Available online at www.sciencedirect.com
Catena 74 (2008) 13 – 21
www.elsevier.com/locate/catena
Above-ground earthworm casts affect water runoff and
soil erosion in Northern Vietnam
Pascal Jouquet a,b,⁎, Pascal Podwojewski b,c , Nicolas Bottinelli a,b , Jérôme Mathieu a ,
Maigualida Ricoy d , Didier Orange b,c , Toan Duc Tran b , Christian Valentin c
a
d
IRD, UMR 137 Biosol, 32 Avenue H. Varagnat, 93143 Bondy Cedex, France
b
SFI - IRD - IMWI, Dong Ngac, Tu Liem, Hanoï, Vietnam
c
IRD, UR176 SOLUTIONS, 32 Avenue H. Varagnat, 93143 Bondy Cedex, France
Universidad de Vigo, Facultad de Biologia, Departamento de Ecologia y Biologia Animal, Campus as Lagoas-Marcosende, 36310 Marcosende, Spain
Received 5 July 2007; received in revised form 7 December 2007; accepted 20 December 2007
Abstract
This manuscript focuses on the effects of above-ground earthworm casts on water runoff and soil erosion in steep-slope ecosystems in Northern
Vietnam. We investigated the effects of Amynthas khami, an anecic species producing above-ground casts of prominent size, on water infiltration
and soil detachment along a land-use intensification gradient: a cultivation of cassava (Mahinot esculenta; CAS), a plantation of Bracharia
(Bracharia ruzziziensis; BRA), a fallow (FAL), a fallow after a forest of Eucalyptus sp. (EUC) and a plantation of trees (Acacia mangium and
Venicia Montana; FOR). Two scales of studies were considered: (i) at the structure scale (cm2), a water runoff simulation was used to
differentiate the effects of casts, free biogenic aggregates that previously belong to casts, and free physicogenic aggregates; (ii) at the station levels,
1-m2 plots were used to determine runoff and soil detachment rates during the rainy season in 2005.
A. khami was sensitive to land-use management. Earthworm density was low in all the fields (0–1 ind m− 2). The highest densities were found in
EUC and FOR and no individual was found in CAS. As a consequence, soil surface in EUC and FOR was covered with casts and free biogenic
aggregates (approximately 22 and 8 kg m− 2, respectively). In FAL and BRA, casts covered the soil only sparsely with b 3 kg m− 2. In CAS, soil surface
was characterized by free physicogenic aggregates that might be produced by human activity or endogeic earthworms through tillage (approximately
1 kg m− 2). Water runoff simulation clearly showed an enhancement of water infiltration with earthworm casting activity. Water runoff was more
decreased with casts (R2 = 0.26) than free biogenic aggregates (R2 = 0.49). Conversely, physicogenic aggregates were not associated with higher water
infiltration. Analyses of runoff and soil detachment rates during the rainy season underlined that the more land-use type have aggregates on soil
surface and the less important is surface runoff (R2 = 0.922). Conversely, no relation occurred between aggregates and soil detachment rate. While
above-ground casting activity decreased surface runoff, they were not involved in soil detachment, and therefore soil erosion.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Erosion; Earthworms; Above-ground casts; Land-use change; Water runoff; Soil detachment
1. Introduction
Soil erosion is a widespread land degradation problem at the
global scale in term of loss of soil fertility and water quality (Lal,
2004, 2005). Land-use change, with the loss of the protective
vegetation cover, is often considered as the main human factor of
soil erosion. In South-East Asia, erosion is regarded as a major
⁎ Corresponding author. IRD, UMR 137 Biosol, 32 Avenue H. Varagnat,
93143 Bondy Cedex, France.
E-mail address: [email protected] (P. Jouquet).
0341-8162/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.catena.2007.12.006
type of environmental damage (Maglinao and Leslie, 2001). Due
to rapid human population growth, the cropping areas have
expanded to more marginal lands such as mountains and the
fallow periods have been shortened or even abandoned (Clement
et al., 2006). In Northern Vietnam, much of the rain forest in the
mountain was lost during the 1970s, and the trend continues
(Sharma, 1992; Castella et al., 2006). Forests were cut to expand
cultivated cassava, arrowroot, taro, maize and Eucalyptus
cropping on the uplands. Due to decreased soil fertility in the
uplands, farmers have gradually converted their plots under
annual crop cultivation into common grazing land, or tree
14
P. Jouquet et al. / Catena 74 (2008) 13–21
plantations (mainly acacias) (Tran Duc et al., 2004; Clement
et al., 2006).
Land-use change is often accompanied with a loss and shift of
soil macrofauna diversity. Usually considered as being one of the
most important macrofaunal organisms in soil, earthworms are
very sensitive to land-use change (Paoletti, 1999; Curry et al.,
2002). Any shifts in earthworm community might have
significant consequences in term of soil erosion in steep-slope
tropical ecosystems. The influence of earthworm on soil
properties is species-specific. Through their burrowing and
casting activities, earthworms living both below and aboveground (anecic and epi-anecic species, sensu Bouché, 1977)
significantly affect soil surface properties within the top layers of
soil (Lavelle and Spain, 2001). In temperate ecosystems with
gentle slopes, casts produced on the soil surface increase soil
roughness and in turn affect water runoff velocity and infiltration
into soil (Binet and Le Bayon, 1999; Le Bayon and Binet, 2001).
Galleries connected with the soil surface might also constitute
preferential flow paths for water infiltration (Bastardie et al.,
2003, 2005; Chan, 2004). However, beneficial effects of these
earthworm species on soil erosion might be offset by the low soil
structural stability of their casts when they are freshly emitted
(Shipitalo and Protz, 1988; Le Bayon and Binet, 1999). Indeed,
Le Bayon et al. (2002) showed that earthworm casts might
increase soil detachment and nutrient transfers during rainstorm
events in Brittany (North-West France) (Le Bayon et al., 2002).
Conversely, earthworm species that live below-ground do not
affect soil surface properties and are assumed to play a less
significant role. Surprisingly, the influence of earthworm
diversity and activity on water runoff and soil erosion have
been only poorly studied in sloping lands of the tropics, which
have been identified as one of the most biogeochemical active
cycling in the world (Koch et al., 1995).
In the mountains of the Northern Vietnam, Amynthas khami
builds water-stable casts that are deposited on the soil surface.
These biogenic structures can reach 20 cm height and are
formed after week of daily deposition of globular casts at the top
edge of the structure. These casts can be broken, probably by
livestock trampling and human walking. Hence, free biogenic
aggregates can be released on the soil surface and constitute a
significant quantity of free macro-aggregates on the soil surface.
This study aims at determining the effect of this accumulation of
casts and free biogenic aggregates on surface runoff and soil
detachment within steep-slope ecosystems. Two experiments
were set up along a land-use intensification gradient: (i) a water
runoff simulation was carried out at the cm2 scale to determine
the effect of earthworm casting activity on water runoff and
infiltration, and (ii) infiltration plots (1-m2) were set up to
determine if above-ground cast production is associated with
soil detachment, and therefore soil erosion.
Institute, IMWI) project of Dong Cao (46 ha) is located in northeast Vietnam, approximately 50 km south-west of Hanoi (20°
57′N, 105° 29′E). This experimental catchment is followed
since 1999 for the measurement of water fluxes, runoff and
erosion rates in relation with land-use change. This watershed is
surrounded by hills with a general slope of 40% in average but
sometimes reaching 100%. The annual rainfall ranges from
1500–1800 mm, of which 80–85% is concentrated from April
to October. The air humidity is always high, between 75 and
100%. The mean daily temperature varies from 15 °C to 25 °C
(Tran Duc et al., 2004).
The dominant soil type is an Acrisol (WRB, 1998) or Ultisol
(SSS, 1999). Soils derived from the weathering of volcanosedimentary schists of Mesozoic age. Soils are over 1.0 m deep
but with marked variation in depth. They have more than 50%
clay content, mainly kaolinite with a low CEC ( b 10 cmol kg− 1),
and are very porous with a bulk density of 1000 kg m− 3. They
have a homogenous brown colour 10YR4/4 to 7.5YR 4/6, and a
weak vertical differentiation.
Before the 1960s, the region was covered with a dense
primary forest. Deforestation and conversion to agricultural
land has led to its disappearance. From the mid 70s until very
recently, villagers have cultivated cassava, taro and maize on the
uplands (Castella et al., 2006). Since 1998, the catchment was
covered mainly with cassava with some area of Eucalyptus
plantation and the previous crop was Maize. From 2002
practices changed very quickly because of the soil erosion and
soil fertility decrease. Annual soil loss recorded through bed
load measurements have decreased from 3.6 t ha− 1 year− 1
before 2002 to 0.1–0.3 t ha− 1 year− 1 in 2004 (Orange et al.,
2007). Finally, five agro-systems were dominant in 2005: (1) a
young fallow after 4 years of cassava from 2002 (FAL), (2) a
plantation of Acacia mangium and Venicia montana planted in
2001 after a cassava plantation (FOR), (3) a fodder plantation
with Bracharia ruzziensis planted in 2003 after cassava (BRA),
(4) a very young regrowth of Eucalyptus sp. trees, following a
Eucalyptus plantation cut in 2003 (EUC), and (5) a small area of
cassava (Manihot esculenta) (CAS). These five land uses
represent the diversity of managements in this region of
Southeast Asia. No special treatment has been applied in these
agro-systems, even under cassava (no herbicide, low fertilization, just superficial tillage and weeding before cultivation in
CAS and BRA). Vegetation cover on the ground was low under
FOR (few small shrubs, lot of leaves). There was no litter in
CAS and very few in FAL and BRA. FAL and BRA were very
similar in term of vegetation and litter cover. Under EUC, litter
and vegetation cover were more important and shrubs grow
with a large density.
2. Materials and methods
Earthworms were hand-sorted from 1 m × 1 m × 50 cm deep
monoliths. Sampling was done during the rainy season (August
2005) when communities were assumed to be at peak of
abundance. Soil samplings were randomly repeated 10 times in
each land-use type. Earthworms were rapidly hand-sorted after
soil excavation and identified at the species level. In this study,
2.1. Study site
The experimental catchment of the MSEC (Managing Soil
Erosion Consortium of International Management of Water
2.2. Sampling of earthworms, surface casts and soil aggregates
P. Jouquet et al. / Catena 74 (2008) 13–21
15
Fig. 2. Water runoff simulation: 1.5 L Methylene blue water was added in a
constant flow (75 mL s− 1) on the top of a 50 ⁎ 30 cm Plexiglas plate. Water
reached the soil with a uniform front of 30 cm width along a 40% slope.
Measured parameters were soil moisture in the surrounding environment,
distance and surface covered by runoff water (determined from dyed soil),
surface occupied by casts (CAST), biogenic aggregates (ROUND) and
physicogenic aggregates (ANG) and vegetation.
the overall density of earthworms and the specific abundance of
A. khami were considered. This earthworm species is considered as anecic sensu Bouché (1977) and create vertical
galleries until more than 60 cm deep. Its size is highly variable
and individuals can probably reach more than 50 cm long at the
adult stage.
Above-ground soil macro-aggregates N 5 mm were collected in
the different land-use types in 25 × 25 cm plots (n = 9, Fig. 1).
According to the definitions described in Bullock et al. (1985) and
Pulleman et al. (2005), we distinguished three groups:
(i) epigeous casts (fresh casts: CAST), (ii) free biogenic aggregates (rounded shape macro-aggregates that were clearly identified as belonging to old casts: ROUND) and (iii) physicogenic
aggregates (angular to subangular blocky macro-aggregates:
ANG). The morphological fractions were air-dried and weighed
to determine their relative mass contribution.
Table 1
Overall density of earthworms and specific density of Amynthas khami
(ind m− 2) in the different land use (CAS: cassava plantation; FAL: fallow;
BRA: bracharia plantation; EUC: fallow and Eucalyptus regrowth; FOR: forest)
(Mean ± standard error, n = 10)
Fig. 1. Examples of soil macro-aggregates: (a) CAST: casts deposited by Amynthas
khami and (b) ROUND: free rounded shape aggregate clearly identified has
belonging to old cast (i.e. biogenic aggregates); (c) ANG: (sub)angular blocky
aggregate (i.e. physicogenic aggregate) (Photos Pascal Jouquet, IRD).
Density (ind m− 2)
CAS
FAL
BRA
EUC
FOR
Total
Amynthas khami
4.29 (± 0.70)
13.50 (± 2.03)
9.60 (± 1.38)
6.99 (± 1.13)
4.03 (± 0.57)
0.00 (± 0.00)
0.25 (± 0.44)
0.30 (± 0.42)
1.11 (± 0.59)
0.63 (± 0.78)
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P. Jouquet et al. / Catena 74 (2008) 13–21
Table 2
Weight of air-dried macro-aggregates on soil surface (kg m− 2): casts (CAST),
free rounded shapes biogenic aggregates (ROUND) and free angular and
subangular aggregates (ANG) in the different land use (CAS: cassava plantation;
FAL: fallow; BRA: bracharia plantation; EUC: fallow and Eucalyptus regrowth;
FOR: forest) (Mean ± standard error, n = 9)
CAS
FAL
BRA
EUC
FOR
CAST
ROUND
ANG
0.00 (±0.00)
0.88 (±1.18)
0.11 (± 0.11)
11.57 (± 2.80)
4.42 (± 1.10)
0.00 (±0.00)
2.15 (±1.74)
0.82 (± 1.03)
11.24 (± 4.42)
3.27 (± 0.77)
0.93 (± 0.59)
0.31 (± 0.28)
0.20 (± 0.28)
0.00 (± 0.00)
0.00 (± 0.00)
2.3. Water runoff simulation
Water runoff simulation was carried out by adding 1.5 L
methylene blue water ( ≈ 5 g L− 1) with a constant flow of 75 mL
s− 1 on the top of a 50 ⁎ 30 cm Plexiglas plate (Fig. 2). The plate
lay on the ground and water reached the soil with a uniform front
of 30 cm width. Given the pronounced impact of slope gradient
on runoff and detachment (Janeau et al., 2003), a same slope was
set, around 40%, which is the mean slope value of the catchment.
Litter was carefully removed from the soil surface to avoid any
influence on water runoff. Methylene blue water dyed the soil
and allowed us to immediately and precisely determine the
distance and surface covered by water runoff. Different parameters were measured to describe soil surface properties. Surfaces
(%) covered by macro-aggregates (CAST, ROUND and ANG)
and vegetation were visually estimated in a 25 ⁎ 25 cm frame
positioned 3 cm downslope from the Plexiglas plate. The maximum
distance covered by water runoff (cm) was measured while the
overall area covered by water runoff (cm2) was visually estimated.
The ratio distance:surface was used as an index to describe the
shape of runoff (i.e. linear surface vs sheet surface runoff). Soil
moisture was measured in the surrounding environment.
This experiment was done in each land use in differentiating
areas without macro-aggregates (control plots: CAS, FAL and
BRA, n = 5), to areas with CAST and ROUND aggregates
(FALcast, n= 6; BRAcast, n = 10; FORcast, n = 12; EUCcast,
n =6), areas with only ROUND aggregates (FALround, BRAround,
FORround, EUCround, n= 6) or only physicogenic aggregates
(CASang, n= 4) on the soil surface.
2.4. Runoff plots
Three 1-m2 plots were set up in each land-use type: CAS,
FAL, BRA, EUC and FOR. The slope ranged from 39 to 44%,
Fig. 3. Principal components analysis (PCA) on the water runoff simulation in the different land use (CAS: cassava plantation; FAL: fallow; BRA: bracharia plantation;
EUC: fallow and Eucalyptus regrowth; FOR: forest): distance (cm) and surface (cm2) covered by coloured runoff dyed water, soil moisture (%), and soil cover
properties (% cover of herbs, % aggregates: casts (CAST), biogenic- (ROUND) and physicogenic aggregates (ANG) in a 25 ⁎ 25 cm square beneath the Plexiglass
plate). Control plots without free macro-aggregates on soil: CAS, FAL, BRA; Plots with CAST and ROUND: FALcast, BRAcast, FORcast, EUCcast; Plots with
ROUND aggregates: FALround, BRAround, FORround, EUCround; Plots with ANG aggregates: CASang. (a) Correlation circle. (b) Ordination of the samples in the
plane defined by axes 1 and 2 of the PCA.
P. Jouquet et al. / Catena 74 (2008) 13–21
17
(PCA) was done using a matrix of 58 samples and 7 variables
(distance and surface of water runoff, percentage of herb,
CAST, ROUND and ANG cover, and soil moisture). All
statistical calculations were carried out using R (R Development
Core Team, 2004). Differences were considered significant,
only when P values were lower than 0.05.
3. Results
3.1. Abundance of A. khami and quantity of surface aggregates
Fig. 4. The relationship between casts (CAST), biogenic (ROUND) and
physicogenic (ANG) aggregates (% soil cover) and dye stained area (cm2)
(n = 66). Linear regression lines describing the relationship between soil
aggregates and dye stained area are fitted (y = − 5.28x + 757.30, R2 = 0.490,
P b 0.001 for ROUND; y = − 7.42x + 595.73, R2 = 0.259, P b 0.001 for CAST;
R2 = 0.01, P = 0.198 for ANG).
The abundance of earthworms and more specifically of
A. khami was site-specific (Table 1). The density of earthworm
was low and earthworms were mainly endogeic species (nonpigmented earthworms). Earthworms were more abundant in
FAL as compared to the other agro-ecosystems. A. khami was
not find in CAS and densities were very low in BRA and FAL
( b 1 ind m− 2), without any significant difference between them
(P N 0.05). Although the density of A. khami in FOR was higher
than in FAL and BRA and lower than that in EUC, the results
were not significantly different (P N 0.05 in all the cases). The
density of A. khami was the highest in EUC (approximately
1 ind m− 2) and significantly different from that in FAL and
BRA (P = 0.011 and 0.012, respectively).
As a consequence, we did not find any biogenic aggregates,
either CAST or ROUND, in CAS (Table 2). The quantity of
CAST in FOR was greater than in FAL and BRA (P b 0.001 in
all the cases) and lower than in EUC (P b 0.001). Conversely, no
significant difference occurred between the quantity of ROUND
in FAL, BRA and FOR (P N 0.05) and the highest value was
found in EUC (P b 0.001 in all the cases). CAS was characterized by the highest quantity of ANG aggregates (P b 0.005 in
all the cases) and no significant difference occurred between
FAL, BRA, EUC and FOR (P N 0.05).
representing the average slope value of the catchment. Plots
were bounded by rigid metal frames inserted to a depth of
0.10 m. Soil detachment and water runoff were collected after
each rainfall event, from May to October 2005, in a collector at
the outlet of the plot, as described by Janeau et al. (2003).
Runoff rate was determined by the ratio of the quantity of water
that runoff in the collector tank to the amount of daily rainfall.
Soil detachment rate was determined through the measurement
of sediment weight after filtration from runoff water and heating
at 105 °C. This sediment weight is assumed to represent the
quantity of soil losses during the rainfall event on 1-m2 plots.
Surfaces occupied by macro-aggregates (CAST + ROUND +
ANG) on these plots were visually estimated in October 2005,
which corresponds to the end of the rainy season.
2.5. Statistical analyses
Prior to analyses data were inspected for homogeneity of
variance using the Levene's test and log-transformed when
required. Differences between treatments were tested through
analysis of variance (ANOVA). Principal component analysis
Fig. 5. Distance:surface runoff ratio in the different land use (CAS: cassava
plantation; FAL: fallow; BRA: bracharia plantation; EUC: fallow and Eucalyptus
regrowth; FOR: forest). Control: bare soil; ANG or ROUND: soil covered by
physicogenic and biogenic macro-aggregates, respectively; CAST: soil with
earthworm casts on the soil surface (bars indicate standard errors).
18
P. Jouquet et al. / Catena 74 (2008) 13–21
3.2. Water runoff simulation
A principal component analysis (PCA) was made (Fig. 3a).
Control plots (i.e. plots without CAST, ROUND and ANG
aggregates) were clearly separated from plots with biogenic
aggregates on the soil surface (i.e. CAST and ROUND) on axes
1 and 2 that respectively explained 50.6 and 15.4% of the total
variance. The correlation circle (Fig. 3b) clearly opposed
variables linked to earthworm activity (the surface occupied by
CAST and ROUND) to those linked to water runoff (the
distance and surface covered by water runoff). More precisely,
runoff magnitude (distance and surface covered by coloured
water) increased in the range CAS b BRA b FAL. We did not
find any plots without ROUND in EUC and FOR. Plots with
CAST and ROUND were also separated with the PCA, except
in the case of BRA, and CAST were associated with high water
infiltration. CAS and CASang were also separated and ANG
were tightly associated with higher water runoff.
Correlations between soil surface occupied by macroaggregates and dye stained area are shown in Fig. 4. Linear
regressions were highly significant for CAST and ROUND
(P b 0.001) and no significant for ANG (P = 0.198).
The ratio of the distance to the surface of coloured water on
soil (i.e., shape of surface runoff) is shown in Fig. 5. This ratio
was significantly the lowest in CAS (P = 0.018 and 0.011,
Fig. 6. Relation between free macro-aggregates (% soil cover), and runoff (L m−2
year−1) and soil detachment (g m−2 year−1) rates in the different land use (CAS: cassava
plantation; FAL: fallow; BRA: bracharia plantation; EUC: fallow and Eucalyptus
regrowth; FOR: forest) in 2005. : CAS, +: FAL, : BRA, : EUC, :FOR. Linear
regression describing the relationship between runoff and free- macro-aggregates is
fitted (y = −4.07x + 428.42, n = 11, R2 = 0.922, P b 0.001). Data for FOR runoff
were not included in the linear regression.
respectively with FAL and BRA) and no difference occurred
between FAL and BRA (P = 0.676). Although this ratio was not
significantly influenced by ANG, ROUND and CAST on the
soil surface (P N 0.05 in all the situations), standard errors were
the highest with CAST (P = 0.017 and 0.009, with control soil
and soil with ROUND, respectively), suggesting an increase of
irregularities of surface runoff. Standards errors were not
significantly different between control soil and soil plus
ROUND (P = 0.864).
3.3. Runoff and soil detachment rates
The relations between soil detachment and runoff rates, and
the surface occupied by free macro-aggregates (CAST +
ROUND + ANG) are shown in Fig. 6. Water runoff was negatively correlated with free macro-aggregates (P b 0.001), except
in FOR where runoff rate was very low and not affected by
macro-aggregates. Conversely, soil detachment rate was not
affected by free macro-aggregates and values were very low
( b 100 g m− 2 year− 1), except in CAS that was characterized by
high loss of soil (300–1300 g m− 2 year− 1).
4. Discussion
4.1. Earthworm activity and soil surface properties
Earthworms are usually considered as interesting indicators
to monitor different farming practices and different landscape
structures and transformations because they respond quickly to
land-use change (Paoletti, 1999; Curry et al., 2002). Fragoso
et al. (1997) found that earthworm biodiversity is modified
when natural (i.e., undisturbed) ecosystems are replaced by
agro-ecosystems. In this catchment, the density of earthworm
was low and species were mainly endogeic species. A. khami
was mainly abundant in EUC and slightly less in FOR. This
species is anecic (sensu Bouché, 1977) as determined through
field observations and δ13C analysis of litter and casts (data not
shown). As a consequence, the presence of this earthworm
species is determined by litter incomes and/or specific microclimatic conditions (e.g., surface temperature, soil humidity)
(Lavelle et al., 1995). Hence, the low input of litter and organic
residues in FAL, BRA, and especially CAS, might be responsible of the low survival and activity of this earthworm
species while it did not affect the density of endogeic species.
Conversely, this species was able to grow in EUC and FOR that
were characterised by high litter content and, although not
measured, probably higher soil moisture than in the other landuse types.
As a consequence, soil in EUC was covered with functional
and old earthworm casts that formed a typical granular horizon
with a high rugosity and macroporosity. The low density of
earthworms (approximately 1 ind m− 2) and the high quantity of
CAST and ROUND covering the soil surface suggest that these
biogenic structures are stable over a long period of time,
probably many years. The high soil structural stability of earthworm casts has been stressed by many authors (e.g., Shipitalo
and Protz, 1988; Marinissen and Dexter, 1990; Barois et al.,
P. Jouquet et al. / Catena 74 (2008) 13–21
1993). Blanchart et al. (1999) suggested that earthworm casts
can last for a long time (N 26 months in an African savannah
ecosystem) in kaolinitic soils owing to their high water stability.
We assume that this is especially true in our study site where
clay content in earthworm cast is important (approximately 50
to 68%), depending on land-use type (Jouquet et al., 2007).
Although A. khami was not found in CAS, free angular
macro-aggregates (ANG) occurred on the soil surface. These
macro-aggregates had a higher soil density (1600 kg m− 3 as
compared to 970 kg m− 3 for the surrounding soil) and water
stability than the surrounding soil (data not shown). The higher
water stability of these aggregates involved the protection of
soil beneath against slaking and thus the formation of pedestal
features (i.e., micro-soil erosion column where the base is more
erodible that the top, see Fig. 7). Since soil macro-aggregates
had a higher density and soil structural stability to water, we
assume that they were likely to be produced by earthworms.
While A. khami could not be suspected to be responsible of
these cast production, endogeic earthworms were found in each
land-use type and they were likely to be able to produce these
aggregates (Blanchart et al., 1999, 2004). We assume that
endogeic earthworms produced below-ground casts that could
be exposed at soil surface during tillage or cassava harvest. As a
consequence, what we have called free physicogenic aggregates
(ANG) so far in this paper might be considered as free biogenic
aggregates of endogeic earthworms. Further analyses are
however necessary to confirm this hypothesis.
4.2. Consequences for water infiltration and soil detachment
Soil aggregate stability in the top-soil is closely linked to
runoff and erosion rates at the landscape scale (Barthès et al.,
2000; Barthès and Roose, 2002; Cotler and Ortega-Larrocea,
2006). Water stable macro-aggregates are known to prevent
detachment of easily transportable particles, and thereby soil
surface clogging and runoff (Le Bissonnais, 1996; Le
Bissonnais and Arrouays, 1997; Barthès et al., 2000; Barthès
Fig. 7. Pedestal features in cassava plantation: The upper part of the soil column
is less prone to erosion decay by splash effect (harder meso-angular free
aggregate: ANG) and protect the lower part from detachment. The origin of
ANG is unknown but might be explained by endogeic earthworm activity (i.e.
below-ground casts exposed at soil surface during tillage or cassava harvest).
(Photo Pascal Podwojewski, IRD).
19
and Roose, 2002). Through the creation of stable aggregates
(CAST and ROUND) on soil surface, A. khami significantly
increased water infiltration into soil. In producing casts,
earthworms enhance surface roughness, modify the circulation
of surface runoff and reduce its velocity, and as a consequence
improve water infiltration. Our data showed that the reduction
of runoff water was more efficient with CAST than with
ROUND. Galleries beneath the casts might partially explain this
more efficient reduction of runoff (Chan, 2004). However, field
observations pointed out that galleries did not always constitute
preferential flow paths for water, especially when galleries were
still functional (i.e. occupied by earthworms) or when casts
occluded galleries. Casts deposited by A. khami are anchored in
soil and there is continuity between below-ground galleries and
above-ground casts. Since casts are not broken, this continuity
impedes the infiltration of water in galleries.
The water runoff simulation clearly showed that surface
runoff decreased with earthworm surface casting activity. This
experiment at the cm2 scale was confirmed by the annual
measure of runoff rate at the 1-m2 scale in all the stations except
FOR. The more agro-ecosystems have macro-aggregates on soil
surface and the less the runoff of water is important. The two
studies were complementary to discriminate the respective
effects of CAST, ROUND and ANG aggregates. Although
water runoff simulation was useful to discriminate the different
effects of CAST and ROUND on runoff magnitude and shape
(i.e., distance:surface ratio), this experiment did not take into
account soil detachment during rainfall events, and then did not
considered splash effect and variation of water runoff velocity.
In addition, the low quantity of coloured water added to soil
might make conclusions difficult. Conversely, although 1-m2
plots really provided runoff water rate in the field, they did not
discriminate the respective effects of CAST, ROUND and ANG
structures from themselves and from the other ecological factors
(e.g., percent litter or canopy cover, soil organic matter content,
non-surface biological activity…). In FOR, CAST and ROUND
aggregates deposited on soil surface were associated with
higher water infiltration during the water runoff simulation.
This trend was not confirmed with 1-m2 plots. We assume that
this difference can be explained by rainfall interception and
preferential stemflows that modified the amount and energy of
raindrops and associated runoff rates (Mosley, 1982; Duran
Zuazo et al., 2004). Effectively, pluviometers located below
Acacia and Venicia plantation showed a decrease of 20% of the
rainfall as compared to the meteorological station.
The influence of earthworms on soil detachment and
subsequent soil erosion might be positive or negative depending
on the age of casts. Aged or dried casts are usually considered as
being more stable than the surrounding soil while freshly
egested casts are highly susceptible to dispersion and transport
by water (Darwin, 1881; Shipitalo and Protz, 1988; Blanchart
et al., 1999). Le Bayon and Binet (1999, 2001) found that
earthworm casts might obviously contribute to soil erosion in
temperate gentle slope ecosystems (slope of approximately
4.5%). On the contrary, although our study was established
under a tropical climate with a steep slope (about 40%) and
strong rainfall intensities, sometimes reaching more than
20
P. Jouquet et al. / Catena 74 (2008) 13–21
100 mm h− 1, our study did not conclude about a significant loss
of soil promoted by earthworms. Owing to these environmental
conditions, a higher soil detachment rate was expected from
earthworm casting activity. We assume that the high stability of
casts, their rapid drying and the low earthworm activity (i.e. low
earthworm density) might explain the low contribution of
earthworm to soil loss even under intense rainfalls.
5. Conclusion
In conclusion, this study points out that earthworm casting
activity plays a significant role in decreasing water runoff
velocity and that biogenic structures are not prone to dispersion
and erosion in steep-slope ecosystems of the Northern Vietnam.
Different studies have demonstrated that earthworm casts can be
washed up and contribute to soil erosion. However, in this
catchment, the effect of earthworm activity was considered as
being rather beneficial in term of soil erosion and positive
effects of casts and free biogenic aggregates (higher water
infiltration) clearly counterbalanced negative ones (cast decay
and dispersion of particles). Favouring anecic earthworms could
therefore be an interesting strategy to decrease soil erosion in
steep-slope tropical ecosystems of the Northern Vietnam.
Acknowledgements
We would like to express our gratitude to Samuel James and
Robert Blakemore for their helps in earthworm identification.
This project is financially supported by the Management of Soil
Erosion Consortium (MSEC) from the International Water
Management Institute (IWMI), the Soils and Fertilisers Institute
(SFI from MARD, Hanoi, Vietnam) and the Institute of Research
for Development (IRD from French Ministry of Research, unit
research UMR137-BIOSOL and UR176-SOLUTIONS).
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