at SciVerse ScienceDirect

Soil Biology & Biochemistry 50 (2012) 214e220

Contents lists available

Soil Biology & Biochemistry

journal homepage: www.elsevier .com/locate/soi lbio

The effect of earthworms on carbon storage and soil organic matter compositionin tropical soil amended with compost and vermicompost

Phuong-Thi Ngo a,b,*, Cornelia Rumpel b,**, Thu-Thuy Doan c,d, Pascal Jouquet d

a Institute of Environmental Technology (VAST), Hanoi, Viet Namb IRD, CNRS, UMR 7618 BIOEMCO (UMR CNRS e UPMC-UPEC e IRD e ENS-AgroParisTech), Campus AgroParisTech, Thiverval-Grignon, Francec Soil and Fertilizers Research Institute (SFRI), Hanoi, Viet Namd IRD, UMR 211 BIEMCO (UMR CNRS e Université Paris VI et XIIe IRD e ENS-AgroParisTech), Centre IRD Ile de France, Bondy, France

a r t i c l e i n f o

Article history:Received 14 November 2011Received in revised form23 February 2012Accepted 24 February 2012Available online 28 March 2012

Keywords:EarthwormsSoil organic matterCompostVermicompostTropical soil

* Corresponding author. Institute of EnvironmentaViet Nam.** Corresponding author.

E-mail addresses: thi-phuong.ngo@grignon.inra.frgrignon.inra.fr (C. Rumpel).

0038-0717/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.soilbio.2012.02.037

a b s t r a c t

The use of organic matter (OM) amendments is widespread in tropical countries and may be beneficialfor soil carbon storage. Interactions between earthworms and OM amendments in tropical soils arelargely unknown. The aim of this study was to investigate the effect of bioturbation on the quantity andchemical composition of OM in soil amended with compost and vermicompost. Our approach includedcomparison of soil samples amended with compost, vermicompost or chemical fertilizers in the presenceor absence of earthworms during a one-year greenhouse experiment. The soils were submitted toa regular cultivation cycle. After one year, we analysed bulk samples for soil OM elemental compositionand characterised its lignin and non-cellulosic carbohydrate components.

Our results showed a decrease of the carbon and nitrogen content in soil amended with chemicalfertilizers. Vermicompost amendment led to unchanged OC content, whereas the compost amendmentincreased the soils OC content compared to initial soil. The addition of earthworms reduced OC and Ncontent in soils with organic amendments. This is in contrast to soil amended with mineral fertilizer only,where the presence of earthworms did not have any effect. Bioturbation influenced the lignin signatureof the soils, and to a lesser extent the non-cellulosic carbohydrate signature. In conclusion, compostamendment combined with bioturbation influenced the quality and quantity of SOM and as result carbonstorage and its biogeochemical cycling in tropical soils. Implications for soil fertility remain to beelucidated.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Since a long time, mineral fertilizers have been widely used toincrease crop yields all over the world. Numerous studies haveshown that their use can have many negative effects on soil such asacidification, increased leaching losses, decline of organic matter(OM) content and reduction of microbial communities (Marschner,2002). An alternative to mineral fertilisation is the amendment ofsoil with organic matter in form of compost. Composting organicmaterials has the advantage to stabilise and to homogenize them.Compost addition has been shown to improve soil fertility(Cantanazaro et al., 1998; Caravaca et al., 2002), plant nutrition and

l Technology (VAST), Hanoi,

(P.-T. Ngo), cornelia.rumpel@

All rights reserved.

vegetation cover (Larchevêque et al., 2005). Moreover, it canimprove other soil functions such as soil hydraulic conductivity(Celik et al., 2004), aggregate stability and resistance to erosion(Bresson et al., 2001). The use of organic materials may be espe-cially appropriate in countries with subsistence agriculture, wherethe use of mineral fertilisers is restricted and organic waste avail-able. For example, in Northern Vietnam buffalo dung, which shouldnot be directly applied to fields due to adverse effects on soilfertility as well as human and animal health (Vu et al., 2007), maybe converted by farmers into more suitable organic amendmentsby composting.

In the perspective of ecological engineering, epigeic earthwormscanbe used to transformOMduring the compostingprocess. The so-called ‘vermicomposting’ is a technique, which takes advantage ofthe presence of epigeic earthworms during composting to generatean organic material that may be physically, nutritionally and bio-chemically improved compared to compost (Pramanik et al., 2007;Ngo et al., 2011). Greenhouse experiments and field trials have

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220 215

shown that vermicompost can accelerate plant growth (Atiyeh et al.,2000a; Jouquet et al., 2010). The reasons for this are not entirelyclear. Canellas et al. (2002) found that plant growth hormones invermicompost improved root growth and lateral roots initiation inmaize, whereas according to Albanell et al. (1988), vermicompost ismore beneficial due to a lower rate of soluble salts, a higher cationexchange capacity and more humic substances than the originalmaterial.Whereas the effect of vermicompost amendments onplantgrowth and nutrient leaching are well documented (Larchevêqueet al., 2005; Singh et al., 2008; Jouquet et al., 2011), less is knownabout its impact on carbon storage and its biogeochemical cycling.Biological activity in soil amendedwith vermicompost was found tobe increased compared to amendment with mineral fertiliser(Mannaet al., 2003;Kaur et al., 2005) or regular compost (Tejada andBenitez, 2011). Ngo et al. (2011) comparing chemical parameters ofcompost and vermicompost used for amendment of degradedtropical soil concluded, that compost might be less available to thesoil microbial biomass due to a higher contribution of recalcitrantcompounds.

In the field, the amendment with OM usually leads to thestimulation of local earthworm populations (Huerta et al., 2007;Birkhofer et al., 2008). When locally present, earthworms createorganic structures (biogenic aggregates and biopores) which mayinfluence microbial activity and diversity as well as OM chemistryand thus the sequestration and mineralization of soil organicmatter (SOM) in temperate regions and in tropical ecosystems(Lavelle and Spain, 2001; Nguyen et al., 2011). Interactions of OMamendments with local earthworm populations in tropical soilwere addressed by Jouquet et al. (2010). The results of this studyindicated, that the presence of the endogeic earthworm speciesDichogaster bolaui tended to decrease plant growth and carboncontent in soil amended with vermicompost while no effect wasnoted for compost amended soils.

In the present study, we tested the effect of compost or vermi-compost amendments in presence or absence of the endogeicearthworm species Metaphire posthuma on carbon storage and OMchemistry in tropical soil after a one-year cultivation cycle andcompared them to amendment with mineral fertiliser. The exper-iment was carried out in a greenhouse during one year. After theend of the experiment, the soils were analysed for elementalcontent. SOM chemistry was addressed by lignin and sugar anal-ysis. The aim of the study was to investigate (1) the effect ofcompost, vermicompost and mineral fertilisers on the quantity andchemical composition of SOM and (2) how the presence of localearthworms influences this effect.

2. Materials and methods

2.1. Soils and organic amendments and pretreatment

The soil has been sampled from a fallow in the red river deltasituated in Hanoi, Vietnam (Tu Liem district). It was classified asEutric Fluvisoils (FAO UNESCO, 1976). It was alkaline (pHKCl ¼ 7.7)and mainly sandy (61.0% sand, 28.7% silt and 10.3% clay). The soilwas sampled in the 0e10 cm layer, air-dried and sieved at 2 mm.

Compost and vermicompost were produced from buffalo(domestic water buffalo) dung after 3 months of maturation in twodifferent and separated units, as reported by Jouquet et al. (2010).Briefly, buffalo manure was placed in 500 L units and covered bya lid, thereby conserving soil humidity and preventing anaerobicconditions due to rainfall. Every month the compost was mixed toincrease aeration. Vermicomposting was carried out in a similarway, the only difference being the addition of earthworms. Theearthworm species used to produce the vermicompost was Eiseniaandrei.

2.2. Greenhouse experiment

The experiment was carried out during one year in a greenhousein the Soil and Fertilizer Research Institute (SFRI), in Hanoi, Viet-nam. The experiment was set up in 10 L containers made of clay. A5 cm layer of white sand and stones was established at the bottomand covered by 5 kg of soil. The soil was mixed thoroughly withcompost, vermicompost (90.5 g per container or 20 t ha�1) or thesame amount of mineral fertilizers (5.2 g of urea CH4N2O, 7.2 g ofpotash K2O, and 2.9 g of phosphate P2O5 per container). Theseamendments were repeated after each vegetation period; in total 3times during the experiment. Thereafter, we planted commonagricultural crops: maize (JuneeSeptember 2008, variety LNS 222),Tomato (November 2008eMay 2009, variety HT14), and maize(JuneeSeptember 2009, variety LNS 222).

The influence of earthworms on SOM dynamic was investigatedthrough an addition of two adults of the endogeic species M.posthuma per container before each cultivation (i.e. before the twocorn and tomato plantations). This species was found in the gardenof the SFRI institute. M. posthuma is a medium size endogeic geo-phagous earthworm (w12 cm in length and 5 mm diameter onaverage at the adult stage) which produces approximately 5e10fold its own weight in casts per day. Its casts range from 2 to3 mm and are mainly found belowground (Bottinelli et al., 2010;Jouquet et al., 2011). At the end of the incubation no earthwormswere found in the fertilised treatment, 1.4 � 0.2 (mean � SD)earthworms were recovered in the soil amended with vermicom-post and 4.8 � 1.5 earthworms were found in the soil amendedwith compost.

In total, 30 containers were used and the number of replicatesper treatment was n ¼ 5. Throughout the experiment, air humiditywas high, between 75 and 100% and the average daily temperaturevaried from 15

�C to 25

�C during the year. After the end of the

experiment, the soil was thoroughly mixed, air-dried and sieved at2 mm. Aliquots of the samples were ground prior to analyses.

2.3. Elemental analysis

Carbon and nitrogenwere determined by a single analysis usinga CHN auto-analyser (CHN NA 1500, Carlo Erba). Before organiccarbon (OC) analysis the samples were subjected to decarbon-isation by fumigation with concentrated HCl (Harris et al., 2001).

2.4. Determination of carbon stocks

Before incubation the carbon stocks of the initial and amendedsoils were calculated after determination of the OC contents by thefollowing formula:

Cstock initial ¼ S � Csi þ A � Ca � 3 , (1)

where S is the weight of soil and Csi its OC content at thebeginning of the experiment and A the weight of the amendmentand Ca the OC content of the amendment.

After one year we determined the carbon stocks of soil of thethree treatments (mineral fertilisation, compost and vermicom-post) by the following formula:

Cstock 1 yr ¼ (S þ 3 A) � Cs, (2)

where S is the weight of soil, A the weight of the amendmentand Cs the OC content of soil at the end of the experiment.

In all treatments, we calculated the OC stock in percent of theinitial soil OC stock.

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220216

2.5. Lignin quantification by alkaline CuO oxidation

Soil lignin content and composition of dried and groundsamples were analysed gas chromatographically after alkalinecupric oxide (CuO) oxidation at high temperature (Hedges andErtel, 1982). CuO products were purified (Kögel and Bochter,1985) and quantified as trimethylsilyl derivatives by gas chroma-tography (GC) with a HP gas chromatograph (HP GC 6890) equip-ped with a flame ionisation detector (FID) and a SGE BPX-5 column(50 m length, 0.25 mm inner diameter, 0.32 mm coating). Sampleswere injected in split mode (1:10). The GC oven temperature wasprogrammed at 100 �C during 2 min, then from 100 �C to 172 �C at8 �Cmin�1, from 172 to 184 �C at 4 �Cmin�1, and from 184 to 300 �Cat a rate of 10 �C min�1.

CuO oxidation yields a suite of single-ring phenol compounds(V-vanillyl, S-syringyl and C-p-coumaryl) with their aldehyde,ketone and acid side chains. Acid-to-Aldehyde ratios (Ad/Al) werecalculated for the lignin monomers V and S. The sum of all units(V þ S þ C) was considered to represent the total lignin content ofthe sample. With increasing decomposition, VSC is usuallydecreasing, whereas the Ad/Al ratios of V and S units are increasingand C/V as well as S/V decreasing.

Three out of five replicates were analysed for lignin content.

2.6. Neutral non-cellulosic carbohydrates

Non-cellulosic sugars were analysed after trifluoroacetic acid(TFA) hydrolysis by gas chromatography as alditol acetates (Rumpeland Dignac, 2006). Briefly, 1 g of soil was hydrolyzed using 10 ml of4 M trifluoroacetic acid (TFA) at 110 �C for 4 h. After the hydrolysis,myoinositol (2 g L�1) was added as internal standard and the soilremoved by filtration using a glass fiber filter (Whatman GF/F).After the evaporation of TFA by a rotary evaporator, sugars werederivatised in screw-top test tubes. These derivatised sugars wereextracted by liquideliquid extraction using dichloromethane. Gaschromatographic measurements of the monosaccharides wereperformed with a HP 6890 gas chromatograph equipped witha flame ionization detector. Separation of themonosaccharide unitswas achieved with a 60 m fused silica capillary column (BPX 70,0.32 mm internal diameter, 0.25 mm film thicknesses) and thefollowing temperature program: the temperature was raised from200 to 250 �C at 3 �C min�1 and then kept isothermal for 15 min.The concentrations of individual neutral sugars were calculatedbased on the total ion currency of the internal standard myoino-sitol. The sum of these monosaccharides gives the total non-cellulosic carbohydrate content of the sample. We calculated theratio between C6 and C5 carbohydrates as ratio between(galactose þ mannose)/(arabinose þ xylose) and the ratio of deoxyto C5 carbohydrates as ratio between (rhamnose þ fucose)/(arabinose þ xylose). These two ratios usually increase withincreasing contribution of microbial-derived carbohydrates.

Three out of five were analysed for sugars content.

2.7. Statistical analysis

Differences in the initial chemical properties between compostand vermicompost were assessed using t-tests. Data obtained after

Table 1Carbon and nitrogen content (mg g-1), OC to N ratios as well as lignin and sugar contenstandard deviation (n ¼ 3). Different letters indicate significant differences (t-test, P < 0

N mg g�1 OC mg g�1

Compost 15.64a (0.17) 162.73a (0.98)Vermicompost 17.22b (0.27) 155.08b (2.03)

the incubation experimentwere tested for homogeneity of varianceusing the Levene’s test before further analyses. Differences inchemical properties between soil treatments were analysed byanalysis of variance (ANOVA) and afterwards through Tukey’smultiple comparison test. A Principal Component Analyses (PCA)was carried out to differentiate the treatments based on thechemical signature of the SOM. All statistical calculations werecarried out using R (R Development Core Team, 2008). Differencesamong treatment were declared at the 0.05 probability level ofsignificance.

3. Results

3.1. Elemental composition of organic amendments and soil as wellas carbon storage before and after the experiment

Elemental compositions are shown in Table 1 for organicamendments and in Table 2 for soil before and after the incubation.Compost was characterized by a higher OC but a lower N content,leading to much higher OC/N ratio compared to vermicompost(Table 1). At the end of the experiment, OC content was signifi-cantly higher in the soil amended with compost than with vermi-compost and the lowest values were recorded in the soil amendedwith chemical fertilizers (Table 2). Interestingly, the utilization ofmineral fertilizers led to a significant decrease of OC and Ncontents, and the utilization of vermicompost without earthwormto significant decrease of N content.

The presence of earthworms led to a decrease of the soil OCcontent in the treatments with organic amendments but nodifference was observed with mineral fertilization. Earthwormactivity also led to a decrease of the nitrogen content in soilsamended with compost, whereas the differences were not signifi-cant for the vermicompost and mineral fertilization treatments. Nodifference in OC/N ratio was noticed between the different treat-ments, except for the mineral treatments which had a lower OC/Nratio than the organic fertilization treatments, with and withoutearthworms.

The total quantities of carbonwere calculated for the initial soilsand the soils after the one-year experiment. They decreased in alltreatments compared to the initial stocks. We recovered between40 and 77% of the initial OC (Table 2). Most OC was lost from thevermicompost-amended soil and least OC loss was recorded for thesoil amended with mineral fertilizers. Despite the high relativedecrease in carbon with regards to the initial, carbon stocks weresignificantly higher in the soils amended with organic materialsafter the end of the experiment, except for the vermicomposttreatment in the presence of earthworms. After the experiment, OCstocks were significantly higher in compost-amended soilcompared to vermicompost-amended soil. In general, the presenceof earthworms significantly decreased the OC stocks of soil amen-ded with organic materials, whereas it had no effect on the soilamended with mineral fertilizer (Table 2).

3.2. Lignin and non-cellulosic carbohydrate signature

Similar lignin contents were recorded in both organic substrates(Table 1). The lignin contents of the organic materials were almost

t (mg g�1 OC) of the compost and vermicompost. Data are presented as means and.05).

OC/N Lignin mg g�1 OC Sugars mg g�1 OC

10.41a (0.07) 30.45a (6.13) 148.56a (14.87)9.01b (0.04) 35.00a (2.18) 158.57a (18.98)

Table 2Carbon and nitrogen content (mg g-1), OC to N ratios, OC stock of soil with and without amendment before and after incubation. The treatments with and without earthwormsare indicated by (EWþ) or (EW�). Data are presented as means and standard deviation (n ¼ 5). Different letters indicate significant differences (ANOVA for soil treatments,P < 0.05).

N mg g�1 OC mg g�1 OC/N OC stock g % of initial

Before incubationSoil 1.29a (0.03) 9.87b (0.14) 7.7a (0.2) 49.35b (1.70) 100Compost-amended soil n.d.* n.d. n.d. 93.54 (1.25) 100Vermicompost-amended soil n.d. n.d. n.d. 91.45 (1.00) 100After 1 yr of incubationMineral fertilizerEW� 0.89c (0.03) 7.28c (0.27) 8.2b (0.3) 36.42c (1.34) 73.79 (2.72)EWþ 0.87c (0.04) 7.46c (0.58) 8.6b (0.1) 38.36c (1.96) 77.73 (3.96)CompostEW� 1.25a (0.07) 11.75a (0.72) 9.4c (0.1) 57.28a (1.72) 61.25 (1.84)EWþ 1.06b (0.06) 9.63b (0.69) 9.1c (0.2) 47.00b (2.67) 50.25 (2.85)VermicompostEW� 1.12b(0.05) 10.11b (0.62) 9.0c (0.3) 50.57b (2.89) 56.66 (3.16)EWþ 0.95bc (0.10) 8.18c (0.12) 9.2c (0.2) 40.90c (0.60) 44.72 (0.66)

*n.d. ¼ not determined.

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220 217

three times higher than those of the initial soil (Table 3). One yearafter incubation, soil lignin content ranged from 17.52 to27.36 mg g�1 OC (Table 3). Lignin contents were significantlyincreased in all treatments compared to the initial soil. Compostand vermicompost amendment led to significantly higher lignincontent (VSC) than the mineral fertilization treatment except forvermicompost without earthworms (Table 3). The presence ofearthworms had no significant influence on the lignin content(Table 3). The (Ac/Al)s ratio did not show any clear trend withregards to OM amendments in presence or absence of earthworms.For soils amended with compost, the (Ac/Al)v ratio was signifi-cantly lower than for soils amended with mineral fertilizer and theinitial soil. Soils amended with vermicompost had intermediate(Ac/Al)v values, without any significant difference between thesoils amended with compost or mineral fertilizer. Whatever thetreatment, the presence of earthworms did not influence the (Ac/Al)v ratio. No difference in C/V and S/V were observed between thedifferent treatments.

A principal component analysis (PCA) was carried out using allindicators of the lignin signature (Fig. 1). The first two axes of thePCA of the lignin parameters account for 88.7% of the observedvariation. Soils were clearly distinguished along the first axisaccording to their amendment. The first axis explained 71.5% of thetotal variability, and it is positively correlated with the (Ac/Al)s and(Ac/Al)v ratios, and negatively correlated with V-vanillyl, S-syringyland C-p-coumaryl units. The compost amended soils (in presenceor absence of earthworms) correlated positively with V, S and Ccontents while soils amended with mineral fertilizers and theinitial soil showed negative correlation with V, S and C. The secondaxis explained 17.2% of the total variability, and it was mainly

Table 3Sugars total, lignin signature and sugar to lignin ratios of initial soil and soil amendedearthworms after one year of incubation. Data are presented as mean with standard dev

(Ac/Al)v (Ac/Al)s C/V S/V

Initial soil 0.56a (0.01) 0.54a (0.01) 0.45a (0.02) 1.10a

Soil after 1 yr of incubationMineral fertilizerEW� 0.57a (0.02) 0.49ab (0.02) 0.44a (0.11) 0.99a

EWþ 0.55a (0.05) 0.52a (0.03) 0.52a (0.03) 1.08a

CompostEW� 0.42b (0.04) 0.39b (0.07) 0.48a (0.04) 1.31a

EWþ 0.45b (0.01) 0.41ab (0.01) 0.60a (0.05) 1.40a

VermicompostEW� 0.48ab (0.02) 0.44ab (0.04) 0.48a (0.12) 1.28a

EWþ 0.47ab(0.01) 0.41b (0.05) 0.57a (0.03) 1.28a

(Ac/Al)s: acid/aldehyde ratio of syringyl units, (Ac/Al)v: acid/aldehyde ratio of vanilyl ucoumaryl units divided by the sum of vanillyl unit.

associated with the C/V ratio in the positive direction. Along theseaxes, the earthworm treatments could be distinguished for thecompost amended soil and to a lesser extent for the vermicompostamended soil, the earthworm treatment being correlated ina positive direction with C/V.

Total sugar content was similar for the two organic substrates(Table 1). Similar to lignin it showed almost three times lowercontent than the initial soil (Table 3). In the amended soils, sugarcontent ranged between 44.13 and 182.79 mg g�1 OC after the one-year experiment (Table 3). The highest content was observed insoils amended with compost and the lowest in soils with mineralfertilizers. Vermicompost treatments had intermediate values.Earthworm bioturbation had no influence on the soil sugar content.

Fig. 2 shows the result of the PCA carried out from all the indi-cators of the carbohydrate signature. The PCA plane defined by thetwo first principal components accounted for 83.1% of the variation.The first axis was related to all individual carbohydrates. Thesecond axis was associated with the C6/C5 ratio. Soils treated withcompost, vermicompost or chemical fertilizers were clearlydistinguished. Fertiliser treatment correlated in the negativedirection with all carbohydrates, while compost treatment showeda positive correlation. Vermicompost treatment was positivelycorrelated with the C6/C5 ratio. The effect of earthworms on thecarbohydrate signature of soil was most obvious for compost-amended soils.

Although the total lignin and carbohydrate content did notallow us to show an influence of earthworm (Table 3), theconsideration of the whole signatures demonstrates an effect ofearthworms on the quality of lignin and carbohydrates in soil(Figs. 1 and 2).

with compost, vermicompost or mineral fertilizer with (EWþ) or without (EW�)iation (n ¼ 3). Different letters indicate significant differences (P < 0.05).

Lignin mg g�1 OC Sugars mg g�1 OC Sugar/lignin

(0.03) 13.74c (1.36) 65.46c (3.40) 4.76d (0.15)

b (0.18) 20.05b (1.32) 44.13d (6.71) 2.20f (0.21)(0.15) 17.52b (1.57) 55.51d (3.02) 3.17e (0.14)

(0.05) 25.14a (1.00) 178.30a (7.55) 7.09a (0.08)b (0.13) 27.36a (1.63) 182.79a (7.08) 6.68b (0.10)

(0.09) 22.33ab (1.44) 115.90b(1.25) 5.19c (0.08)(0.16) 25.91a (1.68) 121.03b (25.32) 4.67d (0.27)

nits, S/V: sum of syringyl units divided by the sum of vanillyl unit, C/V: sum of p-

CEW+

FEW-

FEW+

Initial

soil

VEW+

V

C

(Ac/Al)V

(Ac/Al)S

C/V

S/V

S

a

c

bF2=17.2%

F1= 71.5%

CEW-

V EW-

Fig. 1. Principal Component Analysis (PCA) for lignin analysis of soil with mineral (N:P:K) (F), compost (C) or vermicompost (V) fertilizers with (EWþ) and without earthworms(EW�). (a) Eigenvalue diagram. (b) Correlation circle on F1eF2 plan. Variables are vanilyl units (V), syringyl units (S), Coumaryl units (C), (Ac/Al)s: acid/aldehyde ratio of vanilylunits, (Ac/Al)v: acid/aldehyde ratio of syringyl units, S/V: sum of syringyl units divided by the sum of vanillyl units. (c) Ordination in the plane defined by factor 1 and 2.

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220218

4. Discussion

4.1. Effect of organic amendments on carbon storage

Carbon storage was decreased in all soils compared to the initialstorage after the one-year experiment (Table 2). A decrease of about30% was observed for soil amended with mineral fertilizers. Thisfinding may be related to the preparation of soil, which was used inthe greenhouse experiment. Pretreatment included sieving, whichled to breakdown of aggregates and exposure of occluded organicmatter to microbial decomposition (Six et al., 2000). In soilsamended with organic materials, about 40e50% decreased OMstorage was noted and may illustrate the susceptibility of OM to be

CEW-

CEW+

FEW-

FEW+

Initial soil

VEW-

VEW+

F1=70.38%

F2=12.69% c

Fig. 2. Principal components analysis (PCA) results for sugar analysis of soil amended witearthworms (EWþ). (a) Eigenvalue diagram. (b) Correlation circle on F1eF2 plane. (c) Ordi

degraded in a tropical environment. These results are in line witha study by Mandal et al. (2007), who observed carbon loss between27 and 78% from organic amendments applied to rice-basedcropping systems for 7e36 years.

However, our data indicate increased carbon storage withregards to the control soil by organic amendments. These resultsare in line with other studies, showing that addition of exogenousOM such like compost results in an enhancement of OC storage inaddition to improvement of many other soil functions related to thepresence of organic matter (Lashermes et al., 2009). In tropical soilsthe impact of organic amendments on long-term carbon storagemight be rather small (Mandal et al., 2007). However, many studiesnoted their beneficial effect on biological activity and nutrient

a

b

Mannose

Galactose

Glucose

Fucose Ribose

Arabinose Xylose

C6/C5

Deoxy/C5

Rhamnose

h mineral fertiliser (F), compost (C), and vermicompost (V) without (EW�) and withnation in the plane defined by factor 1 and 2.

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220 219

cycling (Nayak et al., 2007; Kaur et al., 2005). With regards to thedifferent compost types, after one year, the most striking result ofour experiment was the highest OC and N content in soil amendedwith compost as compared to soils which received vermicompost(Table 2). Surprisingly, these data are not in agreement with Ngoet al. (2011) who did not observe any difference between soilstreated with compost and vermicompost in OC and N content.These differences could be due to the difference in the soil natureand because the present experiment lasted longer. This result isalso in contrast to many studies (e.g. Atiyeh et al., 2000b) showingthat vermicompost is more resistant to decomposition thancompost as it is composed of more processed OM, illustrated by themore degraded lignin signature (Ngo et al., 2011). Therefore, wewould have expected higher amounts of this material remaining insoil as compared to compost. In tropical soil with low organicmatter content, microorganisms may be more limited by N than OC(Krashevska et al., 2010). This could explain our results becausemicroorganisms may use preferentially substrates with low OC/Nratios such as vermicompost, which could be more easily decom-posed compared to regular compost (e.g., Leifeld et al., 2002;Dignac et al., 2005). This is supported by the observation of highestcrop yield for the vermicompost treatment (data not shown), whichsuggests that N mineralization and utilization by plants occurred inthis treatment.

4.2. Effect of organic matter addition on biogeochemicalparameters of soil

In soils amended with mineral fertilizer, we observed a lowerlignin and non-cellulosic carbohydrate content compared to soilsamended with organic materials (Table 3). Additionally we noticeda higher state of lignin degradation as indicated by significantlyincreased (Ac/Al)v and (Ac/Al)s ratios in soils amended withmineral fertilizers compared to soils amended with organic mate-rials. This illustrates that OM compounds in tropical soils are highlydecomposed and resistant against decomposition (Tchienkoua andZech, 2004). Organic matter amendments in the form of compostand vermicompost resulted in a significantly increased soil ligninand non-cellulosic carbohydrate contents after the one-yearexperiment. Compost amended soil showed higher contribution ofboth parameters to SOM than vermicompost amended soil(Table 3), despite similar concentrations in the initial substrates(Table 1). This may be explained by lower microbial decay ofcompost compared to vermicompost (see above). In addition todifferences in carbon storage this hypothesis may be supported byPCA analysis of non-cellulosic carbohydrates (Fig. 2), whichshowed, that vermicompost-amended soil was correlated in thepositive direction with a higher contribution of microbial-derivednon-cellulosic carbohydrates (high C6/C5). These compounds mayhave accumulatedmore in vermicompost-amended soils comparedto compost amended soils during more intense decay (see above).This is in accordancewith stimulation of the biological activity (Airaand Dominguez, 2008), due to addition of material high in nitrogen.In compost amended soils on the other hand there may be a highercontribution of root litter input as illustrated by the closer corre-lation of this treatment with lignin phenols (Fig. 1). Generally, morerooting activity in expense of aboveground biomass production(high root/shoot ratio) is observed when soils are lowmineralisableN (Robinson, 1994), which is in line with our data.

4.3. Earthworm effects on soil amended with compost andvermicompost

There are numerous studies on the impact of organic amend-ments (Goyal et al., 1999; Ngo et al., 2011) or earthworms (Nguyen

et al., 2011; Coq et al., 2007) on the dynamic of SOM in the tropics.However, how earthworms influence the quality and quantity ofSOM in soil treated with compost or vermicompost remainsunknown. Our results show that the presence of earthwormsdecreased carbon storage in soils amended with organic material,whereas it had no influence on soils amended with mineral fertil-izers (Table 2). Earthworms, as soil engineers, play a key role in soilfunctioning (Lavelle et al., 1997; Jouquet et al., 2006). They enhancesoil aeration, water infiltration, influence microbial activity anddiversity and stimulate organic decomposition and facilitatenutrient cycling (Lavelle and Spain, 2001). Some studies suggestthat earthworm activity generally leads to incorporation of OM intostable soil aggregates which protect it against decay (e.g., Lavelleand Spain, 2001; Bossuyt et al., 2005), while others conclude onthe opposite (Coq et al., 2007). In our study, M. posthuma waschosen as earthworm model. This species is belonging to theendogeic (sensu Bouche and Al-Addan, 1997) and to the decom-pacting (sensu Blanchart et al., 2004) functional groups species. Aspreviously reported by Bottinelli et al. (2010), this earthwormspecies leads to decreased SOM stocks through direct consumptionand because of the low stability of its casts resulting in a rapidmineralization of the SOM. As we did not use isotope labelling weare unable to say if this OC loss concerns only the added substratesor as well the original SOM. However, the observation of unchangedOM storage and low earthworm survival in soils amended withmineral fertilizers suggest that utilization of soil inherent OM byearthworms may be limited.

Earthworms’ effect seemed to be variable depending on the soilproperties. Earthworms have a negative effect by consuming theOM added and this negative effect might be higher in the presenceof vermicompost. is in agreement with Jouquet et al. (2010) whoconcluded that negative interactions can occur between theendogeic exotic earthworm Dichogaster bolaui and vermicompost.

The presence of earthworms additionally influenced the ligninsignature of SOM (Fig. 1). In the PCA plane, soils amended withorganic matter in presence of earthworms were clearly differenti-ated from thosewithout earthworms, the difference being related tohigher C/V and S/V ratios. Coumaryl and syringyl phenols are moreprone to decomposition than vanillyl phenols (Bahri et al., 2006).Therefore, positive correlation of earthworm treatments with C/VandS/V ratios (Fig.1) togetherwith slightly increased lignin contentscompared to soils without bioturbation (Table 3) could indicate thatOM is likely to be protected through aggregation.

5. Conclusions

The impact of organic matter amendments and activity ofearthworms on carbon storage and biogeochemical parameterswas studied in a tropical soil amended with compost or vermi-compost. Sustained increase of OC storage was dependent on thenature of the organic amendment. Whereas compost amendmentmay lead to sustained increase of SOM contents, vermicompostmay be more accessible to the soil microbial biomass. Therefore,our study suggests that compost would appear as a better candi-date for improving OC sequestration in soil. However, it is worthnoting that, due to its better availability, crop yields were greaterwith vermicompost treatment.

The presence of earthworms decreased carbon storage in soilsamended with organic materials. Highest decrease was observed inthe vermicompost treatment. Moreover, earthworm activity mayalso lead to some extent to protection of OM in soil aggregates asillustrated by positive correlation of earthworm treatments withless degraded lignin signatures. The relationship between the OMchanges induced by earthworms and soil functions yet needs to beelucidated.

P.-T. Ngo et al. / Soil Biology & Biochemistry 50 (2012) 214e220220

Acknowledgement

This project was supported financially by CNRS (SystéMOproject), IRD (unit research UMR-211-BIOEMCO) and CNRS (unitresearch UMR-7618-BIOEMCO) French institutes. We acknowledgewith gratitude Dr. Tran Duc Toan and the SFRI (Soils and FertilizersResearch Institute) Vietnamese institute for providing access to thegreenhouse and laboratories. The authors also thank Gérard Bar-doux and Daniel Billiou for the technical assistance. The first authorwould like to thank Pierre Desesquelles for his helpful comments.

References

Aira, M., Dominguez, J., 2008. Optimizing vermicomposting of animal wastes:effects of rate of manure application on carbon loss and microbial stabilization.Journal of Environmental Management 88, 1525e1529.

Albanell, E., Plaixats, J., Cabrero, T., 1988. Chemical changes during vermicomposting(Eisenia fetida) of sheep manure mixed with cotton industrial wastes. Biologyand Fertility of Soils 6, 266e269.

Atiyeh, R.M., Arancon, N., Edwards, C.A., Metzger, J.D., 2000a. Influence ofearthworm-processed pig manure on the growth and yield of greenhousetomatoes. Bioresource Technology 75, 175e180.

Atiyeh, R.M., Dominguez, J., Subler, S., Edwards, C.A., 2000b. Changes in biochemicalproperties of cow manure during processing by earthworms (Eisenia andrei,Bouche) and the effects on seedling growth. Pedobiologia 6, 709e724.

Bahri, H., Dignac, M.F., Rumpel, C., Rasse, D.P., Chenu, C., Mariotti, A., 2006. Ligninturnover kinetics in an agricultural soil is monomer specific. Soil Biology &Biochemistry 38, 1977e1988.

Birkhofer, K., Bezemer, T.M., Bloem, J., Bonkowski, M., Christensen, S., Dubois, D.,Ekelund, F., Fließbach, A., Gunst, L., Hedlund, K., Mäder, P., Mikola, J., Robin, C.,Setälä, H., Tatin-Froux, F., Van der Putten, W.H., Scheu, S., 2008. Long-termorganic farming fosters below and aboveground biota: implications for soilquality, biological control and productivity. Soil Biology & Biochemistry 40,2297e2308.

Blanchart, E., Albrecht, A., Brown, G., Decaens, T., Duboisset, A., Lavelle, P.,Mariani, L., Roose, E., 2004. Effects of tropical endogeic earthworms on soilerosion. Agriculture and Environment 104, 303e315.

Bossuyt, H., Six, J., Hendrix, P.F., 2005. Protection of soil carbon by microaggregateswithin earthworm casts. Soil Biology & Biochemistry 37, 251e258.

Bottinelli, N., Henry-des-Tureaux, T., Hallaire, V., Mathieu, J., Benard, Y., Tran, T.D.,Jouquet, P., 2010. Earthworms accelerate soil porosity turnover under wateringconditions. Geoderma 156, 43e47.

Bouche, M.B., Al-Addan, F., 1997. Earthworms, water infiltration, and soil stability:some new assessments. Soil Biology & Biochemistry 29, 441e452.

Bresson, L.M., Koch, C., Le Bissonnais, Y., Barriuso, E., Lecomte, V., 2001. Soil surfacestructure stabilization by municipal waste compost application. Soil ScienceSociety of America Journal 65, 1804e1811.

Canellas, L.P., Olivares, F.L., Okorokova, A.L., Facanha, A.R., 2002. Humic acids iso-lated from earthworm compost enhance root elongation, lateral root emer-gence, and plasma Hþ -ATPase activity in maize roots. Plant Physiology 130,1951e1957.

Cantanazaro, C.J., Williams, K.A., Sauve, R.J., 1998. Slow release versus water solublefertilization affects nutrient leaching and growth of potted chrysanthemum.Journal of Plant Nutrition 21, 1025e1036.

Caravaca, F., Hernandez, T., Garcia, C., Roldan, A., 2002. Improvement of rhizosphereaggregate stability of afforested semiarid plant species subjected to mycorrhizalinoculation and compost addition. Geoderma 108, 133e144.

Celik, I., Ortas, I., Kilic, S., 2004. Effects of compost, mycorrhiza, manure andfertilizer on some physical properties of a Chromoxerert soil. Soil and TillageResearch 78, 59e67.

Coq, S., Barthes, B.G., Oliver, R., Rabary, B., Blanchart, E., 2007. Earthworm activityaffects soil aggregation and organic matter dynamics according to the qualityand localization of crop residuesdAn experimental study (Madagascar). SoilBiology & Biochemistry 39, 2119e2128.

Dignac, M.-F., Houot, S., Francou, C., Derenne, S., 2005. Pyrolytic study of compostand waste organic matter. Organic Geochemistry 36, 1054e1071.

Goyal, S., Chander, K., Mundra, M.C., Kapoor, K.K., 1999. Influence of inorganicfertilizers and organic amendments on soil organic matter and soil microbialproperties under tropical conditions. Biology and Fertility of Soils 29, 196e200.

Harris, D., Horwath, W.R., van Kessel, C., 2001. Acid fumigation of soils to removecarbonates prior to total organic carbon or carbon-13 isotopic analysis. SoilScience Society of America Journal 65, 1853e1856.

Hedges, John I., Ertel, John R., 1982. Lignin geochemistry of a Late Quaternarysediment core from Lake Washington. Geochimica et Cosmochimica Acta 10,1869e1877.

Huerta, E., Rodriguez-Olan, J., Evia Castillo, I., Montejo-Meneses, E., de la Cruz-Mondragon, M., Garcia-Hernandez, R., Uribe, S., 2007. Earthworms and soilproperties in Tabasco, Mexico. European Journal of Soil Biology 43, 190e195.

Jouquet, P., Dauber, J., Lagerlöf, J., Lavelle, P., Lepage, M., 2006. Soil invertebrates asecosystem engineers: intended and accidental effects on soil and feedbackloops. Applied Soil Ecology 32, 153e164.

Jouquet, P., Plumere, T., Thu, T.D., Rumpel, C., Duc, T.T., Orange, D., 2010. Therehabilitation of tropical soils using compost and vermicompost is affected bythe presence of endogeic earthworms. Applied Soil Ecology 46, 125e133.

Jouquet, E.P., Bloquel, E., Doan, T.T., Ricoy, M., Orange, D., Rumpel, C., Duc, T.T., 2011.Does compost and vermicompost improve macronutrient retention and plantgrowth in degraded tropical soils? Compost Science and Utilization 19, 15e24.

Kaur, K., Kapoor, K.K., Gupta, Anand P., 2005. Impact of organic manures with andwithout mineral fertilizers on soil chemical and biological properties undertropical conditions. Journal of Plant Nutrition and Soil Science 168, 117e122.

Kögel, I., Bochter, R., 1985. Characterization of lignin in forest humus layers by highperformance liquid chromatography of cupric oxide oxidation products. SoilBiology & Biochemistry 17, 637e640.

Krashevska, V., Maraun, M., Ruess, L., Scheu, S., 2010. Carbon and nutrient limitationof soil microorganisms and microbial grazers in a tropical montane rain forest.Oikos 119, 1020e1028.

Larchevêque, M., Montès, N., Baldy, V., Dupouyet, S., 2005. Vegetation dynamicsafter compost amendment in a Mediterranean post-fire ecosystem. AgricultureEcosystems and Environment 110, 241e248.

Lashermes, G., Nicolardot, B., Parnaudeau, V., Thuries, L., Chaussod, R.,Guillotin, M.L., Lineres, M., Mary, B., Morvan, T., Tricaud, A., Villette, C., Houot, S.,2009. Indicator of potential residual carbon in soils after exogenous organicmatter application. European Journal of Soil Science 60, 297e310.

Lavelle, P., Spain, A.V., 2001. Soil Ecology. Kluwer Academic Publishers, Dordrecht,654 pp.

Lavelle, P., Bignell, D., Lepage, M., Wolters, V., Roger, P., Ineson, P., Heal, O.W.,Dhillion, S., 1997. Soil function in a changing world: the role of invertebrateecosystem engineers. European Journal of Soil Biology 33, 159e193.

Leifeld, J., Siebert, S., Kögel-Knabner, I., 2002. Changes in the chemical compositionof soil organic matter after application of compost. European Journal of SoilScience 53, 299e309.

Mandal, B., Majumder, B., Bandyopadhyay, P.K., Hazra, G.C., Gangopadhyay, A.,Samantaray, R.N., Sishra, A.K., Chaudhury, J., 2007. The potential of croppingsystems and soil amendments for carbon sequestration in soil under long-termexperiments in subtropical India. Global Change Biology 13, 357e369.

Manna, M.C., Jha, S., Ghosh, P.K., Acharya, C.L., 2003. Comparative efficacy of threeepigeic earthworms under different deciduous forest litters decomposition.Bioresource Technology 88, 197e206.

Marschner, H., 2002. Mineral Nutrition of Higher Plants. Academic Press, San Diego.889 p.

Nayak, Dali R., Jagadeesh Babu, Y., Adhya, T.K., 2007. Long-term application ofcompost influences microbial biomass and enzyme activities in a tropical AericEndoaquept planted to rice under flooded condition. Soil Biology & Biochem-istry 39, 1897e1906.

Ngo, P.T., Rumpel, C., Dignac, M.F., Billou, D., Toan, T.D., Jouquet, P., 2011. Trans-formation of buffalo manure by composting or vermicomposting to rehabilitatedegraded tropical soils. Ecological Engineering 37, 269e276.

Nguyen, H.H., Rumpel, C., Tureaux, T.H.D., Bardoux, G., Billou, D., Toan, T.D.,Jouquet, P., 2011. How do earthworms influence organic matter quantity andquality in tropical soils? Soil Biology & Biochemistry 43, 223e230.

Pramanik, P., Ghosh, G.K., Ghosal, P.k., Banik, P., 2007. Changes in organic e C, N, Pand K and enzyme activities in vermicompost of biodegradable organic wastesunder liming and microbial inoculants. Bioresource Technology 98, 2485e2494.

Robinson, D., 1994. The responses of plants to non-uniform supplies of nutrients.New Phytologist 127, 635e674.

Rumpel, C., Dignac, M.-F., 2006. Gas chromatographic analysis of monosaccharidesin a forest soil profile: analysis by gas chromatography after trifluoroacetic acidhydrolysis and reductioneacetylation. Soil Biology & Biochemistry 38,1478e1481.

Singh, R., Sharma, R.R., Satyendra Kumar, S., Gupta, R.K., Patil, R.T., 2008. Vermi-compost substitution influences growth, physiological disorders, fruit yield andquality of strawberry (Fragaria x ananassa Duch.). Bioresource Technology 99,8507e8511.

Six, J., Elliott, E.T., Paustian, K., 2000. Soil macroaggregate turnover and micro-aggregate formation: a mechanism for C sequestration under no-tillage agri-culture. Soil Biology & Biochemistry 32, 2099e2103.

Tchienkoua, M., Zech, W., 2004. Organic carbon and plant nutrient dynamics underthree land uses in the highlands of West. Agriculture Ecosystems & Environ-ment 104, 673e679.

Tejada, M., Benitez, C., 2011. Organic amendment based on vermicompost andcompost: differences on soil properties and maize yield. Waste Management &Research 29, 1185e1196.

Vu, T.K.V., Tran, M.T., Dang, T.T.S., 2007. A survey of manure management on pigfarms in Northern Vietnam. Livestock Science 112, 288e297.