Redistribution of particulate organic matter during ultrasonic dispersion of highly weathered soils

Redistribution of particulate organic matter duringultrasonic dispersion of highly weathered soils

K. OORTSa, B. VANLAUWE

b, S. RECOUSc & R. MERCKX

a

aLaboratory for Soil and Water Management, Faculty of Agricultural and Applied Biological Sciences, K.U. Leuven, Kasteelpark

Arenberg 20, 3001 Leuven/Heverlee, Belgium, bTropical Soil Biology and Fertility Program, UNESCO-Gigiri, PO Box 30597,

Nairobi, Kenya, and cINRA, Unite d’Agronomie Laon-Reims-Mons, Rue Fernand Christ, 02007 Laon Cedex, France

Summary

The use of ultrasonic energy for the dispersion of aggregates in studies of soil organic matter (SOM)

fractionation entails a risk of redistribution of particulate organic matter (POM) to smaller particle-size

fractions. As the mechanical strength of straw also decreases with increasing state of decomposition, it

can be expected that not all POM will be redistributed to the same extent during such dispersion.

Therefore, we studied the redistribution of POM during ultrasonic dispersion and fractionation as a

function of (i) dispersion energy applied and (ii) its state of decomposition. Three soils were dispersed at

different ultrasonic energies (750, 1500 and 2250 J g�1 soil) or with sodium carbonate and were fraction-

ated by particle size. Fraction yields were compared with those obtained with a standard particle-size

analysis. Undecomposed or incubated (for 2, 4 or 6months) 13C-enriched wheat straw was added to the

POM fraction (0.25–2mm) of one of the soils before dispersion and fractionation. Dispersion with

sodium carbonate resulted in the weakest dispersion and affected the chemical properties of the fractions

obtained through its high pH and the introduction of carbonate. The mildest ultrasonic dispersion

treatment (750 J g�1) did not result in adequate soil dispersion as too much clay was still recovered in

the larger fractions. Ultrasonic dispersion at 1500 J g�1 soil obtained a nearly complete dispersion down

to the clay level (0.002mm), and it did not have a significant effect on the total amount of carbon and

nitrogen in the POM fractions. The 2250 J g�1 treatment was too destructive for the POM fractions since

it redistributed up to 31 and 37%, respectively, of the total amount of carbon and nitrogen in these POM

fractions to smaller particle-size fractions. The amount of 13C-enriched wheat straw that was redistrib-

uted to smaller particle-size fractions during ultrasonic dispersion at 1500 J g�1 increased with increasing

incubation time of this straw. Straw particles incubated for 6months were completely transferred to

smaller particle-size fractions. Therefore, ultrasonic dispersion resulted in fractionation of POM, leaving

only the less decomposed particles in this fraction. The amounts of carbon and nitrogen transferred to the

silt and clay fractions were, however, negligible compared with the total amounts of carbon and nitrogen

in these fractions. It is concluded that ultrasonic dispersion seriously affects the amount and properties of

POM fractions. However, it is still considered as an acceptable and appropriate method for the isolation

and study of SOM associated with silt and clay fractions.

Introduction

Physical fractionation of soil samples has been widely used in

studies of the properties and dynamics of soil organic matter

(SOM) (Elliott & Cambardella, 1991; Christensen, 1992). The

results of such a physical fractionation depend largely on the

preceding dispersion of the soil samples. A dispersion procedure

based on ultrasonic energy has proven to be an attractive method

in SOM research (e.g. Anderson et al., 1981; Catroux &

Schnitzer, 1987; Schulten et al., 1993; Amelung et al., 1998), as

it generally attains a good level of dispersion without introducing

chemicals or altering the pH (Christensen, 1992). Therefore,

the properties of the isolated fractions are thought to remain

unaffected after such a dispersion and fractionation procedure.

However, a standard method for ultrasonic dispersion does

not exist. Christensen (1992) mentioned treatment periods

between 3 and 30minutes at a power output varying from 60

to 600W, resulting in applied energies ranging from 480 toCorrespondence: K. Oorts. E-mail: [email protected]

Received 18 July 2003; revised version accepted 4 May 2004

European Journal of Soil Science, February 2005, 56, 77–91 doi: 10.1111/j.1365-2389.2004.00654.x

# 2004 British Society of Soil Science 77

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28 800 J g�1 soil. It is therefore necessary to determine the

ultrasonic energy for complete dispersion for every soil type

studied. Another concern in the use of ultrasonic energy as

a dispersion method is the risk of dissolution effects and

abrasion of fragile minerals (Watson, 1971; Hinds & Lowe,

1980; Christensen, 1992). However, it is generally accepted

that dissolution of minerals during ultrasonic dispersion is

negligible (Gregorich et al., 1988; Escudey et al., 1989).

The largest potential problem associated with ultrasonic dis-

persion lies in the risk of redistribution of organic matter among

the particle-size fractions (Elliott & Cambardella, 1991), which

would obviously seriously compromise its use in SOM research.

There is no consensus on such an effect of ultrasonic dispersion

on SOM in the literature (Gregorich et al., 1988; Morra et al.,

1991; Amelung & Zech, 1999; Schmidt et al., 1999a,b).

Christensen (1992) concluded in his review that there are no

indications for the redistribution of SOM.However, most studies

provided only indirect evidence for the presence or absence of

redistribution of SOM since it was never the objective of the

experiment. Balesdent et al. (1991) studied the effect of ultra-

sonic treatment on the breakdown of isolated particulate

organic matter (POM) fractions in the absence of soil minerals.

They found increasing redistribution of POM when it was

treated for a longer period with ultrasonic energy. Also, other

observations point to POM as the SOM fraction most suscep-

tible to redistribution by an ultrasonic treatment (Amelung &

Zech, 1999; Schmidt et al., 1999b).

Since POM is considered to be a relatively small but extrem-

ely dynamic fraction, it is logical that dispersion methods

should be tuned so that no fragmentation occurs due to the

treatment, or that at least information is on hand that

describes which part of POM is redistributed. It can be

expected that not all POM will be affected to the same extent

by an ultrasonic treatment. As the mechanical strength of

straw decreases with increasing decomposition (Annoussamy

et al., 2000a), it is expected that older, more decomposed POM

will be more redistributed than undecomposed POM during

ultrasonic treatment. So far, neither the effect of the state of

decomposition of POM on its disruption nor its redistribution

during ultrasonic dispersion and fractionation has been

studied.

The objectives of this study were therefore (i) to determine

the redistribution of POM during ultrasonic dispersion at

intensities needed for complete dispersion of highly weathered

soils from West Africa and (ii) to study the effect of the state of

decomposition of POM on its redistribution during ultrasonic

dispersion and fractionation.

Materials and methods

Soils and 13C-enriched straw

The ultrasonic dispersion method was evaluated for samples

from the topsoil (0–10 cm) of three highly weathered soils

representative of major soil types in sub-Saharan West Africa

(Table 1). Samples differ in texture and soil organic matter

content. Soils were air-dried and sieved at 4mm before use to

remove stones and large organic debris.

In order to study the possible redistribution of organic

matter between particle-size fractions during dispersion and

fractionation, 13C-enriched wheat straw was added to the

Ibadan soil before dispersion and fractionation (Table 2).

This uniformly labelled straw was obtained by growing

wheat in a gas-tight growth chamber. The atmosphere in the

chamber was enriched in 13C by injecting CO2 containing 2%

atom-excess 13C-CO2. Air-dry straw with a size up to 1 cm was

milled and sieved to obtain straw with a particle size between

0.25 and 2mm. This fraction was extracted for 1 hour with a

boiling neutral detergent solution and filtered three times with

hot water and twice with acetone (Van Soest & Wine, 1967) in

order to remove the soluble components. After drying, the

remaining organic material (neutral detergent fibre, NDF)

was sieved again at 0.25mm. The properties of this NDF

material are shown in Table 2.

Incubation of 13C-enriched straw

In order to study the effect of the state of decomposition of

POM on its redistribution during ultrasonic dispersion, some

untreated 13C-enriched wheat straw with a particle size between

0.25 and 2mm was incubated in soil for a period of 2, 4 or

6months. One gram of the dry straw was placed in a flat bag

(� 5� 5 cm) made of 0.25mm aperture nylon mesh. The surface

of the bags was large enough to permit a good contact between

the soil and the organic material. Fifteen of these bags were

buried in the equivalent of 4000 g dry loamy soil (pH: 6.64 in

Table 1 General characteristics of the three soils used for evaluating the ultrasonic dispersion method

pH (0.01M Carbon Nitrogen CEC at pHsoil Sandb Silt Clay

Site Country Location Soil typea CaCl2) /g kg�1 /cmolc kg�1 /g kg�1

Ibadan Nigeria 7�300N, 3�540E Ferric Lixisol 5.94 11.65 1.14 5.21 808 75 91

Ferke Ivory Coast 9�360N, 5�120W Ferric Acrisol 5.35 11.74 0.75 4.07 431 395 154

Niaouli Benin 6�400N, 2�100E Rhodic Ferralsol 3.95 3.99 0.26 0.76 911 14 69

aAccording to FAO et al. (1998).bSand: 0.053–2mm; silt: 0.002–0.053mm; clay: < 0.002mm.

78 K. Oorts et al.

# 2004 British Society of Soil Science, European Journal of Soil Science, 56, 77–91

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0.01M CaCl2; carbon content: 8.5 g kg�1) that was already pre-

incubated for 1week at 20% (by weight) water content (corres-

ponding to 60% of its water-holding capacity). Ammonium

nitrate was added (80mg N per g added C) to ensure optimal

decomposition (Recous et al., 1995). The soil and straw were

incubated at 25�C and the water content was checked weekly.

After 2, 4 and 6months, some bags were dug out of the soil and

washed in distilled water to remove any adhering soil, and straw

particles smaller than 0.25mm. The remaining contents of the

bags were dried at 50�C and weighed.

Undecomposed 13C-enriched wheat straw (size 0.25–2mm),

as a control for the incubated straw treatments, was also shaken

for 1 hour in distilled water and filtered. The straw remaining on

the filter was washed three times with distilled water, dried and

sieved again at 0.25mm to remove smaller particles. In this

experiment, shaking the undecomposed 13C-enriched straw in

water was preferred over an extraction with neutral detergent

solution to remove the water-soluble components for this con-

trol treatment, since the incubated straw samples were also not

treated with the neutral detergent solution. Undecomposed and

incubated 13C-enriched straw were analysed for their C and N

concentrations and 13C/12C ratio (Table 2).

Ultrasonic equipment

We used a Misonix Sonicator1 (VWR International, Leuven,

Belgium), model XL2020, with a maximum output of 550W,

equipped with a titanium probe with a flat tip (½ inch dia-

meter) and operating at 20 kHz. In order to compare results

among studies, it is necessary to measure and calibrate the

power output of the ultrasonic equipment since the real

power output applied to the soil suspension during operation

differs from the power output displayed on most sonicators

(North, 1976; Elliott & Cambardella, 1991; Amelung & Zech,

1999; Schmidt et al., 1999a; Roscoe et al., 2000). The sonicator

was calibrated by determining the real power output calori-

metrically (North, 1976; Morra et al., 1991; Roscoe et al.,

2000). The increase in temperature of a known mass (125 g)

of distilled water in a 250-ml glass beaker was measured during

sonication. The probe output energy was calculated from:

P ¼ ðmwcw þ CcontÞ�T

tþH; ð1Þ

where P is the calculated power (W), mw is the mass of water

(g), cw is the specific heat of water (4.18 J g�1 K�1), Ccont is

the heat capacity of the container (JK�1), �T is the tem-

perature change (K), t is the sonication time (s) and H is the

heat loss (J s�1). The heat capacity of the glass beaker (Ccont)

was determined using the method of mixtures (Morra et al.,

1991; Roscoe et al., 2000) according to:

Ccont ¼ m1cwT1 � T2

T3 � T2

� ��m2cw; ð2Þ

where Ccont and cw are as above, m1 is the mass (g) of an

amount of water heated to T1 (K), which is added to the

beaker that already contained an amount of water m2 (g) at

room temperature, T2 (K). The final equilibrium temperature

(K) of the water in the beaker is T3.

In order to determineH, 125ml distilled water was heated to

about 40�C in a glass beaker by sonication and the rate of

cooling was measured at room temperature. The power output

was calculated for the temperature interval 25–35�C since heat

loss was still reasonably small (< 10% of the power input) at

these temperatures.

The measured power output was much smaller than the

displayed power output at all levels of intensity (Figure 1).

Schmidt et al. (1999a) observed the same for five different

sonifiers. The measured power output changed with probe

depth and the absence or presence of soil. In this study, the

probe depth was fixed at 25mm and the intensity level fixed at

80% of the maximum. Ultrasonic dispersion was always per-

formed on a soil suspension of 25 g soil and 125ml distilled

water in a 250-ml glass beaker. The corresponding power

output was 62.5W. Raine & So (1994) calculated the energy,

E (J g�1), applied to the soil from:

E ¼ Pt

ms; ð3Þ

where P is the power output (62.5W), t is the sonication time

(s) and ms is the mass of soil (g).

Table 2 Properties of the 13C-enriched wheat straw added to the Ibadan soil

Carbon Nitrogen Atom-excess 13C Excess 13C added

/g kg�1 C:N ratio /% /mg kg�1 soil

Untreated 417.9� 8.7 7.4� 0.4 56.4� 3.5 1.986� 0.009 0

NDFa 441.8� 5.0 2.8� 0.9 157.2� 5.6 2.007� 0.054 17.7� 0.4

Undecomposedb 459.2� 0.3 2.6� 0.1 175.2� 8.5 2.042� 0.006 18.8� 0.1

2monthsb 433.4� 2.9 13.9� 0.2 31.1� 0.5 2.064� 0.003 17.9� 0.1

4monthsb 421.6� 8.2 33.7� 1.6 12.5� 0.8 1.968� 0.020 16.6� 0.2

6monthsb 466.7� 13.6 32.0� 2.3 14.7� 1.4 1.943� 0.012 18.1� 0.5

aNeutral detergent fibre (washed with a neutral detergent solution).bWashed with distilled water.

Redistribution of organic matter in highly weathered soils 79


Dispersion treatments

As a reference for complete dispersion, a particle-size analysis

was performed on the soils (Gee & Bauder, 1986). First, the

organic matter was removed through oxidation with hydrogen

peroxide: 125ml H2O2 (30% by volume) was added to 50 g soil.

When foaming decreased, another 75ml H2O2 was added and

the samples were heated for 4hours at 75�C. Subsequently, the

samples were dried in the oven at 50�C in order to determine

the weight loss due to removal of organic matter. The samples

were then chemically dispersed by adding 250ml of distilled

water and 50ml of a solution containing 3.57% (w:v) sodium

hexametaphosphate (HMP) and 0.794% (w:v) sodium carbo-

nate and shaking overnight (16 hours) on a reciprocal shaker.

As a comparison with earlier work (Oorts et al., 2000), the

soils were also dispersed with sodium carbonate by adding 0.5 l

of 0.05M Na2CO3 solution to 100 g dry soil and shaking over-

night (16 hours) on a reciprocal shaker.

Soils were treated with the sonicator for 5, 10 or 15minutes at

62.5W, which corresponded to an ultrasonic energy of 750, 1500

and 2250 Jg�1 soil, respectively. The temperature of the suspen-

sion was kept below 30�C by immersing the beaker in an ice bath.

Preliminary experiments showed that there was incomplete dis-

persion below 750 Jg�1 soil (data not shown). For greater energy

levels, there was a risk that too much organic matter would be

affected by the dispersion treatment (Amelung & Zech, 1999).

The NDF material of the 13C-enriched wheat straw was

added to the Ibadan soil at a ratio of 2 g kg�1 soil before

dispersion at the different energies. The straw samples at dif-

ferent stages of decomposition (0, 2, 4 and 6months) were also

added to the Ibadan soil at a ratio of 2 g straw kg�1 soil and

the mixture was then ultrasonically dispersed for 10minutes at

62.5W and fractionated as described below. The 13C-enriched

wheat straw was added to only one soil as it was assumed that

soil properties have a negligible effect on the breakdown and

redistribution of freshly added straw particles during the dis-

persion and fractionation treatments. The amount of energy

consumed by aggregate dispersion is small (<5%) compared

with the total ultrasonic energy applied to the soil (Raine &

So, 1993) and the particle-size distribution does not have a

significant effect on the ultrasonic power applied to the system

(Raine & So, 1994). Therefore, differences in soil properties

have only a negligible effect on the amount of ultrasonic

energy to which the added straw is exposed during dispersion.

Particle-size fractionation

The dispersed soil suspensions were separated into the follow-

ing particle-size classes: > 2mm, 0.25–2mm, and 0.053–

0.25mm, using a wet-sieving shaker (Fritsch1 analysette3,

50Hz, 1.5mm amplitude; VWR International). The fractions

on the sieves were collected and further separated into mineral

and organic material through flotation–decantation on water.

Soil particles < 0.053mm were collected in a bucket and manu-

ally sieved at 0.020mm. The fine silt fraction (0.002–0.020mm)

was separated from a subsample of the material smaller than

0.020mm through four sedimentation cycles (based on Stokes’

Law). The clay fraction (< 0.002mm) in the combined super-

natants was flocculated with CaCl2 (final concentration: about

0.02M), separated from the clear supernatant, transferred to a

dialysis membrane (Spectra/Por1 4, MWCO 12–14 000; VWR

International) and washed free of salts with distilled water. All

fractions were dried overnight at 60�C and weighed.

This fractionation scheme resulted in the following eight

fractions: 2–4mm mineral: ‘gravel’; 0.250–2mm mineral:

‘coarse sand’; 0.250–4mm organic: ‘coarse particulate organic

matter (coarse POM)’; 0.053–0.250mm mineral: ‘fine sand’;

0.053–0.250mm organic: ‘fine POM’; 0.020–0.053mm mineral

and organic: ‘coarse silt’; 0.002–0.020mm mineral and

y = 0.92x – 82.09

R 2

= 0.99

0

10

20

30

40

50

60

70

80

80 90 100 110 120 130 140 150 160 170 180

Power on display / W

Pow

er c

alcu

late

d / W

125 ml water

125 ml water + 25 g soil

Figure 1 Calorimetric calibration of the Misonix

Sonicator1 (model XL2020, maximum output

550W, titanium probe with flat tip of ½ inch

diameter and operating frequency 20kHz).

80 K. Oorts et al.


organic: ‘fine silt’, and < 0.002mm mineral and organic: ‘clay’.

The dry-weight recovery in the eight fractions ranged from

95.7 to 101.0% of the initial amount of soil. As some clay

was lost during removal of the supernatant and dialysis, and in

order to compensate for the weight of adsorbed HMP and

sodium carbonate during the chemical dispersions, the mass

of the clay fractions was calculated by difference.

Analysis of soil samples

The pH was measured after shaking for 1 hour in 0.01M CaCl2at a soil:solution ratio of 1:5. Cation exchange capacity was

determined at the pH of the soil with the silver-thiourea

method (Pleysier & Juo, 1980). Organic carbon and total

nitrogen concentrations and the 13C/12C ratio of the whole

soils and the fractions were determined using a C:N analyser–

mass spectrometer (ANCA-GSL Preparation Module þ20–20 Stable Isotope Analyser; Europa Scientific, Crewe,

Cheshire, UK) after pulverization. Results for the 13C/12C

ratio were expressed as atom% 13C. Excess 13C was calculated

as the difference between % 13C for fractions from the Ibadan

soil with addition of labelled material and % 13C in fractions

of the same soil without labelled organic material (Table 3).

Data analysis

All treatments were replicated threefold, except the dispersion

of the 6month-incubated straw, which was done in duplicate

because of the limited amount of straw available. The meas-

urements were subjected to ANOVA with the MIXED proced-

ure of the SAS system (SAS, 1992). Mean separation was

done with the PDIFF option of the LSMEANS statement.

Differences are considered significant at P 0.05.

Results

Effect of dispersion energy

Fraction weights. The total dry-matter recovery after disper-

sion and fractionation was generally greatest for the particle-

size analysis and the chemical dispersion treatment (Table 4).

The particle-size analysis yielded the largest amount of clay for

all three soils. Only for the Ferke and Niaouli soils did the

2250 J g�1 treatment yield statistically similar amounts of clay.

For the ultrasonically dispersed soils, the clay yield increased

with increasing ultrasonic energy applied. The clay yield of the

sodium carbonate treatment was comparable to the clay yield

of the 750 J g�1 or the 1500 J g�1 treatments.

The peroxide treatment yielded the smallest amount of both

fine and coarse silt. All three sonication treatments yielded

similar amounts of both silt fractions. The chemical dispersion

with sodium carbonate yielded an equal or larger amount of

coarse silt than the other treatments, while its fine silt yield was

generally smaller than for the ultrasonically dispersed samples.

For all three soils, the amount of sand obtained was similar for

all treatments. The chemical dispersion treatment and the ultra-

sonic treatment at 750 J g�1 yieldedmost POM (0.053–4mm), and

POM yield generally decreased with increasing sonication energy.

Amount of carbon and nitrogen in the fractions. Generally,

differences between dispersion methods were small compared

with differences between soils. The C and N concentrations of

both POM fractions did not differ significantly among the

treatments within one soil (Table 5). The coarse silt fractions

of the Ibadan and Niaouli soils showed decreasing C and N

concentrations with increasing sonication energy. The sodium

carbonate dispersion resulted in clay fractions with significantly

larger C and smaller N concentrations compared with the ultra-

sonic dispersion treatments (except for the C concentrations in

the Ferke soil). The sodium carbonate dispersion also yielded

fine silt fractions with a significantly larger C concentration

(Niaouli soil) or a significantly smaller N concentration (Ibadan

and Niaouli soils) compared with the other dispersion treat-

ments. The 1500 and 2250 J g�1 sonication treatments always

resulted in fine silt and clay fractions with similar C and N

concentrations.

For all soils, the amount of C and N in both POM fractions

generally decreased with increasing sonication energy (Figures

2 and 3). The POM fractions of the chemical dispersion treat-

ment contained the largest amount of C and N. For the

Ibadan and Ferke soils, the sodium carbonate treatment had

significantly more C and N in the coarse silt fraction and less

in the fine silt fractions compared with the sonicated samples.

Less C and N remained in the fine silt fraction of the 2250 J g�1

sonication treatment compared with the other ultrasonic dis-

persion treatments. The amount of C and N in the clay frac-

tions increased with increasing sonication energy. However,

the 1500 and 2250 J g�1 sonication treatments were not signifi-

cantly different for any of the soils. The total C and N recov-

ery in the fractions obtained after the chemical dispersion was

for all three soils greater and smaller, respectively, compared

with the ultrasonic dispersion treatments. There were no sig-

nificant differences in C and N recoveries between the ultra-

sonic treatments.

Table 3 Carbon and nitrogen concentrations and atom% 13C for the

whole soil and fractions of the Ibadan soil dispersed at 1500 J g�1,

without addition of 13C-enriched straw

Carbon Nitrogen Atom% 13C

/g kg�1 C:N ratio /%

Whole soil 11.7� 0.2 1.14� 0.16 10.4� 1.5 1.0825� 0.0001

Coarse POM 401.7� 18.4 24.71� 1.66 16.3� 0.8 1.0829� 0.0005

Fine POM 176.7� 21.0 13.26� 1.69 13.3� 0.4 1.0826� 0.0003

Coarse silt 45.5� 1.1 3.38� 0.11 13.5� 0.2 1.0810� 0.0004

Fine silt 57.3� 1.9 5.46� 0.06 10.5� 0.3 1.0819� 0.0002

Clay 36.2� 1.0 4.22� 0.10 8.6� 0.3 1.0836� 0.0003



Table 4 Weight distribution of particle-size fractions obtained after different dispersion methods and sonication energies

Sand Coarse POM Fine POM Coarse silt Fine silt Clay Recovery

Dispersion method /g 100 g�1 soil /%

Ibadan soil

PSAa 80.8 0.00 0.00 2.4 5.1 9.1 101.0

Na2CO3 80.6 0.42 0.50 4.5 6.6 7.5 100.1

750 J g�1 81.3 0.39 0.60 3.2 8.0 6.7 98.9

1500 J g�1 81.2 0.40 0.53 3.0 7.3 7.8 99.5

2250 J g�1 80.4 0.34 0.40 3.3 7.3 8.4 98.6

SEDb 0.3 0.02 0.09 0.4 0.4 0.3 1.2

Ferke soil

PSA 43.1 0.00 0.00 20.7 18.8 15.4 98.2

Na2CO3 43.0 0.20 0.92 27.9 15.5 12.6 98.2

750 J g�1 44.0 0.13 0.52 22.7 19.5 13.2 97.5

1500 J g�1 43.5 0.11 0.36 21.2 20.9 13.9 98.8

2250 J g�1 43.0 0.09 0.33 24.4 17.0 15.2 95.7

SED 0.5 0.01 0.09 0.6 1.1 0.6 1.2

Niaouli soil

PSA 91.1 0.00 0.00 0.6 0.8 6.9 100.0

Na2CO3 90.1 0.06 0.25 1.3 2.4 5.8 99.7

750 J g�1 90.0 0.07 0.31 1.4 3.2 5.0 98.6

1500 J g�1 89.7 0.05 0.15 1.5 3.2 5.4 99.5

2250 J g�1 89.4 0.07 0.22 1.5 2.4 6.4 98.1

SED 0.2 0.02 0.06 0.1 0.3 0.3 0.5

aPSA, particle-size analysis after peroxide treatment.bSED, standard error of the differences.

Table 5 Carbon and nitrogen concentrations of the fractions obtained by the different dispersion treatments

Carbon /g C kg�1 fraction Nitrogen /g N kg�1 fraction

c POMa f POM c Silt f Silt Clay c POM f POM c Silt f Silt Clay

Ibadan soil

Na2CO3 427.6 200.4 50.2 48.5 48.8 10.79 14.64 4.65 4.97 4.11

750 J g�1 402.0 204.1 50.1 50.9 35.1 10.96 13.63 4.65 5.48 4.77

1500 J g�1 402.5 190.6 49.6 57.3 39.2 11.74 12.73 4.00 5.56 5.00

2250 J g�1 394.5 220.8 41.1 53.1 37.5 7.91 13.57 3.27 5.42 4.80

SEDb 19.2 32.4 1.8 1.6 1.3 1.22 2.21 0.21 0.08 0.18

Ferke soil

Na2CO3 364.4 196.4 7.3 19.2 36.0 12.86 10.35 0.44 1.13 2.75

750 J g�1 405.9 215.6 5.7 19.6 31.8 15.25 10.16 0.28 1.28 3.26

1500 J g�1 391.8 310.3 6.0 18.0 35.7 15.58 14.80 0.28 1.13 3.68

2250 J g�1 383.0 247.2 5.8 20.5 34.6 12.50 10.17 0.24 1.36 3.57

SED 37.4 28.8 0.2 0.9 1.1 1.79 1.43 0.04 0.12 0.12

Niaouli soil

Na2CO3 316.4 202.6 21.8 57.8 31.1 10.36 11.43 1.45 1.79 2.71

750 J g�1 292.9 147.7 30.9 41.5 22.5 9.53 7.67 2.08 3.59 2.78

1500 J g�1 392.4 202.2 27.0 45.0 24.8 16.77 10.19 1.69 3.45 3.13

2250 J g�1 280.9 137.2 23.2 44.9 25.3 8.61 6.86 1.38 3.18 3.11

SED 62.2 37.6 2.2 1.5 2.7 1.84 2.14 0.10 0.15 0.16

ac POM, coarse POM; f POM, fine POM; c Silt, coarse silt; f Silt, fine silt.bSED, standard error of the differences.

82 K. Oorts et al.


Redistribution of added POM as a function of dispersion

treatments. The recovery of the added 13C-enriched C in the

fractions from the Ibadan soil ranged from 107 to 122%

(Table 6). These large recoveries were probably due to biased

subsampling of the coarse POM fraction as it was difficult to

completely pulverize the small amounts of sample available.

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0Coarse POM

Am

ount

of c

arbo

n / g

C k

g–1 s

oil

Am

ount

of c

arbo

n / g

C k

g–1 s

oil

Am

ount

of c

arbo

n / g

C k

g–1 s

oil

Coarse silt Fine silt ClayFine POM

Coarse POM Coarse silt Fine silt ClayFine POM


4.5

5.0

5.5

6.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

1.8

2.0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

lbadan

Ferke

Niaouli

Figure 2 Amount of carbon in the particle-size

fractions obtained by the different dispersion

treatments. Error bars indicate the standard

error of the differences (SED).



Therefore, the percentage found in this fraction was recalcu-

lated as (100 – sum of the percentages of added 13C found

in the fractions smaller than 0.250mm). More than 84.7% of

the recovered 13C was found in the coarse POM fraction

irrespective of the dispersion method used. The 1500 and

2250 J g�1 ultrasonic treatments had significantly more added

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0Coarse POM

Am

ount

of n

itrog

en /

g N

kg–1

soi

lA

mou

nt o

f nitr

ogen

/ g

N k

g–1 s

oil

Am

ount

of n

itrog

en /

g N

kg–1

soi

l




0.45

0.50

0.55

0.60

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

0.25

0.20

0.15

0.10

0.05

0

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

Na2CO3

750 J g–1

1500 J g–1

2250 J g–1

lbadan

Ferke

Niaouli

Figure 3 Amount of nitrogen in the particle-size

fractions obtained by the different dispersion

treatments. Error bars indicate the standard

error of the differences (SED).

84 K. Oorts et al.


13C in their fine POM fractions than the other treatments

(respectively 10% compared with 6%). The amount of added13C material found in the coarse silt fractions was significantly

different for all the treatments and increased with increasing

sonication energy (up to 2.9% for the 2250 J g�1 treatment).

The amount of redistributed, labelled material in the fine silt

and clay fractions was less than 1% and 1.6% for all dispersion

treatments, respectively.

Redistribution of added POM as a function of the state of

decomposition

After 6months’ incubation, only 6.6% of the initial straw

material was left in the 0.25–2mm fraction (Figure 4). Incuba-

tion had most effect on the N concentration of the remaining

wheat straw in this fraction (Table 2). The N concentration

increased up to 4months’ incubation and then remained con-

stant. Both treatments to remove water-soluble components

from the straw (with a neutral detergent solution or distilled

water) decreased the N concentration of the straw to the same

extent. Carbon concentration and 13C/12C ratios of the wheat

straw were unaffected by the different pretreatments.

The total amount of C in the coarse POM fraction of the

Ibadan soil decreased significantly with increasing incubation

time of the added straw and reached a minimum when straw

incubated for 4 or 6months was added (Figure 5a). The

amount of C in the fine POM fraction of the 4-month treat-

ment was significantly larger compared with the soil where

undecomposed straw was added. No significant differences

were observed for the amount of N in the coarse POM frac-

tions (Figure 5b). As for C, the 4-month treatment showed a

significant increase in the amount of N in the fine POM

fraction. No significant differences were observed for the

amount of C or N in the particle-size fractions < 0.053mm.

The distribution of the added 13C over the fractions of

the Ibadan soil showed some very pronounced differences

depending on the state of decay of the added straw (Table 7).

When the wheat straw was added as undecomposed organic

matter, more than 90% of the added 13C was still in the

coarse POM fraction. In total, only 3% was redistributed to

fractions < 0.053mm. There were no significant differences

between straw pretreated with a neutral detergent solution

and straw extracted with distilled water in the distribution of

added 13C over the fractions after ultrasonic dispersion at

1500 J g�1.

The partially decomposed straw was far more redistributed

to finer particle-size fractions. Only 66% of the C in straw that

was incubated for 2months remained in the coarse POM

fraction after ultrasonic dispersion and fractionation, and up

to 10% ended up in the clay fraction. More than 90% of the

C in straw incubated for 4months was transferred to smaller

particle-size fractions. After 6months’ incubation, almost all

Table 6 Distribution of added 13C in the particle-size fractions of the

Ibadan soil after the different dispersion treatments

c POMa,b f POM c Silt f Silt Clay Recovery

/% of 13C added

Na2CO3 113.0 91.2 6.4 0.3 0.6 1.5 121.7

750 J g�1 99.3 92.3 5.8 1.1 0.3 0.6 107.0

1500 J g�1 102.6 84.7 10.4 2.3 1.0 1.6 117.9

2250 J g�1 95.4 84.8 9.8 2.9 0.9 1.6 110.6

SEDc 6.1 1.3 1.2 0.2 0.1 0.2 5.7

ac POM, coarse POM; f POM, fine POM; c Silt, coarse silt; f Silt, fine silt.bValues in italic are calculated as 100% – sum of percentages of all other

fractions.cSED, standard error of the differences.

120

100

80

60

Res

idua

l am

ount

/ %

add

ed

40

20

00 50 100

37.4

100.0

7.8 6.6

150 200

Time / days

y = 100exp –0.0164x

R 2

= 0.93

Figure 4 Change in the amount (as weight %)

of wheat straw particles with a size of 0.25–

2mm as a function of time. Error bars

indicate the standard deviations and are

shown above data points for clarity.



the added C was redistributed to smaller particle-size classes

during ultrasonic dispersion and fractionation, the amount

remaining in the coarse POM fraction being 3.2%. The per-

centage of added C recovered in the fine POM fraction

increased with incubation time to a maximum of 33.8% for

straw incubated for 4months. When straw incubated for

4.5

5.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

0.45

0.50

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

Coarse POM

Am

ount

of c

arbo

n / g

C k

g–1 s

oil

Am

ount

of n

itrog

en /

g N

kg–1

soi

l



Undecomposed

2 months

4 months

6 months

Undecomposed

2 months

4 months

6 months

(a)

(b)

Figure 5 Amount of (a) carbon and (b) nitrogen in

the particle-size fractions of the Ibadan soil obtained

after addition of partially decomposed straw and

ultrasonic dispersion at 1500 J g�1. Error bars

indicate the standard error of the differences (SED).

Table 7 Distribution of added 13C in the particle-size fractions of the Ibadan soil after the addition of partially decomposed straw and ultrasonic

dispersion at 1500 J g�1

c POMa f POM c Silt f Silt Clay Recovery

/% of 13C added in partially decomposed straw

Undecomposed 91.5 6.0 0.6 0.6 1.8 100.5

2months 66.0 19.5 5.4 7.4 9.5 107.8

4months 9.1 33.8 15.5 19.0 15.4 92.8

6months 3.2 20.5 13.3 22.9 22.4 82.3

SEDb 2.2 2.3 0.8 0.8 0.5 3.1

ac POM, coarse POM; f POM, fine POM; c Silt, coarse silt; f Silt, fine silt.bSED, standard error of the differences.

86 K. Oorts et al.


6months was added, this percentage of 13C recovered in the

fine POM fraction decreased again to 20.5%. The coarse silt

fraction showed the same trend as the fine POM fraction, but

it contained smaller amounts of added C (maximum of 15.5%

after addition of straw incubated for 4months) and the

decrease after 6months’ incubation was less pronounced, but

still significant. The amount of added 13C found in the fine silt

and clay fractions increased with increasing incubation time up

to 6months. Both fractions contained about 22% of the added

C when this was incubated for 6months. The total recovery of

added 13C in the fractions decreased significantly to, respect-

ively, 93 and 82% when straw incubated for 4 or 6months was

added.

Discussion

Effect of dispersion energy

Degree of dispersion. The results for the particle-size analysis

showed that none of the other dispersion treatments resulted

in a complete dispersion to clay size (Table 4). All treatments

dispersed the soils completely to at least 0.053mm, since no

statistical differences were observed in the amounts of sand

recovered.

Several observations indicated that the sodium carbonate

treatment did not completely disperse the Ibadan and Ferke

soils below 0.053mm. First, this chemical dispersion yielded

significantly more coarse silt and less fine silt compared with

the ultrasonic dispersions (Table 4). Second, the C and N

concentrations of the coarse silt fractions were greater than

for the coarse silt fractions obtained after ultrasonic dispersion

treatments (Table 5), indicating that there was still some clay

or fine silt material, with a greater C and N concentration,

present in this fraction. These differences in C and N concen-

trations were most pronounced for the Ferke soil, due to the

greater distinction in C and N concentration among coarse silt,

fine silt and clay fractions for this soil. Third, the C and N

concentrations of the fine silt fraction of the Ibadan soil were

significantly smaller after the chemical dispersion than values

for the ultrasonic dispersion treatments. This again points to

the fact that there was still some clay material, with smaller C

and N concentrations, residing in the fine silt fraction. Fourth,

the N concentrations of the clay fractions were significantly

smaller compared with the other dispersion treatments. All

these observations indicate that a substantial part of the fine

silt and clay material still remains in the coarse silt fraction

after shaking these soils for 16 hours in a sodium carbonate

solution. This resulted in an overestimation of the amount of

C and N in the coarse silt fraction and an underestimation of

the amount of C and N associated with the fine silt or clay

fraction (Figures 2 and 3).

Contradicting the results for the Ferke and Ibadan soils, the

sodium carbonate treatment gave good dispersion down to

0.020mm for the Niaouli soil. There was still no complete

separation of the fine silt and clay fraction since the clay

content was still significantly smaller than for the particle-

size analysis. The different degrees of dispersion obtained by

the sodium carbonate treatment were explained by the texture

of the soils. Only for the very sandy Niaouli soil could all

particles larger than 0.020mm be completely dispersed by

this method.

Besides the unreliable degree of dispersion, shaking the soil

in a sodium carbonate solution introduced some artefacts in

the SOM fractions obtained. The large C concentration in the

clay fractions of all three soils and the fine silt fraction of the

Niaouli soil (Table 5) was a clear indication that substantial

amounts of carbonate remained in these fractions after disper-

sion and fractionation (3 g carbonate-C added kg�1 soil).

From the differences in the amount of C recovered in the

fractions after chemical or ultrasonic dispersion, it was calcu-

lated that the chemically dispersed soils contained in total

between 0.55 (Niaouli) and 1.05 (Ibadan) g extra C kg�1 soil.

Further, the high pH (c. 10) during chemical dispersion may

also have dissolved some organic matter. This would explain

the significantly lower N recoveries after dispersion with

sodium carbonate in contrast to the high C recoveries. It is

clear that the use of sodium carbonate for soil dispersion

seriously compromises the study of carbon dynamics in the

particle-size fractions obtained.

The three ultrasonic treatments dispersed the soils to the

same degree down to 0.020mm, since no significant differences

were observed in their yields of the coarse silt fraction. Based

on its lower clay yield, it was concluded that the 750 J g�1

ultrasonic treatment resulted in the weakest dispersion for

the three soils. Further, the smaller C concentrations of the

fine silt fractions for the Ibadan and Niaouli soils obtained

after ultrasonic dispersion at 750 J g�1 compared with the same

fractions obtained by the other ultrasonic dispersion treat-

ments (Table 5) indicated that there was still a substantial

amount of clay material, with a smaller C concentration,

residing in the fine silt fraction. It was therefore concluded

that the 750 J g�1 ultrasonic treatment did not give sufficient

separation of the fine silt and clay fractions, resulting in a

lower C and N recovery in the clay fraction.

The 2250 J g�1 treatment gave slightly larger clay contents

and smaller fine silt contents than the 1500 J g�1 treatment, but

differences were not significant except for the Niaouli soil

(Table 4). As both ultrasonic treatments also obtained fine

silt and clay fractions with similar C and N concentrations,

the distribution of C and N over the silt and clay fractions was

similar after both treatments (Figures 2 and 3). Only for the

Niaouli soil did the 2250 J g�1 treatment result in a signifi-

cantly larger amount of C and N associated with the clay

fraction. These observations indicated that there was only a

small increase in dispersion between the 1500 and 2250 J g�1

ultrasonic treatments. As mentioned above, none of the treat-

ments obtained a complete dispersion compared with the

particle-size analysis. However, because of the small differences



between the 1500 and 2250 J g�1 ultrasonic dispersion treat-

ments and the particle-size analysis, it was concluded that both

treatments dispersed the soil adequately. This sonication energy

agrees well with results found in the literature for the ultrasonic

dispersion of soils from temperate regions (Gregorich

et al., 1988; Amelung & Zech, 1999; Schmidt et al., 1999a).

Results for an Oxisol with 62% clay showed that an ultrasonic

energy of 2600 J g�1 was needed to disperse all aggregates

larger than 0.1mm and that aggregate disruption still continued

beyond the maximum energy used (8250 J g�1), without

reaching stabilization (Roscoe et al., 2000). The large amount

of clay in the Oxisol probably explains the differences with our

results for highly weathered soils.

Disruption and redistribution of soil organic matter. The mild-

est ultrasonic dispersion treatment (750 J g�1) did not result in a

significant redistribution of POM to smaller particle-size classes

for both the Niaouli and Ibadan soils as the results for the total

C and N recoveries in the POM fractions did not show signifi-

cant differences compared with the chemical dispersion (Figures

2 and 3). However, for the Ferke soil, the 750 J g�1 treatment

yielded in total about 40% less C and N in the POM fractions

compared with the chemical dispersion. It was assumed that,

due to the larger clay and silt content, this soil contained more

old POM that was protected in aggregates. This older POM

may be more susceptible to disruption and redistribution during

an ultrasonic dispersion treatment.

The 1500 J g�1 ultrasonic dispersion treatment showed a

small, insignificant, decrease in total C and N recoveries in

the POM fractions compared with the 750 J g�1 treatment. The

2250 J g�1 treatment redistributed in all three soils most POM

into finer particle-size fractions. Compared with the mildest

ultrasonic treatment, between 20 and 31% of C and between

35 and 37% of N was removed from the POM fractions. For

all soils, more N than C was removed from the POM fractions.

The coarse silt fraction for the Ibadan and Niaouli soils also

showed a decrease in C and N concentration with increasing

ultrasonic dispersion energy, which could not be explained by

the increasing degree of dispersion obtained since there were

no significant differences between the dispersion treatments in

yield for this fraction (Tables 4 and 5). As for the POM fractions,

the decrease was larger for N than for C. This decrease in C and

N concentrations was an indication that coarse silt-sized SOM

was also affected by ultrasonic dispersion. However, effects

on coarse silt-associated SOM were smaller than on the POM

fractions. The 2250 J g�1 treatment removed, respectively, 14

and 25% of the total amount of C and N in this fraction

compared with the 750 J g�1 treatment (Figures 2 and 3), but

differences were not statistically significant.

It was not clear to which fractions C and N from native

SOM were redistributed since the amounts were small com-

pared with the total amounts of C and N in the silt and clay

fractions. The addition of 13C-enriched straw to the Ibadan

soil before dispersion overcomes these limitations. Both the

1500 and 2250 J g�1 treatments transferred significantly more

added 13C to smaller particle-size fractions than did the 750 J g�1

and the chemical dispersion treatments (Table 6). The

distribution of the added 13C over the fractions indicated

that for the ultrasonic dispersion treatments, redistribution

was the result of two processes: (i) disruption of the straw

particles and redistribution to gradually finer particle-size

classes (coarse POM ! fine POM ! coarse silt ! fine silt

! clay) and (ii) dissolution of C, which then ended up in the

clay fraction after fractionation. The results for the chemical

dispersion with sodium carbonate showed that there was as

much C dissolved as for the 2250 J g�1 treatment, but disrup-

tion was less important than for ultrasonic dispersion.

The total amount of freshly added organic material trans-

ferred to fractions smaller than 0.053mm was < 5.5% for all

treatments. This was significantly less than the 20–31% of C

originally present in the POM fractions of the Ibadan soil that

was redistributed by the 2250 J g�1 treatment. This discrepancy

pointed to differences in mechanical properties between the

freshly added POM and the particulate organic matter natur-

ally present in this soil. The added straw seemed to be more

resistant to disruption during ultrasonic dispersion.

Balesdent et al. (1991) observed that isolated SOM fractions

were increasingly redistributed to smaller particle-size frac-

tions when treated with ultrasonic energy for increasing peri-

ods. After sonication for 2, 10 or 30minutes at 50W, they

found that, respectively, more than 40, 60 and 80% of the

organic matter was disrupted and redistributed to smaller

particle sizes. Results were similar for SOM with different

particle sizes (0.2–2mm, 0.05–0.2mm, 0.025–0.05mm). How-

ever, the ultrasonic power was not calibrated in their study and

SOM was treated in the absence of soil, therefore it was

impossible to compare these results with our observations.

But both studies have shown that ultrasonic dispersion can

cause important damage to SOM fractions with a size of

0.025–2mm. Amelung & Zech (1999) also found some evi-

dence for the redistribution of coarse POM during ultrasonic

dispersion. They observed that the C concentrations in the

coarse sand fraction (> 0.25mm) decreased with increasing

input of ultrasonic energy above 500 J g�1, despite the average

yields remaining constant. At ultrasonic energies < 500 J g�1,

particles with a small C concentration (< 30 g Ckg�1) were lost

from this coarse sand fraction, i.e. clay or silt material. In the

500–2500 J g�1 interval, on average 270 g Ckg�1 particles were

lost, suggesting that particulate plant residues contributed sig-

nificantly to this C. Schmidt et al. (1999a) could not detect

either detachment of natural SOM from primary organo-

mineral complexes or redistribution between particle-size frac-

tions in the range 150–3000 J g�1 based on C concentrations

and the distribution of dissolved C. However, when studying

soils containing appreciable amounts of coal or lignite, they

observed that sand-sized lignite particles could easily be dis-

rupted by relatively little ultrasonic energy (< 500 J g�1) and

redistributed to smaller size fractions (Schmidt et al., 1999b).

88 K. Oorts et al.


The data on the total amount of C and N in the fractions

after the 1500 J g�1 treatment showed no significant indications

of redistribution of POM to smaller fractions. However, the

distribution of added 13C in the Ibadan soil showed that

about 15% of the added straw was transferred to smaller

particle-size classes. As only less than 5% was redistributed to

particle-size classes < 0.053mm and C and N concentrations of

these fractions were not significantly affected, this was still

considered as acceptable for the study of silt- and clay-sized

SOM fractions. Because of the adequate degree of dispersion

and the limited redistribution of SOM to finer particle-size

fractions, the 1500 J g�1 ultrasonic dispersion treatment was

selected as the best dispersion method for the three soils studied.

Effect of state of decomposition of POM on its redistribution

during ultrasonic dispersion

The half-life of wheat straw particles with a size of 0.25–2mm

was approximately 42 days at 25�C and optimal water and N

content (Figure 4). This fast decomposition rate agrees with

results from other authors (Aita et al., 1997) and indicates that

most of the material in the POM fractions will be partially

decomposed and therefore may behave differently compared

with new, undecomposed POM during ultrasonic dispersion.

After dispersion and fractionation, the coarse POM frac-

tions of the Ibadan soil where straw incubated for 4 or

6months was added contained about 1 gCkg�1 dry soil less

than the fractions where washed undecomposed wheat straw

was added (Figure 5). This loss corresponded to the entire

amount of C added to this coarse POM fraction. The Ibadan

soil amended with straw incubated for 2months contained

0.56 gCkg�1 dry soil less in its coarse POM fraction, which

corresponded to a loss of 64% of the amount of C in the added

straw. In contrast to these decreases in the amount of C, there

were no significant differences in the amount of N in the

coarse POM fractions of the Ibadan soil. This was explained

by the larger amount of N initially added to this fraction with

the more decomposed straw due to its larger N concentration,

which compensated for the larger amount of N lost during

ultrasonic dispersion and fractionation after addition of more

decomposed straw. The lack of significant differences observed

for the total amounts of C or N in the particle-size fractions of

the Ibadan soil < 0.053mm indicated that the amounts of C

and N redistributed were still small compared with the original

amounts of C and N in those fractions.

The distribution of the added 13C over the fractions of the

Ibadan soil confirmed the results for the total amounts of C and

made clear that older and more decomposed coarse POM was

much more vulnerable to removal from its original particle-size

fraction during the ultrasonic dispersion and fractionation

(Table 7). This more decomposed material was also more redis-

tributed to smaller particle-size fractions than less decomposed

POM. The smaller recovery of added 13C in the fractions after

addition of more decomposed straw indicated that a substantial

part of the added C was dissolved during the dispersion treat-

ment. All these observations pointed to the fact that the

mechanical properties of the straw particles have decayed

during decomposition and that older POM was much more

susceptible to physical stresses. This was also observed by

Annoussamy et al. (2000a). They found a reduction of 80% in

the maximum shear stress and maximum bending stress of wet

wheat straw internodes after an incubation of 40 days at 25�C

and �10kPa water potential (mass loss: 50%). Physical char-

acteristics (e.g. length, diameter) of the internodes did not

explain the change in mechanical properties. A study of the

changes in biochemical composition of the decomposing wheat

straw showed that the decrease in its mechanical strength was

due mainly to the decay of cell wall polysaccharides by soil

organisms (Annoussamy et al., 2000b).

The difference in mechanical properties between undecom-

posed and partially decayed POM also explained the observed

difference in mechanical strength between the added, unde-

composed straw and the POM originally present in the Ibadan

soil. This further agreed with the fact that for all three soils

relatively more N than C of original POM was redistributed to

smaller particle-size classes during the ultrasonic treatments

since older, more decomposed soil organic matter generally

has a lower C:N ratio. Balesdent et al. (1991) also observed

that POM with a large N concentration, resulting from more

advanced decomposition or containing microbial components,

was more fragile and was more easily redistributed to smaller

particle-size classes during ultrasonic treatment than POM

with a small N concentration.

The observed effect of the state of decay on the redistribu-

tion of POM during ultrasonic dispersion has severe conse-

quences for both the amount and the properties of the isolated

POM fractions. It will result in underestimation of the amount

of POM present in the soil and overestimation of its C:N ratio.

Also, the results for other properties measured on the isolated

POM will be different from the value of the total amount of

POM present in the soil. The extent of the difference is depend-

ent on the proportion of recent and older POM present in a

soil at the time of sampling. Consequently, ultrasonic disper-

sion at the energies studied is not a proper method for the

dispersion and isolation of POM fractions. Some authors have

also proposed removal of the POM fractions after mild ultra-

sonic dispersion (300 J g�1) or shaking the soil with glass beads

before application of greater ultrasonic energy (Balesdent

et al., 1991; Amelung & Zech, 1999).

The more extensive redistribution of older POM during ultra-

sonic dispersion and fractionation does not strongly affect the C

content of smaller SOM fractions (< 0.053mm) due to the rela-

tively small amount of this old POM naturally present in the soil.

When the decrease in the amount of wheat straw left in the

coarse POM fraction after decomposition (Figure 4) was taken

into account, and the amount of 13C found in the fractions of the

Ibadan soil was expressed as a percentage of the amount of 13C

added in undecomposed straw, none of the silt or clay fractions



received more than 3.5% of the added C. This was negligible

compared with the original amount of C and N in these frac-

tions. It is therefore concluded that ultrasonic dispersion at

1500 J g�1 still can be considered as an appropriate method for

the isolation and study of silt- and clay-sized SOM fractions.

Conclusions

Redistribution of native and added particulate organic matter

increased with the application of increasing sonication energy.

For the three soils studied, the 1500 J g�1 ultrasonic dispersion

treatment best met the criteria for a good dispersion method.

It obtained a near-complete dispersion down to clay size

(< 0.002mm), compared with particle-size analysis, and did

not have a significant effect on the total amount of C and N

found in the POM fractions. The milder 750 J g�1 treatment

did not result in complete dispersion below 0.053mm. The

2250 J g�1 treatment gave a slightly better dispersion (but

not significantly so) than the 1500 J g�1 treatment, but it

removed 20–31% and 35–37% of the total amount of C and

N, respectively, in the natural POM fractions during disper-

sion. Chemical dispersion with sodium carbonate should be

discouraged for SOM studies, since this method did not result

in complete dispersion for all soils and introduced changes in

the chemical properties of the fractions obtained. The mechan-

ical strength, and therefore the resistance to disruption during

ultrasonic dispersion, of 13C-labelled wheat straw added to the

POM fraction decreased with increasing state of decompos-

ition. Older, more decomposed, straw was more redistributed

to smaller particle-size fractions during an ultrasonic disper-

sion treatment than undecomposed straw. Therefore, ultra-

sonic dispersion resulted in fractionation of POM, leaving

only the less decomposed particles in this fraction. This had

important consequences for both the amount and properties of

the isolated POM fractions. As the amounts of C and N of

POM redistributed to smaller particle-size fractions during

ultrasonic dispersion were negligible, compared with the total

amounts of C and N in those fractions, ultrasonic dispersion at

1500 J g�1 still is an acceptable and appropriate method for the

isolation and study of silt- and clay-sized SOM fractions.

Acknowledgements

K. Oorts wishes to thank the Fund for Scientific Research –

Flanders (Belgium) for a grant as Research Assistant. The 13C-

enriched straw was kindly provided by INRA Laon, France.

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