organic matter influence on clay wettability and soil aggregate stability

Organic Matter Influence on Clay Wettability and Soil Aggregate StabilityC. Chenu,* Y. Le Bissonnais, and D. Arrouays

ABSTRACTSoil organic matter is thought to increase aggregate stability by

lowering the Wettability and increasing the cohesion of aggregates.In southwest France, thick humic loamy soils (Vermic Haplubrepts)have been intensively cropped for 40 yr, decreasing the soil organicpool and lowering the soil agregate stability. This study assessed (i)the contribution of organic matter to aggregate stability by decreasingaggregate Wettability and (ii) the specific role of clay-associated or-ganic matter. Soil samples with a C content of 4 to 53 g kg"1 weresampled and soil aggregate stability was measured. Aggregate wetta-bility was assessed by measuring water drop penetration times onindividual 3- to 5-inm aggregates. The <2-jxm fractions were extractedwithout organic matter destruction and their wettability was deter-mined by measuring contact angles of water on clay deposits. Aggre-gate stability against slaking was correlated to soil C content (r2 =0.71 for fast wetting). Water drop penetration time increased with Ccontents from 1 to 32 s and was very heterogeneous among individualaggregates from a given soil. The contact angle of water on the clayfraction increased linearly with the C content (r2 = 0.86). This changein clay wettability could partly explain the higher water stability ofsoils rich in C.

AGREGATE STABILITY is generally strongly correlatedwith soil organic matter content (Chancy and

Swift, 1984). Upon cultivation the organic matter con-tent of soils typically decreases with a correspondingdecrease in aggregate stability (Hamblin, 1980; Dor-maar, 1983; Angers and Mehuis, 1989).

The main processes by which soils aggregates aredisrupted upon rainfall are (i) slaking, that is, the disrup-tion of aggregates due to the forces exerted by com-pressed air entrapped during rewetting; (ii) differentialswelling of clays; (iii) mechanical dispersion due to thekinetic energy of rain drops; and (iv) physicochemicaldispersion (Le Bissonnais, 1996). Soil organic matter(SOM) is assumed to stabilize aggregates against thesedisruptive processes by two major actions. First, organicmatter increases the cohesion of aggregates, throughthe binding of mineral particles by organic polymers,or through the physical enmeshment of particles by fineroots or fungi (Tisdall and Oades, 1982; Chenu andGuerif, 1991; Dorioz et al., 1993; Chenu et al., 1994).Second, organic matter may decrease the wettability ofaggregates, slowing their rates of wetting and thus theextent of slaking (Monnier, 1965; Chassin, 1979; Sulli-van, 1990). The second mechanism has received far lessattention than the first one.

In some soils, organic substances induce very severewater repellency, especially in sandy soils (Bond, 1969;

C. Chenu, Unite de Science du Sol, INRA, 78026 Versailles, France;and Y. Le Bissonnais and D. Arrouays, Unite de Science du Sol,Service d'Etudes des Sols et de la Carte Pedologique de France,INRA, 45160 Olivet, France. Received 27 July 1999. "Correspondingauthor ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:1479-1486 (2000).

Wallis and Home, 1992) but also in heavy textured ones(MacGhie and Posner, 1980). Strongly hydrophobic or-ganic coatings can prevent water from entering the ag-gregates or the horizon, restrict infiltration, and causeintense surface runoff (Wallis and Home, 1992).

Apart from the case of strongly hydrophobic soils,SOM may impart partial repellency to soil aggregatesand thereby contribute to their stability. Haynes andSwift (1990) reported that dried aggregates from a pas-ture soil rich in organic matter, were more stable thanfield moist ones, and that it was the opposite for arablesoils with low C content. The slower rewetting of pastureaggregates as compared to arable counterparts was as-cribed to hydrophobic properties of SOM (Sullivan,1990). Furthermore, Capriel et al. (1990) reported goodcorrelations between the aliphatic fraction of a soil ex-tracted with supercritical hexane, and its aggregate sta-bility.

Several organic fractions were shown to be responsi-ble for the hydrophobicity of soils or to be partly hy-drophobic: humic acids (Roberts and Carbon, 1972;Tschapek et al., 1973; Giovannini et al., 1983; Jouanyand Chassin, 1987b), aliphatic fractions (MacGhie andPosner, 1980; Ma'shum et al., 1988), or plant litter debris(MacGhie and Posner, 1981). Using model organic mol-ecules and reference clays it was shown that organicsubstances can render clays hydrophobic (Jouany andChassin, 1987a; Janczuk et al., 1990; Jouany, 1991).However, it has not been established to which extentnatural clay-organic matter associations have hy-drophobic properties, nor whether they contribute tosoil aggregate stability.

In southwest France, thick humic soils developed onloams have been deforested and converted to intensivearable cropping during the last century. This conversionled to a rapid decrease of the organic pool (Arrouaysand Pelissier, 1994) and to an associated decrease inaggregate stability, infiltration and increase of sealing(Le Bissonnais and Arrouays, 1997). These soils thenprovide a unique sequence of soils with the same textureand mineralogy but differing organic matter contentsand physical properties.

The present work aimed (i) to analyze in this soilsequence the possible contribution of SOM to aggregatestability by decreasing their wettability and (ii) to evalu-ate the role of clay-associated organic matter in soilaggregate wettability and water stability.

MATERIALS AND METHODSSoils

The soils were sampled on different terraces of the Py-renean Piedmont: Geaune (G), Hagetmau (H), Adour (A)

Abbreviations: MWD, mean weight diameter; POM, paniculate or-ganic matter; SOM, soil organic matter; WDPT, water drop penetra-tion time.

1479

1480 SOIL SCI. SOC. AM. J., VOL. 64, JULY-AUGUST 2000

Table 1. Soil location, cropping history, and main characteristics.

Period ofSoil namet Terrace cultivation yeai

GIB*KUG1M,Gj7,

Hs*A42aA3J,

L*,,

H27aAuiaHIS,

HIS,,'

GtoULo.

GeauneHagetmauGeauneGeauneHagetmauAire/AdourAire/AdourLacadeeHagetmauAire/AdourHagetmauHagetmanGeauneLacadeeLacadee

10054

100475442323527181518870

•s Sand

11412312417114110191

1191801491181237886

124

Texture

Silt

728725744706737751742701673608731729750720666

Clay

g kg'1 —158152132123122148167180147243151148172194210

Organic C

4.15.98.49.5

10.211.315.317.818.121.524.124.429.730.952.6

C/N

8.58.79.195

10.110.210.513.913.712.914.110.515.114.317.6

PH

6.16.3566.26.86.96.665.75.45.15.15.64.8

CEC± Exchangeable Ca

3.75.64.75.55.97.59.94.57.48.98.67.69.93.14.7

———— cmolt kg '3.75.42.44.95.46.38.65.04.94.73.92.23.33.30.59

Exchangeable Ali

NDt

NDNDNDNDND0.10NDNDNDNDND1.176.57

t The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. Thesubscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer.

I CEC, cation-exchange capacity; ND, not determined.

and Lacadee (L). The samples were collected from a forest siteand from corn cropped fields with different ages of cultivationfrom 7 to 100 yr (Table 1). On cropped sites, samples werecollected from the tilled layer (0 to depth of tillage), and ontwo locations deeper layers were collected (depth of tillageto 0.5 m). Depth of tillage ranged 0.24 to 0.28 m. On the forestsite the sample was collected between 0 and 0.3 m. Details onsampling are reported in Le Bissonnais and Arrouays (1997).

Soils are humic loamy soils classified as Vermic Haplum-brepts (Arrouays et al., 1992). Soil characteristics were deter-mined by standard methods (Baize, 1988), and are presentedin Table 1. The soils had a homogeneous texture with siltcontents ranging from 608 to 751 g kg"1 and clay contentsfrom 122 to 243 g kg"1. Their C contents ranged from 4.1 to52.6 g kg"1 (Table 1). The pHs were moderately acidic andthe cation exchange capacity of the soils was saturated mainlyby Ca in cropped sites and by Al in the forest soil.

The soil clods, while moist, were broken apart by hand intoaggregates <10 mm by following the planes of least resistance,being careful to break them in traction rather than in compres-sion, and then air dried. The 3- to 5-mm aggregates were thenseparated by dry sieving, and coarse plant debris retained onthe 3 mm sieve were discarded. The 3- to 5-mm aggregatesrepresented 20 to 50% of the mass of the soil sieved. The Ccontent of the 3- to 5-mm aggregates was measured by drycombustion and expressed on a 105°C oven-dry weight basis.

Aggregate StabilityAggregate stability was measured according to Le Bisson-

nais (1996) on 3- to 5-mm air-dried aggregates. The methodseparates between the various mechanisms of breakdown:slaking due to fast wetting (Treatment 1), microcracking dueto slow wetting (Treatment 2) and mechanical breakdown ofprewetted aggregates (Treatment 3) (Amezketa et al., 1996).Treatment 1: 5 g of aggregates were immersed in deionizedwater for 10 min. After sucking off the water with a pipette,the soil material was gently transferred on a 0.05-mm sievepreviously immersed in ethanol. The fraction <50 jtm wasrecovered after gentle sieving and oven dried. The fraction>0.05 mm was oven dried and its size distribution was mea-sured by dry sieving with sieves of 2, 1, 0.5, 0.2, 0.1, and 0.05mm. Treatment 2: the aggregates were capillary rewetted ona tension table at 3-cm tension for 30 min before immersionin water. The procedure was then continued as above. Treat-ment 3: the aggregates were rewetted with ethanol, which wasnondestructive. The ethanol was sucked off with a pipette,200 cm3 of deionized water were added and the flask was

agitated end over end 20 times. The procedure was then pur-sued as above. The results are expressed as the resulting frag-ment size distribution and as the mean weight diameter(MWD), which is the sum of the mass fraction remaining oneach sieve after sieving, multiplied by the mean aperture ofthe adjacent sieves. Five replicates were performed for eachtreatment. Calculated MWD values range between 0.025 and3.5 mm for the initial size of aggregates and mesh of sievesused.

Wettability Measurement on AggregatesThe wettability of 3- to 5-mm aggregates was assessed with

the water drop penetration time (WDPT) method of Letey(1969). Results obtained by this method are fairly well corre-lated with other methods to determine the repellency of soils(King, 1981). It is better suited to soils with low degrees ofrepellency, than the Molarity of Ethanol Droplet method(King, 1981). It is simple, rapid and requires only smallamounts of samples.

De-ionized water drops (0.1 ± 0.005 mL) were depositedwith a micro-syringe on the surface of individual air-driedaggregates (3-5 mm diam.), and the time required for thedrop to penetrate the aggregate was recorded. Times less orequal to 1 s were given a value of 1 s. Measurements werereplicated on 100 to 200 individual aggregates for each soil.We performed a one-way variance analysis (ANOVA) withsoil as the main effect. Then we tested the least significantdifference between all WDTP mean values, one to each other,or by grouping them into two groups of organic C contentson the basis of a threshold value of 15 g kg"1. This thresholdvalue comes from a previous study (Le Bissonnais and Arrou-ays, 1997), which indicated a threshold effect for infiltration.

Extraction and Wettability Measurementon the Clay Fraction of Soils

The clay fraction (<2 |xm) of soils was extracted withoutorganic matter destruction, by mechanical dispersion of thesoil and sedimentation according to Balesdent et al. (1991).The C content of the clay fractions was determined by drycombustion and expressed on a 105°C oven-dry weight basis.Total C could be equated with organic carbon (OC) becausethe soils contained no carbonates. For one soil for each terrace,an aliquot of the clay fraction was treated with H2O2 to removethe organic matter. The mineralogy of the clay fraction ofthese samples was determined by x-ray diffractometry usingconventional methods.

CHENU ET AL.: ORGANIC MATTER INFLUENCE ON CLAY WETTABILITY 1481

900 -

jr~ 800| 700

? 600 -o1 50° "£ 400 -o

E 200

100 -n -I

OL

a) -Oa, forest

mm fast wettingKja slow wettingE53 mechanical breakdown

— e-t m — m — d>2 1-2 0.5-1 0.2-0.5 0.1-0.2 0.05-0.1 < 0.05

size fraction (mm)

1000 y900800 -700 -600 -500 -400 -300200 -100 -

b) L7a, 7 years cropping

__^Hsicxx—

>2 1-2 0.5-1 0.2-0.5 0.1-0.2 0.05-0.1 <0.05size fraction (mm)

1000900

_— 800O>

» 70°£ 600I 500I 400S 300

£ 200100

0

c) L35a, 35 years cropping

>2 1-2 0.5-1 0.2-0.5 0.1-0.2 0.05-0.1 < 0.05size fraction (mm)

1-2 0.5-1 0.2-0.5 0.1-0.2size fraction (mm)

Fig. 1. Aggregates size distributions after the water stability tests for four of the studied soils having decreasing C contents in relation to timeof cultivation: (a) L«., forest; (b) L7>, 7 yr cropping; (c) L3S,, 35 yr cropping; and (d) G1Ma, 100 yr cropping.

Oriented deposits were prepared by allowing a 1 mL dropof a 20 g L"1 suspension of the clay fraction to evaporate onglass slides and to dry over silica gel. For two soils, paniculateorganic matter >50 jxm (POM) was also separated (Balesdentet al., 1991; Besnard et al. 1996), air-dried, finely ground andpressed into pellets (Jouany and Chassin, 1987b). Contactangles of water were measured with a Rame Hart telegonio-meter, by depositing de-ionized water drops on the clay depos-its or on the POM pellets with a micro-syringe according toChassin et al. (1986). Contact angles values are an average of25 determinations.

RESULTSCarbon Contents of Soils and Aggregates

The C content of 3- to 5-mm aggregates from culti-vated soils was not significantly different from that ofthe corresponding bulk soil samples (Tables 1 and 2).This was in agreement with previous results on othersilty cultivated soils (Puget et al., 1995). However, theaggregates separated from the forest soil (Loa) had Ccontents of 47.6 ± 0.20 g kg^1 which were significantlylower than that of the bulk sample 52.61 ± 0.62 g kg~!

(Table 1). This was probably due to particulate organicmatter, free from aggregates, which was abundent inthis sample (Besnard et al., 1996).

Aggregate StabilityAs represented on Fig. 1 for a selection of 4 of the

analyzed soils, aggregate size distributions variedwidely, generally becoming dominated by smaller parti-cles with the increasing time of cultivation and decreas-ing C. After fast rewetting, most of the inital aggregates

remained in millimetric size classes for the forest soil(Loa) or the soil cultivated for only 7 yr (L^). Contrast-ingly, for the soil after 35 yr (L35a) and after 100 yearsof cropping (G10oa) most of the initial aggregates haddisrupted to the 0.1 to 0.5 mm size classes. The fastwetting treatment was the most disruptive, and the me-chanical breakdown was the least. The soils ranked be-tween a mean MWD of 2.96 mm, which correspondsto very stable soils to a mean MWD of 0.35 mm thatcharacterizes very unstable soils (Le Bissonnais, 1996)(Table 2). In agreement with previous results on severalof these soils (Le Bissonnais and Arrouays, 1997), thewater stability was significantly correlated with carboncontent of the aggregates (Table 2). Among treatmentsthe MWD after slow and fast rewetting were best corre-lated with C content, and MWD after mechanical break-down the least.

Water Drop Penetration TimeMean WDPT ranged 1 to 32.2 s. Most soils were non

repellent (mean WDPT < Is), had very low repellency(1 < WDPT < 10 s), or low repellency (10 < WDPT< 60 s) according to King's classification (1981) (Table2). For all soils having mean WDPT > 1 s, that is, havingsome degree of repellency, the WDPT varied widelyamong individual aggregates. The hydrophobic charac-ter was due to only a small proportion of the aggregatesexhibiting WDPT > 10 s (Table 2). For example in theLacadee soils, 16.5% of the aggregates had a WDPThigher than 10 s in the soil cultivated for 7 yr (Lya) and61% in the forest soil (Fig. 2). The one way ANOVAanalysis showed a F value of 80.14 (significant for


Table 2. Mean weight diameters (MWD) after the water stability tests (mean of five replicates); water drop penetration times ofaggregates (WDPT), and the determination coefficient of their linear correlation with soil C content. Letters in the WDPT columnsstand for significantly different values (P < 0.05).

MWDfast rewetting

Soil name!

GiootoHs*G1M.Gn,Hsfc

A42.A-32a

L3S,

H1S.Hi8a'G,,uLto

r1

mean

0.260.300.250.400.360.250.290.450.960.530.880.350.471.693.190.70

std

0.010.010.020.040.040.010.010.020.070.050.070.020.030.070.09

MWDslow rewetting

mean

0.340.560.440.580.700.400.651.141.811.221.860.810.892.643.280.73

std

0.030.090.060.090.030.030.040.060.080.090.070.070.070.050.02

MWD mechanicalbreakdown

mean

0.470.890.690.850.660.641.352.411.372.141.830.961.532.682.410.57

std

0.040.080.050.050.050.050.040.070.130.040.070.070.090.070.09

MWDmean

mean

0.350.580.460.610.570.430.761.331.381.301.520.070.962.342.96

std

0.010.020.030.020.020.030.010.030.140.000.230.010.210.030.04

WDPT

mean

l.Oal.Oa1.3cl.Oa1.2bl.OaLib1.4cd5.4e1.5d6.4eLib1.8d5.3e

32.2f0.59

std

0.00.00.90.20.50.00.30.88.90.9

11.40.41.38.9

44.0

Aggregates withWDPT >10 s

o/

00000000

15.60

1808

16.5610.68

Organic C ofaggregates

gkg '3.36.57.9

10.310.211.215.516.417.821.924.124.229.430.247.6


100

10 20 30 40 50 60 70 80 90 100 200WDPT (s)

V*

£

uir

80-

60-

40-

20-

o-

ty • G100b

B H54b

B G100a

0 G47a

D H54a

B A42a

I B A32a

I 1. rfL ^ _1 2 3 4 5 6 7 8 9 1

WDPT (s)Fig. 2. Frequency distribution of water drop penetration time

(WDPT) on individual aggregates of Lacadee soils under forest,and cultivated for 7 and 35 yr: (a) soils from Lacadee terraces and(b) soils from other terraces.

F > 1.69) indicating that the WDPT differs significantlyacross the various soils (Table 2). The grouping of soilsissued from a multiple comparison of WDPT was consis-tent with the range of C values within the groups. Split-ting the samples in two groups according to the thresh-old value of 15 g kg^1 lead to highly significantdifferences in mean WDPT (1.1 for C < 15 g kg~\ 6.3for C > 15 g kg-1).

The mean WDPT and percentage of aggregates hav-ing WDPT > 10 s were both significantly correlatedwith MWD slaking (r2 of 0.88 and 0.94, respectively),showing that the resistance of the aggregates to slakingwas related to their rate of rewetting.

Characteristics and Wettability of the Clayand Particulate Organic Matter Fractions

The fractions <2 u,m of all the soils from all the fourterraces exhibited the same mineralogy. It was a mixtureof predominantly illites, kaolinite and chlorites, withsome quartz (results not shown). There was no smec-tites. The clay fractions separated from the differentsoil samples exhibited a wide range of C contents. TheC content of <2-|jun fractions increased proportionallyto the variations of organic carbon content of the bulksoils (r2 = 0.82) (Fig. 3) and was always higher, as gener-ally found for other silty soils (Balesdent et al., 1991).

The clay fractions exhibited contact angles from 19to 60° (Fig. 4). None of the clay fractions thus exhibiteda truly hydrophobic character (i.e., 0 > 90°). Contactangles increased with the C content of the fraction. Thecurve was fitted with a linear regression (r2 = 0.86),and the contact angle of water on clay without organicmatter was then extrapolated to be of 6 = 16.1 ± 2.71°.We could not measure contact angles directly on claysin which the organic matter had been oxidized by H2O2because the clays then lost their cohesion and it wasthen not possible to make deposits coherent enough tomeasure contact angles. Among the <2-|ji,m fractions


120

b> 100 -

*E 80

1 60M—

| 40

.? 20 -

0

•60 -r

0 20 40 60

C content of bulk soil (g kg"1)Fig. 3. Carbon contents of 2-ixm fractions vs. the C content of soil.

from the different soils the nature of exchangeable cat-ions and the abundance of poorly crystallized Al or Fecompounds differed (Table 3). In the soil under forestthe cation exchange capacity of the bulk soil and of theclay fraction was partly saturated by aluminium (Tables1 and 3). With cultivation and liming of the soil, theclay fraction shifted from Al saturation to Ca saturation,and poorly crystallized Al compounds decreased. How-ever, the nature of exchangeable cations or poorly crys-tallized Al compounds were not significantly correlatedwith the contact angle of the clay fractions (Table 3).

Particulate organic matter from the Lacadee soil culti-vated for 35 yr (L35a) had contact angles of 0 = 54.1 ±1.5° and that from the forest soils (L0a) = 58.0 ± 1.1°.These values are close to those for the clay fractionsfrom the forest soil (L0a) (Fig 4).

DISCUSSIONSoil Organic Matter Imparts SomeHydrophobicity to Clays and Soils

Several authors reported that model organic mole-cules that were adsorbed to clays decreased their wetta-bility (Jouany and Chassin, 1987a; Janczuk et al., 1990;Jouany, 1991). In this study we demonstrated that thisis also true for natural clay-SOM associations, as SOMcompounds associated with clays increased hydropho-bicity. The contact angles values that we measured inthis study were quite comparable to those found withmodel humic substances (Jouany, 1991). The extrapo-lated contact angle of water on clays with no organicmatter showed a hydrophilic behavior of the pure claythat is consistent with literature data (Chassin et al.,1986).

d° = 13.65 (+/-2.71) + 0.35 (+/-0.03) C R2=0.86

C content (g kg"')Fig. 4. Contact angles of water on the clay fractions vs. their C content.

Bars are standard deviations.

The surface energy of clay minerals depends on theirmineralogy and on the nature of their exchangeablecations (Jouany and Chassin, 1987b; Jouany, 1991).Coatings of amorphous Fe and Al compounds decreasethe wettability of clay minerals (Le Souder, 1990). Inthe present study, all the clay fractions had the samemineralogy, and the nature of exchangeable cations orthe presence of noncrystalline Al were not significantlyrelated to hydrophobicity (Table 3). Hence, increasedhydrophobicity of <2 |xm clay fractions was mostly dueto their organic constituents.

The observation of <2-|j,m fractions from other siltysoils with electron microscopy (Robert and Chenu,1992), as well as preliminary observations of the clayfractions from this study have shown that the organicconstituents are small plant or microbial debris, bacte-ria, free amorphous organic matter and organic matterstrongly associated with clay particles, that is, clay coat-ings. Increasing hydrophobicity with increasing SOMcontent in the clay fractions from this study may thuscorrespond to (i) an increasing proportion of organicparticles with hydrophobic character, among mineralhydrophilic ones, (ii) or an increasing coverage of claymineral particles by hydrophobic organic coatings. Withcultivation, the organic inputs to soils change from forestvegetation remnants to maize. Changes in the nature ofSOM in the <2-u,m fractions are then expected andcould also affect the wettability of the clay fractions.

Plant debris could also reduce the wettability of soilaggregates. We found that particulate organic matterwere partly hydrophobic. Several authors demonstratedthe hydrophobicity of plant debris (MacGhie andPosner, 1980; MacGhie and Posner, 1981; Valat et al.,1991; Franco et al., 1995) and MacGhie and Posner(1981) showed that material derived from cereal cropaerial parts was more wettable than that derived fromforest or pasture.


Table 3. Physicochemical characteristics of a selection of clay fractions, r2 is the regression coefficient of variable with contact anglevalue. Non-crystalline Al and Fe were determined after oxalate extraction according to McKeague and Day (1996).

Soilsf

GiMbHs4b

Guxh

Lss.A1SaH27a

L7.HIS,G,.HIS,,'I*r2

Organic Ccontent

gkg~'19.4126.2943.3249.0156.7373.5174.4186.2589.3189.78

112.120.86

CECt

26.1ND*23.121.5ND28.115.920.9ND16.318.20.33

ExchangeableCa

————— cmol, kg"1 —22.6ND16.621.9ND26.214.516.4ND1211.20.42

ExchangeableAl

0ND0.60ND01.10.4ND0.77.50.36

NoncrystallineAl*

————————— gkg'13.0ND4.06.5ND7.17.88.0ND7.97.50.61

NoncrystallineFe*

4.0ND7.68.9ND8.95.97.3ND7.87.00.29

Contactangle

d°19.822.038.233.339.243.943.944.445.545.758.2


t CEC, cation-exchange capacity; ND, not determined.

In the soils of the present study organic matter con-tributed to decrease the wettability of aggregates in twoways: by lowering the wettability of the clay mineralsand by the presence of particulate organic matter.

Relation between Wettabilityand Aggregate Stability

Among the three tests, the MWD after slow and rapidwetting were the best correlated with the C content ofaggregates. Differential swelling of clay and slaking areresponsible for aggregate breakdown in these tests. Inorder to isolate the effect of slaking the difference be-tween MWD values after slow and fast wetting wascalculated (Fig. 5). When cultivated soils only were con-sidered, this difference increased with the C content ofaggregates (r2 = 0.57). Slaking of air-dried aggregateswas thus C content-dependent for cultivated soils. Thevery small difference between MWD after slow or fastwetting observed for the forest soil (Loa) could be as-cribed to the very high water stability of this soil (Fig.1): nearly all aggregates were water-stable to eithertreatment, and the soil was thus given nearly the maxi-mum MWD value in both cases (i.e., 3.19 for fast wettingand 3.28 for slow wetting).

1.2

0.8

I 0.6

1 0.4COQ§: 0.2

forest soil

0 10 20 30 40 50

C content (g kg1)Fig. 5. Difference between mean weight diameter (MWD) after fast

and slow wetting as affected by C content.

Slaking of soil aggregates is due to the pressure devel-oped by air entrapped in pores upon sudden wetting.A reduced rate of entry of water into aggregates allowsair to escape and minimizes slaking (Monnier, 1965).When water is drawn into aggregate pores by capillaryaction, the capillary pressure P is given by

P = 2-y cosO[1]

with r is the pore radius (cm)~y is the surface tension of water (dynes cm"1)6 is the contact angle of water (°)

The rate of water entry into aggregate pores is expressedby Poiseuille's law

r 2 P8-r iz [2]

with v is the rewetting rate of aggregates (cm s"1)r is the pore radius (cm)P is the capillary pressureT| is the viscosity of water (P)z is the length of water penetration in the pore (cm)

Combining Eq. [1] and [2] givesr -v cos 6v = —-———

4 T| z [3]

An increase of the the contact angle of water from16 to 58°, as shown in this study makes cos 6 and thusthe rate of water entry into aggregates to decrease by45%, which would reduce slaking. In this soil sequence,we found that the rate of water entry into agregates(WDPT) increased with C contents. High WDPT maybe ascribed to changes in the contact angle of wateron pore surfaces, to the presence of slightly repellentparticulate organic matter, or to changes in the porediameters. The latter were not investigated in the pres-ent study. An increased hydrophobicity of the clay frac-tions should also reduce the extent of clay swelling, andthereby reduce the extent of aggregate disruption bymicrofissuration.

On the other hand, the resistance of aggregates tomechanical disruption after rewetting with ethanol(Treatment 3) was also related to SOM contents. It


„ 2

E.aI'

0

a) fast wetting 0

0

9 ao "o

00 0 °

3-

I'as i -

n •

b) slow wetting *

a

*i

• *#

>* *

10 20 30 40 50 60Contact angle (d°)

10 20 30 40 SO 60Contact angle (d°)

2.EE.Q

| 1 •

0

c) mean of three testsA

A

A ^A

*A *

40

30

"of

£ 2 00s

10

0.

d) mean WDPT

10 20 30 40 SO 60 10 20 30 40 50 60Contact angle (d*) Contact angle (d°)

Fig. 6. Relation between the wettability of the clay fraction (expressedby the contact angle value) and the stablity and wettability ofaggregates. Bars are standard deviations.

implies that organic matter also acted in this sequenceby increasing the internal cohesion of aggregates. Thiswould also increase the resistance of aggregates to slak-ing and to differential swelling of clays.

Difference of Properties between the ClayFraction and the Aggregates

The water stability of aggregates and their water droppenetration time generally increased with the hydropho-bicity of the clay fractions (Fig. 6). However, there wasno significant linear relationship between clays wettabil-ity and aggregates stability or WDPT. This may be ex-plained by several processes. First, soil fractions otherthan <2 jxm contribute to aggregate properties. Forexample, POM had contact angles of about 55 to 58° andwill contribute to rates of aggregate rewetting. However,POM represents a small proportion of the soil mass:about 6% in the forest Loa soil and 1 to 5% in thecultivated ones (Besnard et al., 1996).

Second, the observed discrepancy may be due to thespatial arrangement of organic matter and organic coat-ings in the soil matrix. Organic matter has an heteroge-neous distribution in the soils matrix, among aggregates(Puget et al., 1995) and at the scale of clay particlesarrangement (Foster, 1981). In the studied soils, we hy-pothesize that the arrangement of the clay fraction andassociated organic constituents within aggregates doesnot expose the organic surfaces to water upon rewetting.

We found that aggregate stability and WDPT werevery variable among individual millimetric aggregates(Fig. 2 and 3). The samples consisted in populations of3- to 5-mrn aggregates with very different water stabilit-ies and wettabilities. As shown on Fig. 1, =90% aggre-gates from the forest soil did not slake and remainedin the >2-mm class. After 7,35, and 100 yr of cultivationrespectively, only 35, 3, and 1% of the soil mass wereaggregates that resisted slaking. Similarly the proportion

of aggregates having a WDPT > 10 s was 61% for thesoil under forest (Lna) and it was of 17% after 7 yr ofcultivation, 15% after 35 yr of cultivation and 0% after100 yr of cultivation. As the C content decreased inthe sequence, the properties of the soil did not changehomogeneously, but rather the proportion of individualaggregates having a high water stability and a highWDPT decreased.

In silty cultivated soils from the Paris basin, the distri-bution of organic matter was found to be heterogeneousas stable aggregates were richer in C and POM thanunstable aggregates (Puget et al., 1995; Puget, 1997). Wehypothesize that organic matter has an uneven spatialdistribution in these soils also and that this explains pro-parte the formation of aggregates with such a rangeof stability and water uptake rates. With cropping andtillage, SOM contents generally decrease due to in-creased SOM mineralization (Balesdent et al., 2000).Aggregate stability under different land uses may beviewed, as suggested by Haynes and Swift (1990), aschanges in the proportion of aggregates with enough Cto be stable.

CONCLUSIONThe detailed analysis of a unique cultivation sequence

from southwest France, in which the organic C contentof soils decreased while soil texture and mineralogyremained constant, permitted evaluation of the contri-bution of organic matter to soils wettability and ag-gregate stability. We have found that organic matterassociated to clay minerals gave them increased hydro-phobicity. The increased water stability of aggregatescould be ascribed to better resistance to slaking, throughincreased hydrophobicity of the aggregates and to in-creased internal cohesion of the aggregates. Both clayfractions and particulate organic matter contributed toincrease hydrophobicity.

Aggregate properties were very heterogeneousamong individual aggregates and were not always re-lated to the properties of the <2-u.m fraction extractedfrom bulk soil. This shows that the spatial distributionof organic matter at the scale of individual aggregatesis of major importance for soil physical properties andshould be analyzed.

ACKNOWLEDGMENTSThe authors thank the technical assistance of P. Berche

for field sampling, H. Gaillard and E. Besnard for the waterstability measurements, M. Perrier for clay fractions analysis,and M. Pernes for x-ray diffractometry.


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organic matter influence on clay wettability and soil aggregate stability

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