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Catena 74 (2008

Above-ground earthworm casts affect water runoff andsoil 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 IRD, UMR 137 Biosol, 32 Avenue H. Varagnat, 93143 Bondy Cedex, Franceb SFI - IRD - IMWI, Dong Ngac, Tu Liem, Hanoï, Vietnam

c IRD, UR176 SOLUTIONS, 32 Avenue H. Varagnat, 93143 Bondy Cedex, Franced 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 NorthernVietnam. We investigated the effects of Amynthas khami, an anecic species producing above-ground casts of prominent size, on water infiltrationand 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 andVenicia Montana; FOR). Two scales of studies were considered: (i) at the structure scale (cm2), a water runoff simulation was used todifferentiate 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 inEUC and FOR and no individual was found in CAS. As a consequence, soil surface in EUC and FOR was covered with casts and free biogenicaggregates (approximately 22 and 8 kgm−2, respectively). In FAL andBRA, casts covered the soil only sparsely with b3 kgm−2. In CAS, soil surfacewas characterized by free physicogenic aggregates that might be produced by human activity or endogeic earthworms through tillage (approximately1 kg m−2). Water runoff simulation clearly showed an enhancement of water infiltration with earthworm casting activity. Water runoff was moredecreased with casts (R2=0.26) than free biogenic aggregates (R2=0.49). Conversely, physicogenic aggregates were not associated with higher waterinfiltration. Analyses of runoff and soil detachment rates during the rainy season underlined that the more land-use type have aggregates on soilsurface and the less important is surface runoff (R2=0.922). Conversely, no relation occurred between aggregates and soil detachment rate. Whileabove-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 theglobal scale in term of loss of soil fertility and water quality (Lal,2004, 2005). Land-use change, with the loss of the protectivevegetation cover, is often considered as the main human factor ofsoil 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: pascal.jouquet@ird.fr (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). Dueto rapid human population growth, the cropping areas haveexpanded to more marginal lands such as mountains and thefallow periods have been shortened or even abandoned (Clementet al., 2006). In Northern Vietnam, much of the rain forest in themountain was lost during the 1970s, and the trend continues(Sharma, 1992; Castella et al., 2006). Forests were cut to expandcultivated cassava, arrowroot, taro, maize and Eucalyptuscropping on the uplands. Due to decreased soil fertility in theuplands, farmers have gradually converted their plots underannual 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; Clementet al., 2006).

Land-use change is often accompanied with a loss and shift ofsoil macrofauna diversity. Usually considered as being one of themost important macrofaunal organisms in soil, earthworms arevery sensitive to land-use change (Paoletti, 1999; Curry et al.,2002). Any shifts in earthworm community might havesignificant consequences in term of soil erosion in steep-slopetropical ecosystems. The influence of earthworm on soilproperties is species-specific. Through their burrowing andcasting activities, earthworms living both below and above-ground (anecic and epi-anecic species, sensu Bouché, 1977)significantly affect soil surface properties within the top layers ofsoil (Lavelle and Spain, 2001). In temperate ecosystems withgentle slopes, casts produced on the soil surface increase soilroughness and in turn affect water runoff velocity and infiltrationinto soil (Binet and Le Bayon, 1999; Le Bayon and Binet, 2001).Galleries connected with the soil surface might also constitutepreferential flow paths for water infiltration (Bastardie et al.,2003, 2005; Chan, 2004). However, beneficial effects of theseearthworm species on soil erosion might be offset by the low soilstructural 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 mightincrease soil detachment and nutrient transfers during rainstormevents in Brittany (North-West France) (Le Bayon et al., 2002).Conversely, earthworm species that live below-ground do notaffect soil surface properties and are assumed to play a lesssignificant role. Surprisingly, the influence of earthwormdiversity and activity on water runoff and soil erosion havebeen only poorly studied in sloping lands of the tropics, whichhave been identified as one of the most biogeochemical activecycling in the world (Koch et al., 1995).

In the mountains of the Northern Vietnam, Amynthas khamibuilds water-stable casts that are deposited on the soil surface.These biogenic structures can reach 20 cm height and areformed after week of daily deposition of globular casts at the topedge of the structure. These casts can be broken, probably bylivestock trampling and human walking. Hence, free biogenicaggregates can be released on the soil surface and constitute asignificant quantity of free macro-aggregates on the soil surface.This study aims at determining the effect of this accumulation ofcasts and free biogenic aggregates on surface runoff and soildetachment within steep-slope ecosystems. Two experimentswere set up along a land-use intensification gradient: (i) a waterrunoff simulation was carried out at the cm2 scale to determinethe effect of earthworm casting activity on water runoff andinfiltration, and (ii) infiltration plots (1-m2) were set up todetermine if above-ground cast production is associated withsoil detachment, and therefore soil erosion.

2. Materials and methods

2.1. Study site

The experimental catchment of the MSEC (Managing SoilErosion Consortium of International Management of Water

Institute, IMWI) project of Dong Cao (46 ha) is located in north-east Vietnam, approximately 50 km south-west of Hanoi (20°57′N, 105° 29′E). This experimental catchment is followedsince 1999 for the measurement of water fluxes, runoff anderosion rates in relation with land-use change. This watershed issurrounded by hills with a general slope of 40% in average butsometimes reaching 100%. The annual rainfall ranges from1500–1800 mm, of which 80–85% is concentrated from Aprilto October. The air humidity is always high, between 75 and100%. 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 volcano-sedimentary schists of Mesozoic age. Soils are over 1.0 m deepbut with marked variation in depth. They have more than 50%clay content, mainly kaolinite with a low CEC (b10 cmol kg−1),and are very porous with a bulk density of 1000 kg m−3. Theyhave a homogenous brown colour 10YR4/4 to 7.5YR 4/6, and aweak vertical differentiation.

Before the 1960s, the region was covered with a denseprimary forest. Deforestation and conversion to agriculturalland has led to its disappearance. From the mid 70s until veryrecently, villagers have cultivated cassava, taro and maize on theuplands (Castella et al., 2006). Since 1998, the catchment wascovered mainly with cassava with some area of Eucalyptusplantation and the previous crop was Maize. From 2002practices changed very quickly because of the soil erosion andsoil fertility decrease. Annual soil loss recorded through bedload 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) ayoung fallow after 4 years of cassava from 2002 (FAL), (2) aplantation of Acacia mangium and Venicia montana planted in2001 after a cassava plantation (FOR), (3) a fodder plantationwith Bracharia ruzziensis planted in 2003 after cassava (BRA),(4) a very young regrowth of Eucalyptus sp. trees, following aEucalyptus plantation cut in 2003 (EUC), and (5) a small area ofcassava (Manihot esculenta) (CAS). These five land usesrepresent the diversity of managements in this region ofSoutheast Asia. No special treatment has been applied in theseagro-systems, even under cassava (no herbicide, low fertiliza-tion, just superficial tillage and weeding before cultivation inCAS and BRA). Vegetation cover on the ground was low underFOR (few small shrubs, lot of leaves). There was no litter inCAS and very few in FAL and BRA. FAL and BRAwere verysimilar in term of vegetation and litter cover. Under EUC, litterand vegetation cover were more important and shrubs growwith a large density.

2.2. Sampling of earthworms, surface casts and soil aggregates

Earthworms were hand-sorted from 1 m×1 m×50 cm deepmonoliths. Sampling was done during the rainy season (August2005) when communities were assumed to be at peak ofabundance. Soil samplings were randomly repeated 10 times ineach land-use type. Earthworms were rapidly hand-sorted aftersoil excavation and identified at the species level. In this study,

Fig. 1. Examples of soil macro-aggregates: (a) CAST: casts deposited byAmynthaskhami and (b) ROUND: free rounded shape aggregate clearly identified hasbelonging to old cast (i.e. biogenic aggregates); (c) ANG: (sub)angular blockyaggregate (i.e. physicogenic aggregate) (Photos Pascal Jouquet, IRD).

Fig. 2. Water runoff simulation: 1.5 L Methylene blue water was added in aconstant flow (75 mL s−1) on the top of a 50⁎30 cm Plexiglas plate. Waterreached 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) andphysicogenic aggregates (ANG) and vegetation.

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the overall density of earthworms and the specific abundance ofA. khami were considered. This earthworm species is con-sidered as anecic sensu Bouché (1977) and create verticalgalleries until more than 60 cm deep. Its size is highly variableand individuals can probably reach more than 50 cm long at theadult stage.

Above-ground soilmacro-aggregatesN5mmwere collected inthe different land-use types in 25×25 cm plots (n=9, Fig. 1).According to the definitions described in Bullock et al. (1985) andPulleman et al. (2005), we distinguished three groups:(i) epigeous casts (fresh casts: CAST), (ii) free biogenic aggre-gates (rounded shape macro-aggregates that were clearly iden-tified as belonging to old casts: ROUND) and (iii) physicogenicaggregates (angular to subangular blocky macro-aggregates:ANG). The morphological fractions were air-dried and weighedto determine their relative mass contribution.

Table 1Overall 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)

Density (ind m−2)

Total Amynthas khami

CAS 4.29 (±0.70) 0.00 (±0.00)FAL 13.50 (±2.03) 0.25 (±0.44)BRA 9.60 (± 1.38) 0.30 (± 0.42)EUC 6.99 (±1.13) 1.11 (± 0.59)FOR 4.03 (± 0.57) 0.63 (± 0.78)

Table 2Weight of air-dried macro-aggregates on soil surface (kg m−2): casts (CAST),free rounded shapes biogenic aggregates (ROUND) and free angular andsubangular 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)

CAST ROUND ANG

CAS 0.00 (±0.00) 0.00 (±0.00) 0.93 (±0.59)FAL 0.88 (±1.18) 2.15 (±1.74) 0.31 (± 0.28)BRA 0.11 (± 0.11) 0.82 (± 1.03) 0.20 (± 0.28)EUC 11.57 (± 2.80) 11.24 (± 4.42) 0.00 (± 0.00)FOR 4.42 (± 1.10) 3.27 (± 0.77) 0.00 (± 0.00)

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2.3. Water runoff simulation

Water runoff simulation was carried out by adding 1.5 Lmethylene blue water (≈5 g L−1) with a constant flow of 75 mLs−1 on the top of a 50⁎30 cm Plexiglas plate (Fig. 2). The platelay on the ground and water reached the soil with a uniform frontof 30 cm width. Given the pronounced impact of slope gradienton runoff and detachment (Janeau et al., 2003), a same slope wasset, around 40%, which is the mean slope value of the catchment.Litter was carefully removed from the soil surface to avoid anyinfluence on water runoff. Methylene blue water dyed the soil

Fig. 3. Principal components analysis (PCA) on the water runoff simulation in the diffEUC: fallow and Eucalyptus regrowth; FOR: forest): distance (cm) and surface (cproperties (% cover of herbs, % aggregates: casts (CAST), biogenic- (ROUND) andplate). Control plots without free macro-aggregates on soil: CAS, FAL, BRA; PlotROUND aggregates: FALround, BRAround, FORround, EUCround; Plots with ANGplane defined by axes 1 and 2 of the PCA.

and allowed us to immediately and precisely determine thedistance and surface covered by water runoff. Different parame-ters 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 framepositioned 3 cmdownslope from the Plexiglas plate. Themaximumdistance covered by water runoff (cm) was measured while theoverall area covered by water runoff (cm2) was visually estimated.The ratio distance:surface was used as an index to describe theshape of runoff (i.e. linear surface vs sheet surface runoff). Soilmoisture was measured in the surrounding environment.

This experiment was done in each land use in differentiatingareas without macro-aggregates (control plots: CAS, FAL andBRA, n=5), to areas with CAST and ROUND aggregates(FALcast, n=6; BRAcast, n=10; FORcast, n=12; EUCcast,n=6), areaswith onlyROUNDaggregates (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%,

erent land use (CAS: cassava plantation; FAL: fallow; BRA: bracharia plantation;m2) covered by coloured runoff dyed water, soil moisture (%), and soil coverphysicogenic aggregates (ANG) in a 25⁎25 cm square beneath the Plexiglass

s with CAST and ROUND: FALcast, BRAcast, FORcast, EUCcast; Plots withaggregates: CASang. (a) Correlation circle. (b) Ordination of the samples in the

Fig. 5. Distance:surface runoff ratio in the different land use (CAS: cassavaplantation; FAL: fallow; BRA: bracharia plantation; EUC: fallow andEucalyptusregrowth; FOR: forest). Control: bare soil; ANG or ROUND: soil covered byphysicogenic and biogenic macro-aggregates, respectively; CAST: soil withearthworm casts on the soil surface (bars indicate standard errors).

Fig. 4. The relationship between casts (CAST), biogenic (ROUND) andphysicogenic (ANG) aggregates (% soil cover) and dye stained area (cm2)(n=66). Linear regression lines describing the relationship between soilaggregates and dye stained area are fitted (y=−5.28x+757.30, R2=0.490,Pb0.001 for ROUND; y=−7.42x+595.73, R2=0.259, Pb0.001 for CAST;R2=0.01, P=0.198 for ANG).

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representing the average slope value of the catchment. Plotswere bounded by rigid metal frames inserted to a depth of0.10 m. Soil detachment and water runoff were collected aftereach rainfall event, from May to October 2005, in a collector atthe outlet of the plot, as described by Janeau et al. (2003).Runoff rate was determined by the ratio of the quantity of waterthat runoff in the collector tank to the amount of daily rainfall.Soil detachment rate was determined through the measurementof sediment weight after filtration from runoff water and heatingat 105 °C. This sediment weight is assumed to represent thequantity 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 ofvariance using the Levene's test and log-transformed whenrequired. Differences between treatments were tested throughanalysis of variance (ANOVA). Principal component analysis

(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). Allstatistical calculations were carried out using R (R DevelopmentCore 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

The abundance of earthworms and more specifically ofA. khami was site-specific (Table 1). The density of earthwormwas low and earthworms were mainly endogeic species (non-pigmented earthworms). Earthworms were more abundant inFAL as compared to the other agro-ecosystems. A. khami wasnot find in CAS and densities were very low in BRA and FAL(b1 ind m−2), without any significant difference between them(PN0.05). Although the density of A. khami in FOR was higherthan in FAL and BRA and lower than that in EUC, the resultswere not significantly different (PN0.05 in all the cases). Thedensity of A. khami was the highest in EUC (approximately1 ind m−2) and significantly different from that in FAL andBRA (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 ofCAST in FOR was greater than in FAL and BRA (Pb0.001 inall the cases) and lower than in EUC (Pb0.001). Conversely, nosignificant difference occurred between the quantity of ROUNDin FAL, BRA and FOR (PN0.05) and the highest value wasfound in EUC (Pb0.001 in all the cases). CAS was charac-terized by the highest quantity of ANG aggregates (Pb0.005 inall the cases) and no significant difference occurred betweenFAL, BRA, EUC and FOR (PN0.05).

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 ANGaggregates) were clearly separated from plots with biogenicaggregates on the soil surface (i.e. CAST and ROUND) on axes1 and 2 that respectively explained 50.6 and 15.4% of the totalvariance. The correlation circle (Fig. 3b) clearly opposedvariables linked to earthworm activity (the surface occupied byCAST and ROUND) to those linked to water runoff (thedistance and surface covered by water runoff). More precisely,runoff magnitude (distance and surface covered by colouredwater) increased in the range CASbBRAbFAL. We did notfind any plots without ROUND in EUC and FOR. Plots withCAST and ROUND were also separated with the PCA, exceptin the case of BRA, and CASTwere associated with high waterinfiltration. CAS and CASang were also separated and ANGwere tightly associated with higher water runoff.

Correlations between soil surface occupied by macro-aggregates and dye stained area are shown in Fig. 4. Linearregressions were highly significant for CAST and ROUND(Pb0.001) and no significant for ANG (P=0.198).

The ratio of the distance to the surface of coloured water onsoil (i.e., shape of surface runoff) is shown in Fig. 5. This ratiowas 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 (gm−2 year−1) rates in the different landuse (CAS: cassavaplantation; FAL: fallow; BRA: bracharia plantation; EUC: fallow and Eucalyptusregrowth; FOR: forest) in 2005. : CAS,+: FAL, : BRA, : EUC,�:FOR. Linearregression describing the relationship between runoff and free- macro-aggregates isfitted (y=−4.07x+428.42, n=11, R2=0.922, Pb0.001). Data for FOR runoffwere not included in the linear regression.

respectively with FAL and BRA) and no difference occurredbetween FAL and BRA (P=0.676). Although this ratio was notsignificantly influenced by ANG, ROUND and CAST on thesoil surface (PN0.05 in all the situations), standard errors werethe highest with CAST (P=0.017 and 0.009, with control soiland soil with ROUND, respectively), suggesting an increase ofirregularities of surface runoff. Standards errors were notsignificantly different between control soil and soil plusROUND (P=0.864).

3.3. Runoff and soil detachment rates

The relations between soil detachment and runoff rates, andthe surface occupied by free macro-aggregates (CAST+ROUND+ANG) are shown in Fig. 6. Water runoff was nega-tively correlated with free macro-aggregates (Pb0.001), exceptin FOR where runoff rate was very low and not affected bymacro-aggregates. Conversely, soil detachment rate was notaffected by free macro-aggregates and values were very low(b100 g m− 2 year−1), except in CAS that was characterized byhigh 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 indicatorsto monitor different farming practices and different landscapestructures and transformations because they respond quickly toland-use change (Paoletti, 1999; Curry et al., 2002). Fragosoet al. (1997) found that earthworm biodiversity is modifiedwhen natural (i.e., undisturbed) ecosystems are replaced byagro-ecosystems. In this catchment, the density of earthwormwas low and species were mainly endogeic species. A. khamiwas mainly abundant in EUC and slightly less in FOR. Thisspecies is anecic (sensu Bouché, 1977) as determined throughfield observations and δ13C analysis of litter and casts (data notshown). As a consequence, the presence of this earthwormspecies is determined by litter incomes and/or specific micro-climatic conditions (e.g., surface temperature, soil humidity)(Lavelle et al., 1995). Hence, the low input of litter and organicresidues in FAL, BRA, and especially CAS, might be re-sponsible of the low survival and activity of this earthwormspecies while it did not affect the density of endogeic species.Conversely, this species was able to grow in EUC and FOR thatwere characterised by high litter content and, although notmeasured, probably higher soil moisture than in the other land-use types.

As a consequence, soil in EUC was covered with functionaland old earthworm casts that formed a typical granular horizonwith a high rugosity and macroporosity. The low density ofearthworms (approximately 1 ind m−2) and the high quantity ofCAST and ROUND covering the soil surface suggest that thesebiogenic structures are stable over a long period of time,probably many years. The high soil structural stability of earth-worm casts has been stressed by many authors (e.g., Shipitaloand Protz, 1988; Marinissen and Dexter, 1990; Barois et al.,

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1993). Blanchart et al. (1999) suggested that earthworm castscan last for a long time (N26 months in an African savannahecosystem) in kaolinitic soils owing to their high water stability.We assume that this is especially true in our study site whereclay content in earthworm cast is important (approximately 50to 68%), depending on land-use type (Jouquet et al., 2007).

Although A. khami was not found in CAS, free angularmacro-aggregates (ANG) occurred on the soil surface. Thesemacro-aggregates had a higher soil density (1600 kg m−3 ascompared to 970 kg m−3 for the surrounding soil) and waterstability than the surrounding soil (data not shown). The higherwater stability of these aggregates involved the protection ofsoil beneath against slaking and thus the formation of pedestalfeatures (i.e., micro-soil erosion column where the base is moreerodible that the top, see Fig. 7). Since soil macro-aggregateshad a higher density and soil structural stability to water, weassume that they were likely to be produced by earthworms.While A. khami could not be suspected to be responsible ofthese cast production, endogeic earthworms were found in eachland-use type and they were likely to be able to produce theseaggregates (Blanchart et al., 1999, 2004). We assume thatendogeic earthworms produced below-ground casts that couldbe exposed at soil surface during tillage or cassava harvest. As aconsequence, what we have called free physicogenic aggregates(ANG) so far in this paper might be considered as free biogenicaggregates of endogeic earthworms. Further analyses arehowever necessary to confirm this hypothesis.

4.2. Consequences for water infiltration and soil detachment

Soil aggregate stability in the top-soil is closely linked torunoff 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 preventdetachment of easily transportable particles, and thereby soilsurface clogging and runoff (Le Bissonnais, 1996; LeBissonnais and Arrouays, 1997; Barthès et al., 2000; Barthès

Fig. 7. Pedestal features in cassava plantation: The upper part of the soil columnis less prone to erosion decay by splash effect (harder meso-angular freeaggregate: ANG) and protect the lower part from detachment. The origin ofANG 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).

and Roose, 2002). Through the creation of stable aggregates(CAST and ROUND) on soil surface, A. khami significantlyincreased water infiltration into soil. In producing casts,earthworms enhance surface roughness, modify the circulationof surface runoff and reduce its velocity, and as a consequenceimprove water infiltration. Our data showed that the reductionof runoff water was more efficient with CAST than withROUND. Galleries beneath the casts might partially explain thismore efficient reduction of runoff (Chan, 2004). However, fieldobservations pointed out that galleries did not always constitutepreferential flow paths for water, especially when galleries werestill functional (i.e. occupied by earthworms) or when castsoccluded galleries. Casts deposited by A. khami are anchored insoil and there is continuity between below-ground galleries andabove-ground casts. Since casts are not broken, this continuityimpedes the infiltration of water in galleries.

The water runoff simulation clearly showed that surfacerunoff decreased with earthworm surface casting activity. Thisexperiment at the cm2 scale was confirmed by the annualmeasure of runoff rate at the 1-m2 scale in all the stations exceptFOR. The more agro-ecosystems have macro-aggregates on soilsurface and the less the runoff of water is important. The twostudies were complementary to discriminate the respectiveeffects of CAST, ROUND and ANG aggregates. Althoughwater runoff simulation was useful to discriminate the differenteffects of CAST and ROUND on runoff magnitude and shape(i.e., distance:surface ratio), this experiment did not take intoaccount soil detachment during rainfall events, and then did notconsidered splash effect and variation of water runoff velocity.In addition, the low quantity of coloured water added to soilmight make conclusions difficult. Conversely, although 1-m2

plots really provided runoff water rate in the field, they did notdiscriminate the respective effects of CAST, ROUND and ANGstructures 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 ROUNDaggregates deposited on soil surface were associated withhigher water infiltration during the water runoff simulation.This trend was not confirmed with 1-m2 plots. We assume thatthis difference can be explained by rainfall interception andpreferential stemflows that modified the amount and energy ofraindrops and associated runoff rates (Mosley, 1982; DuranZuazo et al., 2004). Effectively, pluviometers located belowAcacia and Venicia plantation showed a decrease of 20% of therainfall as compared to the meteorological station.

The influence of earthworms on soil detachment andsubsequent soil erosion might be positive or negative dependingon the age of casts. Aged or dried casts are usually considered asbeing more stable than the surrounding soil while freshlyegested casts are highly susceptible to dispersion and transportby water (Darwin, 1881; Shipitalo and Protz, 1988; Blanchartet al., 1999). Le Bayon and Binet (1999, 2001) found thatearthworm casts might obviously contribute to soil erosion intemperate gentle slope ecosystems (slope of approximately4.5%). On the contrary, although our study was establishedunder a tropical climate with a steep slope (about 40%) andstrong 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 lossof soil promoted by earthworms. Owing to these environmentalconditions, a higher soil detachment rate was expected fromearthworm casting activity. We assume that the high stability ofcasts, their rapid drying and the low earthworm activity (i.e. lowearthworm density) might explain the low contribution ofearthworm to soil loss even under intense rainfalls.

5. Conclusion

In conclusion, this study points out that earthworm castingactivity plays a significant role in decreasing water runoffvelocity and that biogenic structures are not prone to dispersionand erosion in steep-slope ecosystems of the Northern Vietnam.Different studies have demonstrated that earthworm casts can bewashed up and contribute to soil erosion. However, in thiscatchment, the effect of earthworm activity was considered asbeing rather beneficial in term of soil erosion and positiveeffects of casts and free biogenic aggregates (higher waterinfiltration) clearly counterbalanced negative ones (cast decayand dispersion of particles). Favouring anecic earthworms couldtherefore be an interesting strategy to decrease soil erosion insteep-slope tropical ecosystems of the Northern Vietnam.

Acknowledgements

We would like to express our gratitude to Samuel James andRobert Blakemore for their helps in earthworm identification.This project is financially supported by the Management of SoilErosion Consortium (MSEC) from the International WaterManagement Institute (IWMI), the Soils and Fertilisers Institute(SFI fromMARD, Hanoi, Vietnam) and the Institute of Researchfor Development (IRD from French Ministry of Research, unitresearch UMR137-BIOSOL and UR176-SOLUTIONS).

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