Mechanisms of aggregate stabilization in Ultisols from subtropical China

Ž .Geoderma 99 2001 123–145www.elsevier.nlrlocatergeoderma

Mechanisms of aggregate stabilization in Ultisolsfrom subtropical China

Bin Zhang a,), R. Horn b,1

a Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008,People’s Republic of China

b Institute of Plant Nutrition and Soil Science, CAU, 24118 Kiel, Germany

Received 28 October 1999; received in revised form 14 March 2000; received in revised form 15May 2000; accepted 15 May 2000

Abstract

The quantification and interpretation of aggregate stability depend on internal soil propertiesand external factors such as measurement method and aggregate size. The objectives of this study

Ž .were to: i determine the aggregate stability in Ultisols from subtropical China applying the LeŽ . Ž .Bisssonais Method; ii determine the effect of initial aggregate size on its stability, and iii

Žinterpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges 5–3, 3–2.and 2–1 mm were obtained by dry sieving. After the wetting treatments, the dominant fraction of

fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdownwas in the order, slaking)mechanical breakdown)micro-cracking. They differed with soil type

Ž .and composition. The normalized mean weight diameter NMWD of the aggregates after fastwetting and wet stirring were more correlated with soil properties, such as degree of micro-aggre-

Ž . Ž .gation DOA , cation exchange capacity CEC , K O, Fe O or Al O rather than clay and soil2 2 3 2 3Ž .organic carbon SOC content. The binding force by soil organic matter was smaller than the force

caused by entrapped air or the force of combination of mechanical stress by stirring anddifferential swelling of minerals.

) Corresponding author. Current address Institute of Soil Science, Chinese Academy of Sci-ences, P.O. Box 821, Nanjing, 210008, People’s Republic of China. Tel.: q86-25-3369284; fax:q86-25-3353590.

Ž . Ž .E-mail addresses: [email protected] B. Zhang , [email protected] R. Horn .1 Permanent address: Institute of Plant Nutrition and Soil Science, University Kiel, Olshausen-

str. 40, 24118 Kiel, Germany. Tel.: q49-431-880-3190; fax: q49-431-880-2940.

0016-7061r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 00 00069-0

( )B. Zhang, R. HornrGeoderma 99 2001 123–145124

The smaller the aggregate, the larger was the aggregate stability according to NMWD. Therankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams

Ž . Ž .from sandstone Sc and Sw were the weakest soils while the purple mudstone Pp was thestrongest. All the cultivated soils decreased in aggregate stability compared with the comparableuncultivated soils or parent materials irrespective of the cultivation time and the changes in SOCcontent after cultivation. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: mechanisms of aggregate stabilization; Le Bissonnais method; normalized meanŽ .diameter NMWD ; ultisols; subtropical China

1. Introduction

ŽImproper land use leads to degradation of red soils in China Gong and Shi,. Ž1992 . After agricultural cultivation, water-stable macro-aggregates )0.25. Ž .mm are segregated to some extent Yao and Yu, 1982 . Breakdown of unstable

aggregates results in pore collapse and produces finer and more-easily trans-portable particles and micro-aggregates. These particles and micro-aggregatesare very important in the processes of infiltration, seal and crust formation,runoff and soil erosion, and subsequently lead to deterioration of soil structure

Ž .and plant drought stress Levy and Miller, 1997 . Aggregate stability is,therefore, an important property to explain, quantify, or to predict these pro-cesses or changes in some basic soil properties with respect to soil water erosionand soil sealing, and to propose appropriate land use or restoration method ofdegraded soils.

Generally, aggregate stability depends on soil properties such as organicmatter, clay and oxide contents. But all organic compounds in soils are not

Ž .responsible for aggregation Oades, 1984 and different kinds of organic matterstabilise aggregates of different sizes and they may have no effect on swelling

Ž . Ž .soils Coughlan et al., 1973 . Hempfling et al. 1990 described the positiveŽ .effect of unsaturated fatty acids at the same carbon content in sharply

increasing soil strength. Red soils in subtropical China are rich in sesquioxideŽ . Ž .and poor in soil organic carbon SOC Li, 1985 . Organic carbon and sesquiox-

ide are regarded as the key factors, which play a very different role in theaggregation of red soils. Organic carbon has a greater effect on macro-aggrega-

Ž .tion Yao et al., 1990; Zhang et al., 1996 , while sesquioxide has a greater effectŽ .on micro-aggregation Yao et al., 1990 . Free and weakly bound carbon and

carbon combined with clay are the dominant organic cementing material inŽ .aggregates Yao et al., 1990 . Clay mineralogy is another important factor in

aggregation. Stability of clayey soils depends on the physicalrchemical proper-ties of the clay. Smectitic clays are more dispersible than kaolinitic claysŽ .Goldberg and Glaubig, 1987 and illitic and kaolinitic soils, which containsmall amounts of smectite, may be dispersible and as susceptible to sealing as

Ž .smectitic soils Stern et al., 1991 .

( )B. Zhang, R. HornrGeoderma 99 2001 123–145 125

Aggregate stability is also affected by external factors, such as initial soilwater content, rate of wetting, initial aggregate size and method of determina-tion. Common methods for aggregate-stability determination include the crush-

Ž .ing test, wet sieving Kemper and Rosenau, 1986 and the drop-test techniquesŽ . Ž .Farres, 1980 , application of ultrasonic energy North, 1979 and controlling

Ž . Ž .rates of wetting Pierson and Mulla, 1989 . Le Bissonnais 1996 has proposed amethod that accounts for breakdown processes and size distribution of break-down products. The method uses initial dry soil, which is regarded as the most

Ž .sensitive indicator of variability of aggregate stability Martinez et al., 1998 ,Žand three rates of wetting to differentiate the breakdown mechanisms slaking,

.mechanical breakdown, micro-cracking . The method has been applied in theefficient assessment of the relationship between aggregate stability and crusting

Ž .of silty soils Amezketa et al., 1996 and initial moisture content on interrillŽ .erosion Le Bissonnais et al., 1995 as well as organic carbon content on

Žaggregate stability and crusting of humic soils Le Bissonnais and Arrouays,.1997 .

Most methods of aggregate-stability measurement have used a specific soilsize fraction to allow a comparison between soils with different initial condi-

Ž .tions. Le Bissonnais 1988 reported that final fragment size distribution afterimmersion in water of fairly unstable silty soil was not affected by initialaggregate size ranging from 2 to 20 mm in diameter. Le Bissonnais and

Ž . Ž .Arrouays 1997 used 5–3 mm aggregates while Amezketa et al. 1996 usedŽ .2–1 mm aggregates. However, Garey 1954 reported that the results from

selected water-stable aggregate sizes differed from those of the whole soil.Stability of soil aggregates differs with aggregate size due to the principle of

Ž .porosity exclusion Currie, 1966; Dexter, 1988a , and partly because of theŽdifferent mechanisms responsible for aggregation Tisdall and Oades, 1982;

.Truman and Bradford, 1990 .Ž .The objectives of this study were to: i determine the aggregate stability in

ŽUltisols from subtropical China applying the Le Bissonnais Method Le Bisson-. Ž .nais, 1996 method; ii determine the effect of initial aggregate size on its

Ž .stability, and iii interpret mechanisms of aggregate stabilization of the soils.

2. Materials and methods

2.1. Soils

The soil samples were geographically representative for the soils in JiangxiProvince. They covered the most soil parent materials in subtropical China. Thesoil parent materials were Quaternary red clay, sandstone, granite and purple

Ž .mudstone Table 1 . According to the US Soil Taxonomy Classification, the


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soils are in Ultisols except those from purple mudstone, which are in Aquepts.Land uses had been taken into consideration when sampling. The soils withsubscript p indicated that they were originated from eroded parent materials.Those with subscript w were from uncultivated grass upland and with subscriptc from cultivated land, respectively.

Ž .Samples were taken from the surface layer 0–15 cm of nine soils. Some ofthe soil samples were carefully reserved for macro-and micro-aggregation tests.The air-dried soils were packed in plastic bags. Soil physical and chemical

Ž .properties were determined using the routine method ISSAS, 1978 . Particle-sizedistribution was measured by the pipette method. Determination of the soil

Ž .mineral elements was carried out on homogenous samples -0.149 mm fusedby sodium carbonate and extracted with hydrochloric acid. SiO was measured2

using the mass method, Fe O , MnO, TiO and P O using the colorimetry,2 3 2 5

Al O using the volumetric method with EDTA extract substituted by potassium2 3

fluorine, CaO and MgO using the volumetric method with EDTA extract. SOCwas determined using the method of oxidation with potassium dichromate inheated oil bath, total nitrogen using the semi-, micro-Kjeldahl method, cation

Ž .exchange capacity CEC using the ammonium acetate method.Samples for clay mineralogy were treated with 30% H O to remove organic2 2

matter and with dithionite–citrate–bicarbonate to remove free iron and alu-minum oxides. The clay samples were saturated with 0.25 M MgCl solution2

and glycerol. The samples were finally examined using a Phillips diffractometerwith CoK radiation. Relative proportions of the different mineral phases werea

given by the relative heights of peaks in X-ray diffraction traces.

2.2. Methods of measurement of aggregate stability

Macro-aggregate stability was measured with two methods, the routine dry-Ž . Žand wet-sieving ISSAS, 1978 and the Le Bissonnais method Amezketa et al.,

.1996; Le Bissonnais, 1996 .In the routine method, the air-dried soil samples were sieved by hand on a

column of six sieves: 5, 3, 2, 1, 0.5 and 0.25 mm. The mass percentage of eachsize fraction was calculated. Based on these percentages, composite soil sampleswere made for wet sieving. The soils were wetted slowly by adding water up tosaturation and kept for 10 min so as to drive entrapped air from the aggregates.After corking and agitating end over end 10 times in water, the soil sampleswere placed on the same column of sieves, which were raised and lowered 3–4cm under water surface. The material remaining on the sieves was oven driedand weighed to give a stable aggregate mass.

In the Le Bissonnais method aggregates of 5–3, 3–2 and 2–1 mm in diameterwere obtained by dry sieving and oven dried at 408C. Five grams of each samplewere used and repeated five times in a wetting treatment. The aggregates weretreated with fast wetting by water, wet stirring in water after being immersed in


ethanol for 10 min, and slow wetting at a tension of 3 hPa. Segregation of thepre-treated aggregates was conducted by moving 0.063 mm sieve immersed inethanol up and down 20 times by hand. After oven-drying, the )0.063 mmfraction was dry sieved by hand 20 times on a column of six sieves: 2, 1, 0.63,0.2, 0.1 and 0.063 mm.

Ž .Aggregate stability was expressed using the fragment-size distribution FSD ,Ž .and the mean weight diameter MWD in both methods, and the percentage of

Ž .aggregate destruction PAD in the routine method.

nq1 r qriy1 iMWDs =m 1Ž .Ý i21

Ž .where r saperture of the ith mesh mm , r sr and r sr ; m smassi 0 1 n nq1 i

fraction of aggregates remaining on ith sieve; nsnumber of the sieves.

m ymd wPADs =100 2Ž .

md

where m smass fraction of aggregates )0.25 mm after dry sieving; andd

m smass fraction of aggregates )0.25 mm after wet sieving.w

To differentiate the effects of the wetting treatments, relative slaking indexŽ . Ž .RSI and relative mechanical breakdown index RMI were defined as follows

MWD yMWDSW FWRSIs =100 3Ž .

MWDSW

MWD yMWDSW WSRMIs =100 4Ž .

MWDSW

where MWD sMWD in the treatment of slow wetting; MWD sMWD inSW FW

the treatment of fast wetting; and MWD sMWD in the treatment of wetWS

stirring.In order to compare the stability of different sized aggregates, the MWD was

normalized with the following method.

MWDNMWDs =100 5Ž .

r yrmaxyj min

where, NMWDsnormalized mean weight diameter, mm; r smaximummaxyj

sieve aperture for the jth initial aggregate, i.e., 5, 3 and 2 mm for the sizedaggregates from the big to the small, respectively; r sminimum sieve-aper-min

ture, i.e., 0.063 mm.Ž .The microaggregate- -0.25 mm size distribution was measured using the

Ž .pipette method ISSAS, 1978 . The method was the same as being applied to


Ž .soil particle-size analysis except that dispersant sodium hydroxide was notused. Micro-aggregates were separated into four classes, i.e., 0.25–0.1, 0.1–0.05and 0.05–0.002 mm, -0.002 mm. Microaggregate stability was expressed as

Ž .the degree of aggregation DOA .

w ywa bDOAs =100 6Ž .

wa

where w sproportion of particles between 0.25 and 0.05 mm from microaggre-a

gate size analysis; and w sproportion of particles between 0.25 and 0.05 mmb

from particle size analysis.

2.3. Statistical analysis

Ž . ŽA multi-factor analysis of variance ANOVA was performed Statistical.Analysis Systems Institute, 1988 . Significance of the effects of wetting treat-

ment, initial aggregate size and soil type on FSD and MWD were determinedusing Student–Newman–Keuls test procedures. Statistically different averageswere identified by a letter following. Linear regression analyses between the

Ž .parameters of aggregate stability PAD, NMWD and between them and somesoil properties and DOA were also performed using LSD.

3. Results

3.1. Soil characteristics

The physical and chemical properties of the soils are summarized in Table 2.ŽThe soils ranged from sandy loam soils from granite, Gc and Gw, and

. Žsandstone, Sc and Sw , clayey loam soils from Quaternary red clay, Qc, Qp and. ŽQw , to silty clay and silty loam cultivated soil and soil parent material from

.purple mudstone, Pc and Pp, respectively . All soils except Pp contained some)2 mm quartz.

All soils except the purple soils were strongly acidic, the pH ranging fromŽ .4.58 to 5.53 Table 2 . They were rich in sesquioxide, but poor in soil organic

matter and relatively low in total exchangeable bases. These properties variedwith the soil parent materials as well as the land-use history. The purple soilshad the highest exchangeable bases and the sandstone soils had the lowest. Thesoil parent materials contained very low content of organic carbon. The culti-vated soils except the granite soil had higher SOC content and pH valuescompared with the corresponding uncultivated soils. Clay mineralogy is shownby X-ray diffraction patterns in Fig. 1 and summarized in Table 1. The purple


Tab

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Ž . Ž .Fig. 1. X-ray diffractograms for oriented samples Mg–ethylene glycol of clay -0.002 mm forŽthe nine soils. K skaolinite, Smssmectite, HMshydromica, Vs vermiculite, QzsQuartz,

.HBTshydrobiotite, CHTschlorite .


soils had abundant hydromica and smectite clay minerals while the others weredominant in kaolinite, containing additionally some hydromica and vermiculiteŽ .Table 1 .

3.2. Aggregate stability

3.2.1. Aggregate stability measured by routine methodŽ .The purple soils Pc, Pp had the highest fraction at the size of )5 mm

aggregate after dry sieving, whereas the others showed the highest fraction at theŽ .size of -0.25 mm aggregates after wet sieving Table 3 . The soils were

ranked differently according to the parameters of soil aggregate stability.Ž .According to PAD, the soil parent materials Pp and Qp , the uncultivated

Ž . Ž .granite soil Gw , and the cultivated soil from Quaternary red clay Qc wereŽ .most stable while the sandstone soils Sc and Sw were the least stable. It was

also found that the cultivated soils were weaker in aggregate stability comparedwith the corresponding uncultivated soil or its soil parent material.

3.2.2. Aggregate stability measured by Le Bissonnais methodIn general, fragments dominated from 0.1 to 2 mm in the fast-wetting

treatment, )0.2 mm in the wet-stirring treatment and )1 mm in the slow-wet-Ž .ting treatment Table 4 . The products of the three sized aggregates were

different in the fraction of the fragments )0.2 mm. The soil behaved differ-ently after wetting, causing different FSD pattern varied with initial aggregate

Ž .size data not shown . On the average, the highest fraction was from 0.63 to 0.2mm for the sandstone soils and from 2 to 1 mm for the other soils.

The total averages MWD and NMWD agreed strongly with each other withrespect to the effect of wetting treatment and to the rankings of the soils

Ž .according to their aggregate stability Table 4 . Aggregates were weaker in thefast-wetting treatment than in the wet-stirring treatment and than in the slow-wetting treatment. The aggregate stability of the soils were ranked in the order,Pp)Gw)Qp)GcrQw)PcrQc)Sw)Sc, which was not the same as thatby the routine method although the most and the least stable soils were

Ž .distinguished Tables 3 and 4 . It was also shown that cultivated soils wereweaker than the comparable uncultivated soils or the soil parent material.

Initial aggregate size had an effect on measured stability. MWD averageincreased with an increased initial aggregate size, while NMWD decreased with

Ž .an increased initial aggregate size Table 3 . The soils were ranked not in thesame order according to the NMWD of the different initial aggregate sizes in

Ž .different treatment Fig. 2 . The weaker the soils, the smaller were the differ-ences between the NMWD of the different initial aggregate sizes in eachtreatment.

Ž .Breakdown mechanisms varied with soil Table 5 . The soil parent materials,Pp and Qp, had the lowest mechanical breakdown effect indicated by RMI,

()

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Table 3Aggregate stability measured by the dry- and wet-sieve method and microaggregate stability of the soils studied

Ž .Soil Sieving Fragment size distribution % MWD MWD PAD DOAdry – wetŽ . Ž . Ž . Ž .method mm mm % %)5 mm 5–3 mm 3–2 mm 2–1 mm 1–0.5 mm 0.5–0.25 mm -0.25mm

Gc Dry 26.75 13.81 14.42 13.19 13.9 7.15 10.78 2.61 1.52 36 59Wet 5.95 4.67 6.48 10.24 17.93 11.54 43.19 1.09

Gw Dry 28.52 10.95 14.70 12.09 15.37 7.70 10.67 2.58 1.79 17 72Wet 14.74 9.01 9.66 14.17 19.17 7.15 26.10 1.79

Pc Dry 42.7 16.92 15.80 9.55 9.00 2.95 3.08 3.44 2.25 23 93Wet 1.62 7.24 10.06 16.51 29.29 9.75 25.53 1.19

Pp Dry 46.60 28.85 15.10 5.45 1.85 0.54 1.61 3.96 0.91 6 74Wet 5.61 53.74 15.76 11.22 4.89 0.98 7.80 3.05

Qc Dry 24.62 8.74 7.15 8.78 14.7 13.49 22.52 2.11 1.67 54 62Wet 0.14 0.78 1.39 3.62 11.79 17.96 64.32 0.44

Qp Dry 29.7 27.7 20.2 8.47 5.91 2.40 5.62 3.29 2.04 23 61Wet 2.93 9.57 9.05 11.65 26.79 12.87 27.14 1.25

Qw Dry 29.31 8.77 7.67 6.56 9.5 9.73 28.46 2.29 1.47 34 62Wet 3.73 3.57 4.06 5.89 14.48 15.36 52.91 0.82

Sc Dry 36.08 9.91 5.78 4.5 5.73 7.46 30.54 2.56 2.09 56 15Wet 1.59 1.16 0.98 1.18 8.51 16.86 69.72 0.47

Sw Dry 34.48 9.52 6.16 4.78 5.46 9.00 30.6 2.48 1.56 47 26Wet 8.34 4.38 1.51 2.67 5.74 13.94 63.42 0.92

MWDsmean weight diameter; MWD sdifference between dry sieving and wet sieving; PADspercentage aggregate destruction; DOA sdegreedry – wet

of micro-aggregation.

()

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Table 4Ž . Ž .Multiple comparison of the effects of wetting treatment, aggregate size and soil on fragment-size distribution FSD , mean weight diameter MWD and

Ž .normalised mean weight diameter NMWD

Ž .Factor Level FSD % MWD NMWDŽ . Ž .mm mm)2 mm 2–1 mm 1–0.63 mm 0.63–0.2 mm 0.2–0.1 mm 0.1–0.063 mm -0.063 mm

Wetting SW 41.36a 41.76a 5.66b 5.63c 2.50c 0.73c 2.37c 1.53a 0.52aTreatment WS 22.29b 37.31b 11.39a 15.26b 7.32b 2.35b 4.08b 1.18b 0.40b

FW 9.31c 26.27c 11.63a 26.48a 15.43a 4.66a 6.22a 0.82c 0.28cAggregate S 54.82a 13.68a 16.04a 8.39a 2.58a 4.49a 1.02c 0.52aSize M 27.59b 34.08b 7.85b 15.54b 8.40a 2.59a 3.95b 1.21b 0.41b

B 45.38a 16.43c 7.15c 15.79ab 8.46a 2.57a 4.23a 1.30a 0.26cSoil type Gc 28.24c 36.92d 9.64d 11.41f 5.48e 2.96c 5.34b 1.26d 0.43d

Gw 39.03a 41.80b 6.24f 5.66g 2.72g 1.55e 3.00e 1.49b 0.51bPc 15.60f 38.79c 17.21a 19.24d 3.98f 1.28f 3.90d 1.12e 0.38fPp 33.74b 56.89a 4.91g 1.87h 0.481 0.26h 1.85f 1.58a 0.54aQc 19.13d 32.66e 11.51b 21.55c 8.11c 2.70d 4.30cd 1.07f 0.37fQp 33.2b 39.43c 10.65c 11.84f 2.36h 0.86g 1.66f 1.40c 0.47cQw 27.36c 33.41e 9.02d 15.72e 7.14d 2.74d 4.61b 1.20d 0.41eSc 5.11g 14.75g 9.42d 31.04a 25.63a 6.12a 7.94a 0.58h 0.20hSw 17.50e 21.35f 7.42e 23.76b 19.85b 4.76b 5.36b 0.87g 0.30g

SWsslow wetting; WSs wet stirring; FWs fast wetting; Bsbig aggregate size; Msmedium aggregate size; Sssmall aggregate size. Lettersfollowed the values indicate the significance at the 0.05 probability level.


Ž . Ž .Fig. 2. NWMD of the soils after the treatments of fast-wetting a , wet-stirring b , andŽ . Ž .slow-wetting c means of five replicates; bars indicate standard deviation . The soils are ranked

by the NMWD of 2–1 mm aggregates in each wetting treatment.


Table 5Ž . Ž .Relative slaking index RSI and relative mechanical breakdown index RMI by the Le

Bissonnais method

Soil 2–1 mm 3–2 mm 5–3 mm

RMIr%Qp y8 2 4Pp 1 3 4Pc 3 10 33Gw 12 13 17Qw 19 27 22Qc 22 27 31Gc 22 29 33Sc 50 39 39Sw 55 59 69

RSI%Pp 12 25 27Gw 20 22 33Pc 39 36 50Gc 45 48 53Qp 47 43 35Qw 56 51 49Qc 60 61 65Sc 67 62 56Sw 73 74 76

while Gw and Pp had the smallest slaking effect indicated by RSI. The minusvalue of RMI for Qp of 2–1 mm aggregate size showed that the effect ofmechanical breakdown was even smaller than the micro-cracking effect. RMIand RSI generally increased with increased initial aggregates with an exceptionfor the soils, Sc and Qw, as well as an exception in RSI for the soil, Qp.

3.3. Aggregate stability and soil properties

The parameters of aggregate stability from the two methods were linearlyŽ .correlated to each other with some exceptions Table 6 . PAD by the routine

method was more correlated to the NMWD by the Le Bissonnais method. Nosignificant relationships were found between the NMWD values of some sizedaggregates in the treatments of slow wetting and fast wetting, or in thetreatments of wet stirring and slow wetting.

In both methods, the parameters of aggregate stability were not significantlyrelated to clay content or to SOC content, but significantly to silt content and to

Ž . Ž .degree of micro-aggregation DOA Table 7 . CEC, Fe O and Al O and2 3 2 3

K O contents may have contributed to aggregate stability as it evidenced by2

significant correlation coefficients. In the routine method, PAD was correlated

()

B.Z

hang,R.H

ornr

Geoderm

a99

2001123

–145

137

Table 6Correlation coefficients for the linear relationships between the parameters of aggregate stability in different treatments

NMWD Correlation coefficientsby the Le Routine method NMWD by the Le Bissonnais methodBissonnais

PAD MWD by FW21 FW32 FW53 WS21 WS32 WS53 SW21 SW32Methodthe westseivingmethod

)) ))FW21 0.930 0.897)) )) ))FW32 0.897 0.836 0.974)) ) )) ))FW53 0.812 0.794 0.897 0.975) ) )) )) ))WS21 0.651 0.661 0.813 0.882 0.931) ) )) )) )) ))WS32 0.726 0.734 0.859 0.918 0.959 0.989) )) )) )) )) ))WS53 0.661 Ns 0.811 0.889 0.955 0.989 0.987) ) ) ) )SW21 0.653 Ns 0.643 0.654 Ns 0.656 0.650 Ns) ) ) ) ) ))SW32 0.658 Ns Ns 0.661 Ns 0.692 0.691 0.637 0.970

) ) ) ) ) )) ))SW53 Ns Ns Ns 0.678 0.671 0.751 0.720 0.686 0.796 0.885

FWs fast wetting; MBsmechanical breakdown; SWsslow wetting; RMIs relative mechanical breakdown index; RSIs relative slakingindex:Ž .MWDsmean weight diameter mm ; NMWDsnormalized mean weight diameter; PADspercentage of aggregate destruction.

Values followed FW, WS, SW, RMI, or RSI indicate the aggregate sizes, namely, 21saggregates from 2–1 mm. 32saggregates from 3–2 mm and53saggregates from 5–3 mm.Nssnot significant.

) Significant at 0.05 level.) ) Significant at 0.01 level.


Tab

le7

Cor

rela

tion

coef

fici

ents

for

the

line

arre

lati

onsh

ips

betw

een

the

para

met

ers

ofag

greg

ate

stab

ilit

yan

dth

eso

ilpr

oper

ties

Par

amet

ers

ofC

orre

lati

onco

effi

cien

tsa

bb

aggr

egat

est

abil

ity

Sil

tC

EC

SO

CK

OK

OM

gOM

nOM

nOF

eO

Al

OD

OA

22

23

23

))

))

))

))

))

))

))

))

DO

A0.

671

Ns

0.79

50.

8360

0.80

80.

661

0.82

80.

819

0.92

90.

934

1.00

0)

))

))

))

))

))

)

Sil

tco

nten

t1.

000

0.92

00.

777

Ns

Ns

0.92

00.

809

0.81

50.

698

Ns

0.67

1)

,b)

Cla

yco

nten

tN

s0.

715

Ns

Ns

Ns

Ns

Ns

0.72

6N

sN

sN

s)

))

))

))

))

)

Rou

tine

met

hod

PA

D0.

736

0.70

9N

s0.

715

0.84

20.

718

0.65

5N

s0.

719

0.66

10.

723

))

))

MW

Dof

dry

siev

e0.

799

0.75

6N

sN

sN

s0.

789

0.75

1N

sN

sN

sN

s)

))

MW

Dof

wet

siev

e0.

751

0.84

1N

sN

sN

s0.

765

Ns

Ns

Ns

Ns

Ns

))

))

))

)

NM

WD

ofL

eF

W21

0.63

20.

757

Ns

Ns

0.78

80.

670

Ns

Ns

0.63

50.

636

0.65

5)

))

))

))

Bis

sonn

ais

Met

hod

FW

32N

s0.

671

Ns

0.67

20.

854

Ns

Ns

Ns

0.71

40.

719

0.71

2)

))

))

))

))

)

FW

53N

s0.

662

Ns

0.73

70.

896

Ns

Ns

0.72

80.

783

0.76

30.

722

))

))

))

))

))

))

)

WS

210.

653

0.70

5N

s0.

720

0.86

2N

sN

s0.

915

0.91

30.

907

0.79

9)

))

))

))

))

))

)

WS

320.

6440

0.72

7N

s0.

667

0.84

7N

sN

s0.

893

0.86

40.

843

0.74

5)

))

))

))

))

))

)

WS

530.

641

0.68

3N

s0.

720

0.87

8N

sN

s0.

887

0.88

60.

872

0.75

9)

))

)

W53

Ns

Ns

Ns

Ns

Ns

Ns

Ns

0.71

30.

732

0.72

90.

715

))

))

))

))

))

Ave

rage

MW

DN

s0.

687

Ns

0.67

90.

766

Ns

Ns

0.81

60.

822

0.81

80.

744

))

))

))

)

Le

Bis

sonn

ais

Met

hod

RS

I21

0.67

30.

750

Ns

Ns

0.77

60.

746

Ns

Ns

0.66

20.

652

0.71

7)

))

))

)

RS

I32

Ns

Ns

Ns

0.73

90.

803

0.68

4N

sN

s0.

698

0.63

50.

760

))

)

RS

I53

Ns

0.63

1N

sN

s0.

788

Ns

Ns

Ns

Ns

Ns

Ns

))

))

))

))

))

))

))

)

RM

I21

0.67

90.

643

0.73

300.

794

0.80

6N

s0.

731

0.92

10.

944

0.91

40.

858

))

))

))

))

))

))

)

RM

I32

0.67

80.

676

0.72

00.

720

0.74

2N

s0.

696

0.79

00.

856

0.82

00.

786

))

))

))

RM

I53

Ns

0.64

00.

752

Ns

0.75

2N

sN

s0.

727

0.74

10.

741

Ns

Ž.

SO

Cs

soil

orga

nic

carb

on;

CE

Cs

cati

on-e

xcha

nge

capa

city

;D

OA

sde

gree

ofm

icro

-agg

rega

tion

;M

WD

sm

ean

wei

ght

diam

eter

mm

;N

MW

Ds

norm

aliz

edm

ean

wei

ght

diam

eter

;P

AD

spe

rcen

tage

ofag

greg

ate

dest

ruct

ion.

FW

sfa

stw

etti

ng;

WS

sw

etst

irri

ng;

SW

ssl

oww

etti

ng.

RM

Isre

lati

vem

echa

nica

lbr

eakd

own

inde

x;R

SIs

rela

tive

slak

ing

inde

x;V

alue

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MB

orS

Win

dica

teth

eag

greg

ate

size

s,na

mel

y,21

sag

greg

ates

from

2to

1m

m,

32s

aggr

egat

esfr

om3

to2

mm

and

53s

aggr

egat

esfr

om5

to3

mm

.a

Ž.

Soi

lor

gani

cca

rbon

SO

Cex

lcud

ing

Pp

and

Qp.

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Oex

clud

ing

Pc

and

Pp.

2)

Sig

nifi

cant

at0.

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))

Sig

nifi

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at0.

01le

vel.


with more soil physical and chemical properties than MWD. In the Le Bisson-nais method, NMWD of all sized aggregates in the wet-stirring treatment wasaffected by silt content. NMWD of all aggregates in the treatments of fastwetting and wet stirring had significant relationships with DOA, CEC andcontents of K O, Fe O and Al O . In the slow-wetting treatment, NMWD of2 2 3 2 3

the large aggregates had a significant relationship with sesquioxide contents andDOA. Total average NMWD was not correlated with silt content.

Ž .When the purple soils Pp and Pc with dominant smectite were not included,NMWD were significantly related to MnO content and the correlation coeffi-cient between NMWD and K O increased for all sized aggregates in the2

Ž .wet-stirring treatment and for the big aggregate 5–3 mm in the treatments ofŽ .fast wetting and slow wetting. When the soil parent materials Qp and Pp with

low SOC were not included, the RMI was found to be negatively correlated withSOC content. In addition, the RSI had less significant relationships with the soilproperties for the bigger aggregates. The RMI had less significant relationshipswith soil properties than the RSI. This indicates that aggregate porosity and itstortuosity determined slaking effects in the fast-wetting treatment, whereas soilmicroaggregate stability and soil chemical properties determined mechanicalbreakdown in the wet-stirring treatment.

4. Discussion

4.1. Soil aggregation processes and soil properties

In soils with high clay content under field conditions, the mineral particlestend to form soil aggregates. During consecutive swelling and drying cycles,

Ž .stronger and smaller aggregates develop Horn, 1994 . Their compaction de-pends on water surface tension, mineral particle mobility and tensile strength ofaggregated particles. Aggregation is further enhanced by such biological andchemical processes as flocculation, cementation by organo-mineralic bondingŽ .Dexter et al., 1988 . During drying and wetting, the amount of coarser particles,both silt and sand, increases in the centre of the aggregates and the outer skin ofaggregates is clay enriched as compared to the inner part due to particle

Ž .transport caused by changes in water surface tension Horn, 1987 . Therefore,Ž . Ž .aggregate stability depends on i capillary forces, ii intensity of shrinkage,

Ž . Ž .iii number of swelling and shrinkage cycles, iv mineral particle mobility, andŽ .v binding energy between particles inror between aggregates in the bulk soilsŽ .Horn, 1994 .

Thus, mechanisms can be also detected by the results obtained for varioussoils from China. After the treatments, the highest fraction of fragments for eachsoil was at the fragment size of 2–1 or 0.63–0.2 mm rather than the fragments-0.1 mm. However, even if the smallest fragment were not dominating the

Ž .formation of seals and crusts Le Bissonnais, 1988; Loch, 1994 observed in the


Žfield confirmed the severity of these processes Lu et al., 1986; Xu and Yao,.1990 , even induced by greater fragments. The relationship between seal

formation and aggregate breakdown needs further investigation study in field orby simulation of rainfall.

Micro-aggregates of clayey soils are well developed due to drying andwetting dynamics. Silt particles between these clay-coated water stable aggre-gates might only be weak binding points due to relative smaller surface area of

Ž .silt in bigger water-stable aggregates 2–1 and 0.63–0.2 mm in size . Also dueto the positive chemical binding effect pseudo silt originated from aggregatedclay particles resulted in higher strength and reduced slaking intensity. There-fore, silt rather than clay content were significantly related to aggregate stabilityin the wet-stirring treatment, in which the effect of silt could not be shadowedby the slaking effect as in the fast-wetting treatment.

Ž .According to Kezdi 1969 , the mechanical strength increased with thenumber of layer and longer distances even if clay mineralogy could not totallyexplain soil aggregate stability. It could be proved that the purple soils weredominating smectite minerals but were not among the weakest. Those soils withhigher amount of smectite minerals were weaker than those with less smectite ofvermiculite minerals were. However, we have to consider the effect of CEC,

Ž .which is higher in soils with expanding minerals e.g., smectite and vermiculiteand with increased organic matter. The CEC affects aggregate stability as couldbe also verified.

Soil oxides and SOC acted as very important bonding agents in aggregationof the soils. Fe and Al oxide have large total surface area and they reacts with

Ž .clay particles through Coulombic interactions Sumner, 1992 , resulting in aŽ .determinative binding effect of the microaggregate Yao et al., 1990 . Potassium

oxide favourably contributes to the aggregate stability in a low concentrationŽ .due to its little ability to cause soil welling Helfferich, 1962 . Levy and van der

Ž .Watt 1990 showed that when K was the complementary cation to Ca,Ž .increasing the exchangeable potassium percent EPP resulted in a decrease in

Ž .infiltration rate IR if EPP ranges from 6.5 to 14.6 based on three soils fromSouth Africa. Infiltration rate increased when EPP increased at a low range, e.g.,from 5.3 to 6.5 for a kaolinitic Plinthustalf. The authors concluded that K has anintermediate effect between that of Ca and Na on the infiltration rate of the soilsexposed to rain.

Soil organic matter enhances soil aggregate stability or soil strength in aŽ .complex way Soane, 1990 . It increased the friction between particles and

binding effect due to increased menisci forces because of increased finer porearea at the same pore water pressure as experimentally demonstrated by Zhang

Ž . Ž .and Hartge 1995 . According to Goldberg et al. 1990 , soil organic matter canact as an aggregating or segregating material or have no noticeable influence onaggregate stability, depending on its composition in soil andror the relative

Ž .contributions of other aggregating-stabilizing substances. Oades 1984 reported


that in soils with high amounts of sesquioxide, the contribution of organiccarbon to aggregate stability is diminished. The binding force due to low SOCcontent or friction due to soil organic matter may be negligible compared withthe stress exerted by entrapped air, or combination of mechanical stirring anddifferential swelling. It is supported by the significant relationship between RMIand SOC content when the soil parent materials were not included. The numberof drying events and the degree of dryness reached in each even increases

Žaggregate stability and reduce aggregate size Horn, 1994; Junkersfeld and.Horn, 1997; Kay et al., 1994 . However, the processes of aggregation due to

drying and wetting or shrinkage and swelling are completely interrupted byfrequent tillage due to a macroscopic homogenization of the original structure. Ifthese processes are eventually repeated no equilibrium can be reached and only

Žcoarser aggregates are created. However these aggregates even more dense i.e.,.higher aggregate bulk density but weaker as compared with the undisturbed

soils. In addition to SOC decline, amorphous sesquioxide is leached downwardŽ .and active mineral particles increase after agricultural cultivation Yao, 1996 .

These reasons are responsible for lower aggregate stability of the cultivatedŽ .soils. Soil organic matter, clay content or other chemical properties Table 2Ž .could not explain high stability of the uncultivated soil from granite Gw . The

sampling site of the soil was well covered by dense grasses. Besides the mineralquality of kaolinite, biological activity andror quality of SOC might have greatinfluence on aggregate stability. In addition the soils contained high content ofgravel being coated with clays, which was difficulty to be differentiated fromaggregates. These might influence the results, which were not further investi-gated.

4.2. Mechanisms of soil fragmentation by water

When wetting the soil aggregates, their stability was controlled by differentmechanisms. Slaking by fast wetting is caused by entrapped air, which depends

Žon initial soil water content, wetting rates and water uptake rates Dexter,. Ž .1988b and air thrust Gaeth and Frede, 1995 . Mechanical breakdown by wet

stirring has to overcome soil strength for segregation at the same applied energyŽ .Le Bissonnais, 1996 . Micro-cracking or differential swelling by slow wettingdepends on clay mineralogy and its properties such as cation size, valence, ionicconcentration and composition of soil solution.

Ž .Mechanical breakdown was more affected by soil texture silt content thanŽ .slaking as demonstrated by Le Bissonnais 1996 . Because of the complexity of

soil clay minerals and their impurity, microcracking indicated by the NMWD ofaggregates in the slow-wetting treatment was not correlated with the soilproperties such as CEC. The potential of differential swelling was too small inorder to overcome the capillary tension and its effect on aggregate stability wasobscured by the effect of capillary tension.


4.3. Aggregate size and rankings of the soils by aggregate stability

Ž .In contrast to the results described by Le Bissonnais 1988 , the results of theNMWD confirmed that the smaller the aggregate the higher its stability, which

Žcan be explained by the porosity exclusion principle Currie, 1966; Dexter,.1988a . In addition, it also confirms the general finding that aggregate stability

or tensile strength increases with decreasing aggregate size, as being demon-Ž . Ž .strated, e.g., by Braunack et al. 1979 and Kay et al. 1994 . This has also been

Ž .experimentally proved by Hallett et al. 1998 , indicating that fewer pores areavailable for crack propagation with decreasing aggregate size.

Ž .By the definition of MWD Eq. 1 , the weight given to the fraction of biggerfragments increased with the initial bigger aggregates, causing higher values ofthe MWD for the bigger initial aggregates. It, therefore, has a misleadingevaluation of soil aggregate stability when comparing the soils by the calculatedMWD from the different sized aggregates. This can be avoided after normalisa-tion of the MWD as shown in Table 4.

Since different soil properties affected breakdown processes as shown above,the rankings of the soils differed for the three treatments although the weakestand the strongest were distinguished. To relate aggregate stability with soilerosion and soil crustability, it needs further experimental observation.

5. Conclusions

Ž .1 After the wetting treatments, the dominant size-fraction of fragments foreach soil was at the fragment size of 2–1 or 0.63–0.2 mm. The mechanisms ofaggregate breakdown were ranked in the order, slaking)mechanical breakdown)micro-cracking. The effects differed with soil.

Ž .2 NMWD of the aggregates after fast wetting and wet stirring were moreŽ .correlated with soil properties, such as degree of micro-aggregation DOA ,

CEC, K O, Fe O or Al O content rather than clay content and SOC content.2 2 3 2 3

However, silt content correlated with the effect of mechanical breakdown andRMI, respectively. SOC content was only correlated with RMI. The resultsindicate that the binding force of soil organic matter was smaller than the forcecaused by entrapped air or the force of combination of mechanical stress bystirring and differential swelling of minerals.

Ž .3 The smaller the aggregate the higher was the aggregate stability accordingto NMWD. The rankings of the soils differed with the soil aggregate sizes and

Ž .the wetting treatments. Sandy loams from sandstone Sc and Sw were theŽ .weakest soils while the purple mudstone Pp was the strongest. All the

cultivated soils decreased in aggregate stability compared with the uncultivatedsoils or parent materials irrespective of cultivation time and change in SOCcontent after cultivation.


Acknowledgements

We thank the Alexander von Humboldt Foundation for the fellowship pro-Ž .vided to Zhang Bin and the National Foundation of Sciences in China NSFC

Ž .Grant No. 49701008 to Zhang Bin for the funded research project. Weappreciate Dr. Wu Weidong for his help in soil sample collection, Mr. WangFuxiong for his help in measuring aggregate stability using the routine methodand Mr. Gou Xingdu for his help in chemical analysis. Thanks are also given toDr. T. Baumgartl for his helpful discussion.

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