DSpace at VNU: Impact of fodder cover on runoff and soil erosion at plot scale in a cultivated catchment of North Vietnam

DSpace at VNU: Impact of fodder cover on runoff and soil erosion at plot scale in a cultivated catchment of North Vietna...

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Impact of fodder cover on runoff and soil erosion at plot scale in a cultivated

catchment of North Vietnam

Hai An Phan Haa,b, Sylvain Huonb,⁎ , Thierry Henry des Tureauxc, Didier Orangec, Pascal Jouquetc,

a

Vietnam National University (VNU), Faculty of Chemistry, 19 Le Thanh Tong, Hanoi, Viet Nam

b

Université Pierre et Marie Curie (UMPC), UMR 7618 Bioemco, Case 120, 4 place Jussieu, 75 252 Paris Cedex 05, France

c

IRD, UMR 7618 Bioemco, 32 Avenue Henri Varagnat, 93143 Bondy Cedex, France

d

Soil and Fertilizer Research Institute (SFRI), Dong Ngac Tu Liem, Hanoi, Viet Nam

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 21 May 2010

Received in revised form 19 January 2012

Accepted 23 January 2012

Available online 5 March 2012

Keywords:

Soil conservation

Paspalum atratum

Panicum maximum

Stylosanthes guianensis

Slope length

Plant cover

In Vietnam soil erosion is a major environmental problem with respect to soil fertility, water quality and downstream property damages and involves 40% of total land surface Due to a continuous and persistent de-crease of soil quality under annual crops, farmers gradually convert theirﬁelds to grazing lands and their crops to fodder cultures or tree plantations Experimental 1-m2ﬁeld plots with three replicates each were monitored for two years (2006–2007) to evaluate the impact of three different fodder treatments (Paspalum atratum, Panicum maximum and Stylosanthes guianensis) on runoff and soil detachment in a cultivated catch-ment of North Vietnam These expericatch-ments were designed to monitor at local scale the protective effect of vegetation cover against splash and rain-impacted erosion The lowest runoffs (ca 3.0–4.4%), sediment yields (ca 14–19 g m− 2yr− 1) and soil organic carbon losses (ca 0.7 g C m− 2a− 1) were obtained for P maximum that provided the best soil protection with respect to the two other treatments These values were low as compared to cultivated crops (cassava and rainfed rice) Soil surface characteristics (mainly biological activity and crusting) did apparently not play a key role, most likely because each plant cover provided, with its own

efﬁciency, protection against rainfall erosivity and rapid plant regrowth wiped out traces of ﬂow detachment The extent of soil detachment and sediment export, mainly controlled by cut and carry operations of fodder management, was reduced by increasing slope length from 1 to 5 m The choice of dense fodders such as P maximum appears to be, in terms of improved livelihood and environment sustainability, an interesting issue for uplands farmers

1 Introduction

In South-East Asian regions runoff and soil erosion are signiﬁcantly

related to agricultural land use in particular on sloping lands of

head-water catchments (i.e.,Sidle et al., 2006; Valentin et al., 2008) The

am-plitude of erosion seems to be more related to anthropogenic factors

such as land use change, deforestation, cultivation practice and crop

type than to climatic conditions (Chaplot et al., 2005; Craswell and

Niamskul, 1999) In Vietnam, a country covered at 75% by hills and

mountains, erosion involves 13 × 106ha, that represents 40% of total

land surface (Vezina et al., 2006) A large part of the former rain forest

was lost between the 1970s and 1990s to expand cultivation of

cassa-va, arrowroot, taro, maize or tree plantations (De Koninck, 1999;

Meyfroidt and Lambin, 2008; Sharma, 1992) Soil erosion involved

with cultivation affects the livelihood of farmers and thoroughly

hinders the economic development of upland catchments (Bui, 2003; Pimentel, 2006) Due to the continuous decrease of soil organic carbon with soil erosion, farmers gradually tend to convert theirﬁelds for-merly under annual crops, into grazing land, forage cultures or tree plantations (Castella et al., 2006; Horne and Stür, 1997; Tran et al.,

2004) The introduction of fodders opens new perspectives in sustain-able development as it responds to a political desire to integrate crops and livestock in upland farming systems of South East Asian countries (Clement and Amezaga, 2008; Orange et al., 2008)

Conversion of cultivated lands to forage is considered as a tool for conservation and stabilization of soil resources (Karlen et al., 2006) as well as for soil structure improvement and maintenance (i.e.,Juo et al., 1995; Stone and Buttery, 1989; Tisdall and Oades, 1979) Perennial forages strengthen the soil structure and stability by reducing rainfall kinetic energy, soil aggregates breakdown, splash and inter-rill ero-sion due to more efﬁcient leaf cover whereas higher root density in-creases soil carbon storage capacity (Conant et al., 2001; Gebhart et al., 1994; Lal, 2003; Reeder et al., 1998; Uri and Bloodworth, 2000) Fodder cultivation is also effective for the reduction of runoff and

⁎ Corresponding author Tel.: +33 1 44 27 72 82; fax: +33 1 44 27 41 64.

E-mail addresses: phanhahaian@gmail.com (H.A Phan Ha), sylvain.huon@upmc.fr

(S Huon).

Contents lists available atSciVerse ScienceDirect

Geoderma

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / g e o d e r m a

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rill erosion (Gilley et al., 2000; Rachman et al., 2008; Raffaelle et al.,

1997) by increasing surface roughness, lowering runoff velocity and

favoring inﬁltration and sediment deposition (Dabney et al., 1993;

Karlen et al., 2006) It is well established that soil physical properties

(Auzet et al., 1995) as well as surface characteristics such as crusting,

soil roughness and crop cover are major factors controlling runoff and

interrill erosion (Arnau-Rosalén et al., 2008; Durán-Zuazo and

Rodríguez-Plequezuelo, 2008; Le Bissonnais et al., 2005; Ribolzi et

al., 2011) Additional features particularly active in tropical

environ-ments, such as earthworm casts also decrease runoff (Bochet et al.,

1999; Jouquet et al., 2008; Podwojewski et al., 2008)

If the connections between soil surface characteristics, sediment

yield and runoff are well established (i.e.,Janeau et al., 2003; Ribolzi

et al., 2011), the protective role of plant cover is not well constrained

at plot's scales where splash and rain-impactedﬂows are the

domi-nant soil detachment processes (Bellanger et al., 2004; Chaplot and

lengths are also required to assess the impact of soil erosion at

land-scape's scale, mainly because additional processes such as runoff–

runon due to local variability of inﬁltration, rill and gullies erosion,

deposition along slopes as well as stream bank erosion control

sedi-ment transport and export across catchsedi-ments (Chaplot et al., 2009;

Van Noordwijk et al., 2004; Wang et al., 2010) The main objective

of this study was to assess, within a small agricultural catchment in

North Vietnam, the impact of three different fodder types on runoff

and soil erosion and soil organic carbon erosion using experimental

ﬁeld plots with two different slope lengths (1 m and 5 m) These

ex-periments were designed to monitor at local scale the protective

ef-fect of plant type against splash and rain-impacted sediment

mobilization using 1 × 1 m microplots (splash and inter-rill erosion)

and to monitor with a slightly longer slope length (5 × 1 m

micro-plots) the extent ofﬂow detachment and sediment transport

(incipi-ent rill erosion) Because the comparison between the behavior of

fodders in terms of improved livelihood (i.e., biomass production for

cattle) and environment sustainability (i.e., soil protection) was the

major issue of this experimental study for uplands farmers, two

main questions have been addressed: 1) what fodder type among

the chosen species insured the best soil protection against splash

and rain-impacted erosion? and, 2) what was the local impact of

slope length on runoff and sediment yield?

2 Materials and methods

2.1 Site location and soil main characteristics

Experiments were carried out in the Dong Cao watershed, located

in the Hoa Binh Province of North Vietnam, approximately 50 km SW

of Hanoi (20°57′N, 105°29′E,Fig 1) Stream discharge, soil erosion

rates and land use/cover changes are monitored since 1999 in

the framework of the MSEC activity (Valentin et al., 2008) The

catch-ment covers 46 ha of cultivated sloping lands, surrounded by hills,

with average and steepest slopes of 40% and 120%, respectively Annual

rainfall ranges 1200–1800 mm of which 80–85% takes place during the

rainy season between April and October The mean daily temperature

is 15–25 °C with a high 75–100% relative humidity and the annual

potential evapo-transpiration reaches ca 960 mm

The dominant soil types are kaolinite-bearing Acrisol and Ultisol

(WRB, 2006) set up on Mesozoic volcano–sedimentary schist formations

They are very porous with a bulk density close to 1 g cm−3, a thickness

that generally exceeds 1 m with high local variability and have low pH

(b5.0) and low cation exchange capacity (Jouquet et al., 2007)

2.2 Treatments

The efﬁciency of soil protection by plant cover was monitored

using three palatable species, two grasses and one leguminous

sub-shrub Their main characteristics are resumed in the following Atra-tum (Paspalum atraAtra-tum, PA) is a leafy upright perennial tussock grass from South America usually less than 1 m high (up to 2 m during ﬂowering) that is used for long-term pastures and can be combined with agro-forestry applications (Kalmbacher et al., 1997; Quarín et al., 1997) This fodder easily grows on sandy to clay soils and tolerates low fertility and pH conditions It also responds to improved nitrogen fertility and is adapted to heavy rainfalls (1500–2000 mm a−1) Guinea grass (Panicum maximum, PM) is a perennial tufted grass from Africa that produces large leaves and whose height varies from 1 to 3 m (Bogdan, 1977) It may be used for long-term pastures and soil conser-vation if its fertility is maintained The advantages of PM are a high pro-duction rate, a high quality food for cattle and a good resistance to grazing and trampling This forage, suitable for clay and sandy soils, is also resistant to drought, i.e., cumulated rainfall lower than 400 mm for eight months Fine stem Stylo (Stylosanthes guianensis, SG) is a pas-ture legume originating from South America (Mannetje, 1992) that can grow to 1.2 m-heights with 0.5–4.5 cm long- and 0.2–2.0 cm wide-tri-foliate leaves on acid soils under a large range of rainfall conditions (700–5000 mm a−1) Stylo responds well to improved soil fertility and can intercrop with other cultivated plants such as rice, cassava or maize

2.3 Experimental setting Sediment yield, runoff and soil surface characteristics wereﬁrst monitored in 2006 using nine 1-m2 microplots (1× 1 m) set up on 15% slopes for the three treatments, with three replicates each (Fig 2) All plants were seeded in April 2006 (day 96) During the course of the study in 2007, PM and PA were cut three times (July 9— day 190, August 31— day 243 and October 21 — day 294) at 20 cm above ground level whereas SG was only cut two times (June 19— day 170 and September 27— day 270) at 5 cm

Fig 1 Location of the Dong Cao catchment in North Vietnam (top) and SE view of the surrounding hills with main streams and watershed limits (bottom).

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Each plot was surrounded by metal cut-off walls inserted to a soil

depth of 10 cm on three sides and a sediment–runoff collection system

installed at the down slope side (e.g.,Felipe-Morales et al., 1977;

Parsons et al., 2006) In 2007, six additional 5-m2 plots (1 × 5 m)

planted with PA and PM (three replicates each) were added using the

same design but with a longer slope length Runoff and sediment yields

were determined after each rainfall following a procedure described by

Janeau et al (2003)

2.4 Analytical procedures

The rainfall characteristics (amount, duration or average intensity)

were computed with 6-min time lags using the automatic rain gauge

of the weather station installed in the Dong Cao catchment near the

experimental plots (Valentin et al., 2008) Runoff coefﬁcients were

calculated by dividing the volume of water collected in the tank by

total rainfall recorded by the rain gauge, i.e., for PA 1-m2microplots

in 2006, 191.6 l m− 2of water were recovered with a yearly rainfall

of 1252 l m− 2(mm) providing a runoff coefﬁcient of 15.3% Student's t

tests were used to determine whether correlation coefﬁcients or

dif-ferences between mean values differ or not signiﬁcantly from zero

(Snedecor and Cochran, 1980) The amount of sediment recovered in

the collection tanks after each rainfall was determined fromﬁltered

sample aliquots (Millipore® 0.2μm cellulose ﬁlters) dried at 60 °C

This load represented the total loss at plot's scale for each rainfall

and accounted for the delivery of soil particles and vegetation debris

Topsoil surface characteristics together with vegetation cover

were described ﬁve times in June, July, August, September and

October 2007 for each 1-m2plot following the procedure ofCasenave

and Valentin (1992) All other soil properties were estimated in

September 2007 with nine samples per treatment Bulk soil densities

were determined after drying at 105 °C Cation exchange capacities

(CEC) were measured by exchange with cobaltihexamine cations at

natural soil pH (AFNOR NF X31-130, 1993) Soil textures were

deter-mined for each treatment after sieving soil aliquots to four grain-size

fractions, clay (b2 μm), ﬁne silt (2–20 μm), coarse silt and ﬁne sand

(20–200 μm) and coarse sands (200–2000 μm) Hydraulic conductivity

was measured in September 2007 for each treatment with

30× 30× 25 cm samples of crusted soils, soils located slightly above

pedestals and soils located immediately below earthworm casts using

a Decagon mini-disk inﬁltrometer (Coquet et al., 2000) Each measure-ment was repeated three times at 0.05 kPa (labeled as K5)

Total Organic Carbon (TOC) and Total Nitrogen (TN) concentra-tions were determined for topsoil (0–5 cm) horizons at the end of the 2007 rainy season using nine soils per treatment and forﬁltered sediment loads exported after each erosive rainfall by each treat-ment using a Carlo-Erba NA-1500 NC Eletreat-mental Analyser facility of UMR Bioemco Measurements were carried out for aliquots of soils previously sieved at 2 mm and aliquots of total detached sediment loads recovered in the collection tanks All samples were previously dried at 105 °C,ﬁnely grounded and weighted for analysis All TOC and TN concentrations are reported in mg g− 1of dry sample Analyt-ical precision was ± 0.1 mg C g− 1and ± 0.05 mg N g− 1, for TOC and

TN, respectively Data reproducibility was checked with a tyrosine laboratory standard (Girardin and Mariotti, 1991) and by replicate analyses of selected samples No preliminary removal of carbonates was required

3 Results 3.1 Soil main characteristics All soils displayed equivalent characteristics for most of mea-sured parameters, i.e., mean TOC contents of 27.2 ± 1.3, 30.4 ± 5.2 and 30.2 ± 1.3 mg C g− 1for PA, PM and SG, respectively (Table 1)

As for soil organic matter composition, texture was relatively ho-mogenous with clay contents ranging ca 47.3–49.5% and there was

no signiﬁcant difference between treatments with respect to CEC, bulk soil density and root density

Topsoil surface observations performedﬁve times in 2007 showed that less than 10% of plot surfaces were crusted and between ca 52% and 92% were occupied by free soil aggregates with limited occur-rence of vegetation residues (b6%) The surface covered by free aggre-gates, neither anchored to soil surface such as earthworm casts nor embedded in a crust (Casenave and Valentin, 1992) Hydraulic con-ductivity was always lower for SG than for PA and PM treatments with a high variability between replicate plots (high standard devia-tions from mean values,Table 1) The fodder growth rates, estimated

by mean plant height measurements, were 1.0–1.5 cm day− 1for PA and PM treatments (i.e., 1.2 cm day− 1between days 190 and 241) Fig 2 Experimental 1-m 2 and 5-m 2 plot designs for the three treatments.

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and 0.2–1.0 cm day− 1for SG treatment (i.e., 0.6 cm day− 1between

days 170 and 241) until the end of September (Fig 3)

At the end of the rainy season in October plant growth rates

de-creased to half of their former values During the course of this study,

cut and carry operations controlled the extent of soil cover in each

treatment as shown by changes in the percentage of soil surface cover

3.2 Rainfall characteristics

Total rainfall amounted 1252 mm and 1409 mm in 2006 and 2007,

respectively Theﬁrst year precipitation intensity was rather low with

only one event exceeding 60 mm h− 1whereas 15 were recorded the

following year (Table 2) In 2006, 74% of total annual rainfall occurred between May and August against 41% during the same period in 2007 Summer monsoon was still active in September and October 2007 providing 43% of total annual rainfall

3.3 Runoff and soil loss measurements for 1-m2plots in 2006 and 2007 Although rainfall intensity and distribution patterns were different, similar mean runoffs were obtained for 2006 and 2007 (Table 2) The lowest coefﬁcients were obtained for PM treatments with 3.5±0.5% and 4.4 ± 1.0% in 2006 and 2007, respectively These values were

sig-niﬁcantly different (Pb0.001) from those obtained for the other treatments with 15.3 ± 1.8% in 2006 and 11.2 ± 0.7% in 2007 for PA, 12.4 ± 3.3% in 2006 and 14.5 ± 1.5% in 2007 for SG In 2006, the high-est runoff data followed the heavihigh-est rainfalls, 262 mm and 439 mm for July and August 2006, respectively, 331 and 267 mm for September and October 2007, respectively However, maximum runoff did not strictly match total precipitation as it also depended on daily rainfall maximum As for runoff, higher soil losses were measured for

PA treatments (252.1 ± 105.1 g m−2in 2006 and 98.8 ± 58.7 g m−2in 2007) and SG treatments (69.8 ± 51.0 g m−2in 2006 and 118.5± 98.2 g m−2 in 2007) than for PM treatments (13.8 ± 4.3 g m−2 in

2006 and 18.9± 8.4 g m−2in 2007) that provided the lowest sediment export However, the variability of soil loss for each treatment was high (high standard deviations from mean values,Table 2) For in-stance during the October 5 rainfall event in 2007, the three SG plots provided equivalent runoff coefﬁcients, 91.5%, 91.5% and 84.7%, but highly contrasted sediment yields, 114.3, 12.9 and 6.6 g m− 2, respectively Such a high variability was also observed for other 2007 rainfall events as for June 26, July 5, September 7 and

24 Runoff and sediment yields were positively correlated for each treatment The relationship was better deﬁned for PA (r=0.97,

Pb0.001) and PM (r=0.93, Pb0.001) than for SG (r=0.71,

Pb0.001) Sediment yields were also characterized by thresholds (ca 5 g m− 2) above which the amount of sediment exported tends towards a plateau with respect to runoff (Fig 4)

Mean monthly runoff and sediment yields were thus thoroughly reduced to less than 10% of their respective values, when vegetation cover exceeded 40% of total soil surface

3.4 Runoff and soil loss measurements for 5-m2plots in 2007 With increasing slope length mean runoff coefﬁcients decreased from 11.2 ± 0.7% (1-m2 plots) to 2.4 ± 1.0% (5-m2plots) and from 4.4± 1.0% (1-m2plots) to 3.0± 0.8% (5-m2plots), for PA and PM, re-spectively (Table 2) In contrast to PA, the PM treatment did not pro-vide a signiﬁcant difference Mean sediment yields followed the same trend with a drastic drop in soil loss per surface unit, from 98.8 ± 58.7 g m− 2(1-m2plots) to 10.3 ± 3.6 g m− 2(5-m2plots) for

PA but only a slight decrease for PM (Table 2)

3.5 Total organic carbon export for 1-m2and 5-m2plots There was no signiﬁcant difference between the TOC contents of detached sediments exported from 1-m2and 5-m2plots whatever the plant cover was (Table 3) The low TOC/TN ratios (range 10–11) showed that particulate organic matter was mainly released by the breakdown of soil free aggregates (characterized by low TOC/TN values) that represented between ca 52–92% of soil surface cover rather than by coarse organic matter with high contents in vegetation debris (that should display high TC/TN values) The TOC enrichment ratios of detached sediments (ratios of mean TOC concentration in sediments to mean TOC concentration in topsoil 0–5 cm layer) were rather high (1.4–1.7,Table 3)

As TOC yields were proportional to sediment export a decreasing trend, SG–PA>PM, can also be inferred with 1-m2plot measurements

Table 1

Main soil characteristics for the three treatments.

Mean σ a

Mean σ a TOC b

mg C g− 1 27.2 1.3 30.4 5.2 30.2 1.3

TN c mg N g − 1 2.5 0.1 2.0 0.1 2.5 0.3

CEC d c + mol kg− 1 13.5 1.4 12.8 1.0 14.4 1.2

[200–2000 μm] wt.% e

2.9 0.1 3.5 0.6 2.6 0.2 [20–200 μm] wt.% e

19.7 2.3 16.6 1.6 17.9 2.0 [2–20 μm] wt.% e

30.2 2.1 30.3 2.3 30.0 2.5 [b2 μm] wt.% e

47.3 1.4 49.7 1.5 49.5 0.6 Soil density g cm− 3 0.8 0.1 0.9 0.1 0.9 0.0

Root density g cm − 3 0.9 0.1 0.9 0.1 1.1 0.1

K5 f

10− 5m s− 1 7.5 3.2 7.2 3.0 4.6 0.7

PA = Paspalum atratum, PM = Panicum maximum, SG = Stylosanthes guianensis Nine

soils per treatment.

a

Standard deviation.

b Total organic carbon concentration.

c Total nitrogen concentration.

d

Cation exchange capacity.

e

Weight percent.

f

Hydraulic conductivity at 0.05 kPa.

Fig 3 Evolution of plant height and average vegetation cover area for the three

treat-ments in 2007 Vertical bars refer to plant height and open circles to plant cover area.

Standard deviations are reported for each cover area measurement or included in open

circle symbols.

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But the difference between PA and PM treatments is no longer ob-served with an increasing slope length to 5 m

4 Interpretation and discussion 4.1 Comparison between treatments at 1-m2microplot scale Some discrepancies between replicate plots of the same treatment were found They were not linked to differences between soil surface characteristics or properties (Table 1) When cumulated values are plotted against time, one of the PM plots (PM3 inFig 5) provided much lower runoffs and soil losses than the two others, apparently

reﬂecting more efﬁcient protection against rainfall erosivity and soil detachment Runoff thoroughly increased for the two other plots (PM1 and PM2), following the September 9 (day 252) and October

5 (day 278) heavy rainfalls

Theﬁrst major rainfall took place only nine days after the second cutting operation (day 243) Runoff increased from 2.1 to 24 mm, 3.2 to 20 mm and 2.0 to 18 mm whereas, sediment yields increased

Table 2

Total rainfall, average runoff coefﬁcient and total sediment yield for 1-m 2 and 5-m 2 plots for the three treatments (3 replicates each).

1-m 2 plots Runoff coefﬁcients (%) Sediment yield (g m − 2 )

Mean σ a

August 439.5 27.0 1.9 6.2 1.9 17.4 4.9 197.3 84.0 1.4 1.2 14.8 9.1

2006 1252 15.3 1.8 3.5 0.5 12.4 3.3 252.1 105.1 13.8 4.3 69.8 51.0

September 330.5 17.6 1.5 8.8 0.6 16.1 0.8 35.9 8.3 8.7 4.7 9.5 4.0 October 276.5 28.8 6.1 7.2 2.8 38.9 1.9 55.5 53.5 4.2 4.3 46.7 62.7

2007 1409 11.2 0.7 4.4 1.0 14.5 1.5 98.8 58.7 18.9 8.4 118.5 98.2 5-m 2

plots Runoff coefﬁcients (%) Sediment yield (g m− 2)

Mean σ a

–=no runoff or sediment yield, PA=Paspalum atratum, PM=Panicum maximum, SG=Stylosanthes guianensis.

a Standard deviation.

Fig 4 Plot of the relationship between runoff and sediment yield at 1-m 2

plot's scale (log scale) for each rainfall of 2007 and for the three treatments PA= Paspalum atratum,

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from 1.0 to 3.2 g m− 2, 1.3 to 11.8 g m− 2and 0.5 to 1.6 g m− 2for

PM1, PM2 and PM3 plots, respectively In terms of soil detachment,

PM1 and PM2 plots provided the highest cumulated amounts of

sed-iments Because the survey of soil surface characteristics did not show

evidence for different evolutions between plots, the high amount of

sediments collected for PM2 corresponded to pre-accumulation in

the collection pipe, most likely after day 243 These sediments were

ﬂushed out during the following heavy rainfall event (day 252) A

similar observation could be drawn for PM1, following another

heavy rainfall event (October 5, day 278) (Fig 5) As shown for this

treatment discrepancy between replicates could not be avoided but

the average and cumulated trends that could be derived from our

data still remain reliable

For all treatments the highest runoffs and sediment yields were

found when heavy rainfalls followed cutting operations, i.e., August

31 (day 243) followed by heavy rainfalls in September 9 (day 258

with 114.5 mm) and October 5 (day 278 with 118 mm) It was not

the case for earlier cuttings, June 19 (day 170) and July 9 (day 190)

for SG and PA, respectively, only followed by low and much less

ag-gressive precipitations (Fig 6)

However a better management of the soil erosion risk by pro-gramming cutting operations during periods of low rainfall remains difﬁcult as the distribution of precipitation during the wet season changes from year to year, i.e as shown in this study for 2006 and

2007 Because of its tufted and dense structure (Fig 2), PM acted as

a physical barrier that reduced runoff and improved water inﬁltration

Table 3

Mean TOC and TN content of suspended sediments exported from 1-m 2 and 5-m 2 plots for the three treatments (3 replicates each) during the 2007 rainy season.

(g m− 2) (mg C g− 1) (g C m− 2) (mg N g− 1) 1-m 2

PA 70.9 ± 48.9 46.4 ± 9.0 3.29 ± 2.27 1.7 4.5 ± 0.9 10.3 ± 1.0

PM 13.6 ± 8.5 47.8 ± 9.4 0.65 ± 0.41 1.6 4.7 ± 1.2 10.2 ± 1.4

SG 97.2 ± 82.4 48.2 ± 10.6 4.69 ± 3.97 1.6 4.8 ± 1.1 10.0 ± 0.7 5-m 2

PA 18.2 ± 19.5 40.9 ± 6.4 0.74 ± 0.39 1.5 3.8 ± 0.7 10.8 ± 1.2

PM 16.4 ± 15.4 42.8 ± 13.1 0.70 ± 0.66 1.4 3.9 ± 1.2 11.0 ± 1.0

a

Total suspended sediment load.

b Total organic carbon concentration.

c Total organic carbon yield

d

TOC enrichment ratio sediment vs soil.

e

Total nitrogen concentration.

f

Ratio of total organic carbon to total nitrogen concentrations.

Fig 5 Precipitation (vertical bars) and cumulated runoff (top) and sediment yield

(bottom) for the three PM 1-m 2

microplots in 2007 Dashed lines correspond to cutting

Fig 6 Plots of precipitation (vertical bars), cumulated runoff and sediment yield for PA,

PM and SG 1-m 2

microplots in 2007 Open and closed squares refer to runoff and sediment yield, respectively Dashed lines correspond to cutting days PA= Paspalum atratum,

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by channeling rainwater via its root system This fodder also provided

the best soil protection with respect toﬂow detachment and

sus-pended particles transport as pictured by low sediment yields (ca

14–19 g m− 2a− 1,Table 2) and reported in other studies (Nyangito

et al., 2009; Rietkerk et al., 2000) Although its growth rate was

equiv-alent, the distribution of PA sprouts (Fig 2) had more limited inﬂuence

on runoff and soil detachment than PM Accordingly interrill erosion

was responsible of higher sediment yields (ca 100–250 g m− 2a− 1,

Table 2) Lower plant height, leaf size and pedestal thickness for SG

provided even more limited shelter against raindrop impacts (Fig 2)

With total sediment yields of ca 70–120 g m− 2a− 1(Table 2) soil

pro-tection of SG was intermediate between the two others This behavior

was supported by lower conductivities (K5,Table 1) in regard to those

measured in PA and PM plots, favoring runoff with respect to in

ﬁltra-tion during rainfall events The best soil protecﬁltra-tion was thus obtained

for PM with 5–20 times and 2–8 times lower yields than for PA and SG,

respectively

The results of these experiments can be compared to those carried

out with 1-m2microplots in 2004–2005 in the Dong Cao watershed

(Podwojewski et al., 2008) and in 2003 in the Houay Pano watershed

in Laos (Chaplot and Poesen, 2012) Plot of annual mean runoff

against soil detachment highlights the impact of soil protection by

different plant covers and for different rainfall conditions but with

slightly steeper slopes than in our study (Fig 7)

Mean annual runoff and sediment yield determined for PM are

consistent with those reported for Bracharia, another fodder that

pro-vides high plant cover of soil surface The two other fodders, PA and

SG, with higher values fell in the range of young fallow and cassava

(for one of the two years) known to favor, due to weeding and to

its architecture, more soil erosion than other cultivated plants

(Putthacharoen et al., 1998; Valentin et al., 2008) However all three

fodders had much lower sediment yields than for rice, extensively

cultivated by slash and burn along steep slopes in Laos (Chaplot and

Poesen, 2012) As expected, tree covers provided a better soil

protec-tion than fodders because the canopy intercepts part of the raindrops

and reduces as much splash at ground level (Kozak et al., 2007),

ex-cept for some species with large leaves (i.e teak,Nair, 1993) In the

Dong Cao watershedPodwojewski et al (2008)also showed a

posi-tive correlation between soil surface crusting and annual runoff

under cassava and Bracharia Negative correlations between runoff

and earthworm cast surface cover were also reported byJouquet et

al (2008)for various soils under cassava, Bracharia and Eucalyptus

In our study, soil surface properties were not signiﬁcantly different

between PA and PM treatments although contrasted runoff and

sedi-ment yields were determined For both treatsedi-ments, soil surfaces were

mainly occupied by free soil aggregates and the percentages of soil surface cover by crusts, casts and vegetation residues were rather low It is also likely that root density (0.9–1.1 g cm− 3,Table 1) did not play a key role with respect to inﬁltration and sediment transport

in our study as shown elsewhere with experimental and modeling approaches (De Baets and Poesen, 2010; Zhou and Shangguan,

2008) The overall experiments showed that runoff and soil detach-ment were linked both to rainfall intensity (with a threshold effect for soil detachment with respect to runoff) and vegetation cover

Fig 3shows that erosion was detachment limited due to a densi fica-tion of vegetafica-tion cover Conversely, cutting operafica-tions of the fodder enhanced soil losses but rapid regrowth limited soil losses over the cycle (e.g.,Durán-Zuazo and Rodríguez-Plequezuelo, 2008) It is pos-sible that temporary rills not observed during soil surface surveys appeared after major erosive rainfall events but werefilled up or hid-den by plant growth The impact of fodder cultivation on soil erosion

is also supported by decreasing annual sediment yields at the outlet

of the Dong Cao watershed, from 3.6 Mg ha− 1a− 1 before 2002 to 0.1–0.3 Mg ha− 1a− 1after 2004, following the replacement of cassava

by fodder and acacia in the catchment (Orange et al., 2008)

Low TOC/TN ratios for suspended organic matter recovered at the outlet of the plots indicated that most of soil detachment originated from the breakdown of surface soil aggregates These ratios rather matched the composition ofﬁne sized clay-bound organic matter than coarse vegetation debris (Feller and Beare, 1998) and were con-sistent with high occurrences of free soil aggregates These aggregates tend in turn to be embedded in packing crust when soil is bare or in-sufﬁciently covered (Janeau et al., 2003; Podwojewski et al., 2008; Ribolzi et al., 2011) but resist here to crusting due to the dense vege-tation cover of fodder crops With high TOC enrichment ratios (1.4–

with respect to soil average TOC contents as shown for the Houay Pano catchment in Laos (e.g.,Rumpel et al., 2006) However in con-trast, particle size sorting by rain-impacted erosion and runoff was more likely responsible for the TOC enrichment of detached sedi-ments than residual “charcoal fragments” Light organic matter bound to clay size fractions with low TOC:TN ratios was preferentially released during the breakdown of soil aggregates (i.e.,Bellanger et al., 2004; Legout et al., 2005; Palis et al., 1990; Wan and El-Swaify, 1997) Because TOC releases were proportional to sediment yields (e.g.,

Gregorich et al., 1998), lower TOC deliveries were found in 2007 for

PM treatments (ca 0.65 g C m− 2a− 1, Table 3) than for the two others (ca 3.3 and 4.7 g C m− 2a− 1, for PA and SG, respectively,

Table 3) All soil organic carbon losses were low when compared to 1-m2 experiments under rainfed rice carried out in Laos (11.2–

Fig 7 Mean annual runoff and soil detachment determined for 1-m 2

microplots with different plant covers in the Dong Cao watershed Data for cassava, Bracharia, fallow,

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Euca-30.8 g C m− 2a− 1,Chaplot and Poesen, 2012) These results indicate

that the decline of soil fertility (in terms of soil organic carbon

con-tent) will be more limited with fodders and in particular with PM as

compared to the two other plants

4.2 Local impact of slope length on runoff and sediment yield

Field observations carried out in the course of this study showed

that the extension of plant cover was similar at 1-m2and 5-m2plot

sizes Therefore the observed deviations were mainly linked to slope

length It is known that when transport distance increases inﬁltration

along slope is favored, reducing as much runoff (Le Bissonnais et al.,

1998; Poesen et al., 2003) Moreover, grass pedestal bands also

con-tribute to a reduction of runoff velocity and to trapping or sorting of

detached sediments The scale ratios of 5-m2to 1-m2plots runoffs

and soil detachment are reported inFig 8together with precipitation

and plant cover change

Because scale ratios were always lower than 1.0 for the two

treat-ments, our results showed that runoff and sediment yield were both

reduced when slope length increased from 1 m to 5 m This effect

was more pronounced for PA (in average 80% and 90% for runoff

and sediment yield, respectively,Table 2) than for PM (in average

30% for runoff and sediment yield,Table 2) The combined impact of

rainfall intensity and plant cover on scale ratios is difﬁcult to assess

because only a limited number of surface cover surveys was

per-formed (Fig 8) However, the lower ratios were found for PA at the

end of the rainy season when vegetation growth and cover were

low and precipitation still important These results are in agreement

with those displayed byStomph et al (2002)who observed a

reduc-tion of runoff on bare soil experiments with increasing 1.5, 3, and 6 m

slope lengths Equivalent conclusions are drawn for splash erosion

and sediment yields reported byChaplot and Poesen (2012)when

plot size increased from 1 to 2.5 m2 However lower runoff and soil

detachment values for 1-m2than for 5-m2experiments were also

ob-served, when vegetation cover was reduced and accompanied by the

extension of structural crusts and small temporary ponds along slopes (Chaplot and Le Bissonnais, 2003) It was not the case in our study because crusted areas always represented less than 10% of soil surface within each plot favoring inﬁltration whereas plants acted

as physical barriers against splash and rain impacted soil detachment and sediment transport

5 Conclusion

A two years' monitoring of runoff and sediment yield from 1-m2

microplots under different fodder cover, set up on moderate 15% slopes of the Dong Cao catchment in North Vietnam, showed that P maximum provided the best soil protection with respect to splash and rain-impacted soil detachment compared to P atratum and S guianensis, the two other fodders tested These differences are most likely linked to the morphological characteristics of the plant species

in terms of soil cover Because total soil organic carbon losses were proportional to sediment yields, soil quality (in terms of total organic carbon content) should also be better preserved with P maximum This fodder also provided a better protection compared to other veg-etation covers (young fallow, cassava and rainfed rice) tested with comparable experiments In this study, runoff and sediment yields were mainly controlled by rainfall intensity and soil cover extension with a threshold when plant covers exceeded 40% of plot surface Changes in soil surface characteristics (mainly biological activity and soil surface crusting) did apparently not play a key role, most likely because plant covers favored inﬁltration and reduced soil detachment

by rainfall Maximum runoff and suspended sediments yields were recorded when the cut and carry operations of fodder management were followed by heavy rainfalls Cutting of the fodder enhanced soil losses but rapid regrowth limited soil losses over the cycle The increase from 1 to 5 m in slope length contributed to reduce runoff and sediment yield by favoring sediment deposition and water in ﬁl-tration, in particular for P atratum that involved higherﬂow and soil detachments than P maximum The integration of crops and

Fig 8 Plots of precipitation and plant cover (upper graphs), runoff (middle graphs) and sediment yield (bottom graphs) 5-m 2

to 1-m 2

scale ratios in 2007 for PA and PM treat-ments Vertical bars are for precipitation, runoff and sediment yield, open and black circles for plant cover The dashed line refers to cutting operations PA = Paspalum atratum,

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livestock in upland farming systems should be better supported by

the use of dense fodders with high soil cover capacities

Acknowledgments

This study is part of H.A Phan Ha doctorate thesis (UMPC, Paris,

France) and was supported by a PhD grant from AUF (Agence

Uni-versitaire de la Francophonie) This work was also integrated in the

Management of Soil Erosion Consortium (MSEC) activity and the

DURAS program supported by the French Ministry of Foreign Affairs

The authors are grateful to Dr P Podwojewski (IRD), Nicolas Péchot

(UMR Bioemco), MSEC team members for their help during

ﬁeld-work and two anonymous reviewers for their comments of a former

version of this manuscript

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