleries and chambers

belonging to the core, once filled, become part of the

base. The observed increased in mesoporosity further

supported this hypothesis. We observed an increase in

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260258

the proportion of irregular pores (especially the larger

ones) from 4 years to 6 years old, and then a decrease at

15 years old, especially so, for the summit of the ant-

hills across ages. This change in the proportion of

irregular pores is a clear indication that ant would fill

with particles of soil part of the macropores. We hy-

pothesize that ants filled the galleries and chambers in

order to stabilize the anthill.

4.3. Stabilization of the mound by wall gallery’s

compaction

The micromorphological analysis points out that

the soil particles around the galleries were more

compacted than at other parts of the mound (matrix).

The soil particles were closer among each other

producing a reduction in the porosity. Under a mag-

nifier, we observed on the walls of the chambers, the

imprinting of ant mandibles, which ensure to us the

origin of the arrangement of the soil particles that

produce the compaction (see Fig. 2e,f). This com-

paction suggests that the ants were trying to stabilise

the walls of the galleries, necessary to resist the

increasing weight of the soil that is being accumu-

lated while the anthill is growing. In fact, many

myrmecologists have found behaviours related to

wall galleries stabilisation. They suggest that the

walls may have more resistance due to the re-orga-

nization of soil particles mixed with ceraceous sub-

stances, coming from the body of the ants

(Holldobler and Wilson, 1990; Wang et al., 1995).

This should be particularly the case as the anthills of

C. punctulatus are among the hardest observed

among ants (Folgarait, personal communication). A

similar behaviour has been observed in termites,

which made the wall chambers more resistant and

persistent through time by changing the mineralogy

of clay that is deposited for building the wall cham-

bers (Jouquet et al., 2002). It would be very inter-

esting to check for this kind of change in clay

mineralogy also for the anthills of C. punctulatus.

4.4. Changes in the compacted topsoil under ant

activity

The old ploughed horizon remained almost un-

modified below the 4 year old anthill. The macro-

pores were almost absent and there was a small

proportion of mesopores. In beginning a colony C.

punctulatus queens develop their nests at the base of

tussocks or rosettes, and most of the time at the

btaipasQ (name given locally for the ridges that sep-

arate parcels in the rice field) (personal observation).

We do not know how long it may take to the queen

to have a sufficient worker force to build the above-

ground nest. We think that C. punctulatus ants do

not excavate the old ploughed horizon until they are

a huge worker force. Meanwhile the ants may take

the soil from the upper few centimetres of the soil

surface (0–10 cm) and build aerated structures that

they consolidate by mixing with ceraceous sub-

stances coming from their body, and compacting

them with their mandibles and legs. Buhl et al.

(2004) have shown experimentally that the volume

of galleries excavated by the ant Mesor sancta Forel

was positively correlated with the number of work-

ers. On the other hand, at the beginning of the

anthill building, the ant C. punctulatus might not

need to dig a lot of new galleries, all the ants may

re-use the already excavated galleries. The tendency

for an individual to dig where other individuals have

previously dug has been shown in several ant spe-

cies (Hangarten, 1969; Rasse, 1999; Buhl et al.,

2004). After the statement of these hypotheses, we

conclude that C. punctulatus did not modify the old

ploughed horizon below the 4 year old anthill, which

was massive and compact after rice plantation and

cattle trampling during the fallow period. However

an increase in the macropores proportion through

time was clear from 4 to 6 years and then there

was a decrease at 15 years (Table 2), due to the

activity of filling the galleries and chambers as seen

by the increase in the proportion of mesopores,

especially the irregular ones from 4 to 15 years.

Despite some increment in soil porosity in the old

ploughed horizon, it was surprising to find this layer

almost unmodified 15 years after rice field abandon-

ment. The presence of the old ploughed horizon

gave us an idea of the degree of persistence of the

effects of some agricultural practices, which cannot

be rapidly modified by the biological activity. This

results contrasts with observations made in central

Amazonia by, e.g., Barros et al. (2001) who showed

restoration of soil physical properties of a degraded

pasture soil 1 year after exposure to the diverse soil

macrofauna of the original rainforest.

N.B. Gorosito et al. / Geoderma 132 (2006) 249–260 259

5. Conclusions

Anthills of C. punctulatus ants are very hard when

dry, and they persist through time even when aban-

doned. On average the percentage of soil porosity

(macroporosity) observed in the anthills was ~24.6%

in comparison to 8.3% found in the surrounding soil

(Gorosito et al., unpublished). The effect of C. punctu-

latus on soil physical properties was also observed

through the increase of porosity shown in the old

ploughed horizon as the anthills aged. Moreover, the

ditches surrounding old anthills also demonstrate an

enormous indirect effect on the surrounding soil. The

great compaction surrounding galleries and chambers

as well as the filling of those observed at the base of the

anthill additionally demonstrates the capacity of this

ant to consolidate the soil structure. Considering the

high density of anthills registered in abandoned rice

fields (1800 nests ha�1), their enormous size, high

resistance to erosion (we have seen 25 year old active

anthills), the ditches they produce, the enormousmove-

ment of soil particles from the soil to the surface, C.

punctulatus surely affects soil properties such as water

infiltration, soil aeration, and soil chemical properties,

and further affects life conditions for a large number of

organisms. Therefore, this study gives more evidence

to support the idea ofC. punctulatus as a soil ecosystem

engineer (Folgarait, 1998; Folgarait et al., 2002). These

effects are so important that they do not seem to be

naturally reversible in 10–30 years, the time period

covered by our observations. More detailed informa-

tion on the physical and chemical mechanisms in-

volved at the microaggregate scale should explain

how ants and termites manage to build long lasting

structures from apparently fragile soil materials.

Acknowledgements

We are indebted to the staff of the Agricultural

Experimental Station of INTA-Mercedes especially

to Rafael Pizzio for his continual support and logistic

help. We also thank the owners of the establishment

Aguaceritos, especially their managers J.C. Calfuan,

and P. Preliasco for their continuous help. We would

like also to thank Yannick Benard (INRA-Rennes)

for preparing the soil sections. This work was funded

by Institut de Recherche pour le Developement, IRD

(fellowship to NBG), by ECOS-SEPCyT (grant

A99B01 to PJF and PML), by Universidad Nacional

de Quilmes (grant 827-0201/99 and 0235 to PJF)

and IFS (grant C/2437-2 to PJF).

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