Abstract

Wetting and drying (W-D) cycles can cause strong modifications of the structure of a

soil, especially in pore distribution, which reflects the temporal and spatial distribution

of soil water and, consequently, these processes can affect soil water and nutrient

retention and movement. These alterations have important practical consequences when

calculating soil water storages and matric potentials, widely used in irrigation

management. The present paper has as objective to use gamma ray computed

tomography (GCT) as a tool to investigate possible modifications in soil structure

induced by W-D cycles and to analyze how these alterations can affect soil water

retention. The GCT scanner used was a first generation system with a fixed source-

detector arrangement, with a radioactive gamma ray source of 241

Am. Soil samples were

taken from profiles of three different soils characterized as Xanthic Ferralsol (Fx),

Eutric Nitosol (Ne) and Rhodic Ferralsol (Fr). Eighteen samples (50 cm3), six from each

soil, were submitted to none (T0), three (T1), and nine (T2) wetting/drying cycles. Based

on image analysis it was possible to detect modifications in soil structure in all samples

after wetting/drying cycles for all treatments. Tomographic unit profiles of the samples

permitted to identify an increase on soil porosity with the increase in the number of

wetting/drying cycles and it was also possible to quantify the average porosity values.

The statistical test (Duncan test) indicates that there are significant differences between

treatments for all samples at the 5% significance level.

Keywords: computed tomography; wetting/drying cycles; soil water retention; soil

structure; gamma radiation.

2

Introduction

Soil structure is influenced by several phenomena such as organic matter

dynamics, soil genesis, human action, wetting/drying (W-D) cycles, and other [1]. An

important aspect of soil structure is the porosity, which consists of a continuous

branching of pores of sizes classified in different categories like macro, meso and

micropores [2]. Adequate soil porosity is very important for soil aeration, water

infiltration, and root distribution, permitting a better crop development. W-D cycles can

cause strong modifications of the structure of a soil, especially in pore distribution,

which reflects the temporal and spatial distribution of soil water and, consequently,

these processes can affect soil water and nutrient retention and movement. These

alterations have important practical consequences when calculating soil water storages

and matric potentials, widely used in irrigation management.

Many soil scientists have investigated alterations in soil structure induced by

different phenomena [3, 4, 5, 6, 7]. These authors describe as main modifications those

caused by the swelling/shrinkage phenomenon in clayey soils, W-D cycles, compaction

caused by agricultural machinery traffic, and freezing/thawing cycles. Swelling and

shrinkage behavior induces the formation of cracks in clayey soils affecting soil water

movement and causing preferential flow [8]. Compaction due to machinery traffic has

as consequence serious alterations in soil bulk density affecting soil water storage

capacity and matric soil water potential. Freezing/thawing cycles cause soil structural

modifications, also in soil bulk density and surface roughness, depending on soil

mineralogical composition.

Gamma ray computed tomography (GCT), a noninvasive imaging technique, has

been shown as an important tool to investigate possible modifications in soil structure

and other soil physical properties. Vaz et al. [9] used miniscanner CT to study soil

compaction caused during plowing operations with heavy equipment. Phogat et al. [10]

showed the potential of the use of dual-energy gamma computerized tomography for

nondestructive studies of the structural status and stability of soils. Rasiah and Aylmore

[11] used CT to analyze the influence of wetting and differences in structural stability

on the spatial continuity of soil parameters. Perret et al. [12] determined by CT the

geometry and topology of macropore networks. Macedo et al. [13, 14] and Appoloni et

3

al. [15] applied X-ray microtomography to investigate thin layers of soil clod particulate

systems and porous microstructures. Rogasik et al. [16], through X-ray CT, made 3-D

analysis of macropores and Pires et al. [17, 18] investigated through GCT soil

compaction induced by agricultural machinery traffic and modifications in soil structure

caused by soil sampler devices.

The present paper has as objective to use GCT as a tool to investigate possible

modifications in soil structure induced by W-D cycles and to analyze how these

alterations can affect soil water retention.

Material and Methods

CT scanner

The CT scanner is a first generation system with a fixed source-detector

arrangement, with translation/rotational movements of the sample. The radioactive

gamma ray source consists of 241

Am with an activity of 3.7 GBq emitting

monoenergetic photons of energy 59.54 keV. Circular lead collimators of 1 mm

diameter were adjusted and aligned between source and detector. Angular sample

rotation steps were 2.25° until completing a scan of 180°, with linear steps r of

0.14 cm. The pixel size was 1.14 x 1.14 mm, calculated by the ratio between the inner

diameter of the soil sample and number of pixels of the reconstruction matrix. Acquired

data were stored in a PC and a reconstruction algorithm called Microvis [19] developed

by Embrapa Agricultural Instrumentation (CNPDIA – São Carlos, Brazil) was used to

obtain GCT images. The calibration of the GCT system was obtained through linear

correlation between linear attenuation coefficients and tomographic units of different

materials [20]. The tomographic images of the soil samples were taken on vertical

planes crossing the center of each cylindrical sample.

Since GCT was performed on soil samples with constant water content, the soil

porosity () was obtained combining the equations used to calculate soil bulk density by

CT [20], and utilized to evaluate soil porosity by traditional methods [1]:

4

pmwms ).U..(

TU1 (1)

where represents the correlation coefficient between and TU; ms and mw (cm2.g

-1)

the mass attenuation coefficients of soil and water, respectively; p (g.cm-3

) the soil

particle density; U (g.g-1

) the soil water content; and TU1 the tomographic unit.

In order to avoid effects of possible artifacts or fluctuations in the images the

quantitative analyses to determine the soil porosity was made selecting areas inside the

cylinder smaller than the real soil sample size.

Soil preparation

Core samples were taken from profiles of three soils, different in texture,

characterized as: Xanthic Ferralsol (soil 1), Eutric Nitosol (soil 2) and Rhodic Ferralsol

(soil 3) from an experimental field of ESALQ/USP, Piracicaba, SP, Brazil (22°4’ S;

47°38’ W; 580 m above sea level). Eighteen samples (3.0 cm high, 4.8 cm i.d., 55 cm3

volume), six from each soil were collected at the soil surface (0 – 10 cm) with

aluminum cylinders. The chosen wetting process was the capillary rise, which is used

during soil water retention determinations. The procedure of wetting consisted initially

in maintaining a level of water 0.3 cm from the bottom of the cylinder during 2 hours

and after this water was applied at a level just below the top of the cylinder. Twenty

four hours were necessary to obtain complete saturation of soil samples by capillary rise

in order to avoid entrapped air bubbles, which can cause slaking of soil aggregates,

changing soil structure. The procedure used to dry samples was the application of 4.0

MPa of pressure (P) on the saturated soil sample, driving away soil water retained at

pressures below P [21]. Soil samples were submitted to none (Treatment T0), three (T1),

and nine (T2) W-D cycles. GCT images were obtained at fixed water contents, to avoid

differences in soil images due to differences in the residual water content after each

treatment. The procedure used to obtain fixed water contents, after W-D cycles, was to

maintain samples in contact with air until a residual value of U was reached. When

1 TU takes the air as the media with the minimum possible value. It is linearly related to Hounsfield Unit (HU) that takes the water as a reference media for which HU=0.

5

samples, after T0, T1, and T2, reached this residual water content they were involved in

plastic film, to minimize water loss, and submitted to tomography.

Results and Discussion

Table 1 presents some soil characteristics for the three soils used in the

experiment.

Table 1. Physical characteristics of the 0-10 cm soil layer for Xanthic Ferralsol (soil 1),

Eutric Nitosol (soil 2) and Rhodic Ferralsol (soil 3).

Characteristic

1

Soil

2

3

Sand (%) 66 24 26

Silt (%) 6 33 26

Clay (%) 28 43 48

Particle density (g.cm-3

) 2.55 2.68 2.54

Bulk density (g.cm-3

)* 1.56 1.62 1.33

Organic matter (g.dm-3

) 16.0 20.2 27.0

pH (in CaCl2) 3.9 5.3 4.9

Ca (molc.m-3

) 13.0 29.0 27.0

Mg (molc.m-3

) 4.0 20.0 15.0

K (molc.m-3

) 2.6 4.3 3.4 * Soil bulk density was obtained through the traditional gravimetric method [1].

Mass attenuation coefficients were obtained using the method described in

Ferraz and Mansell [22]: 0.24922 0.00001 (soil 1), 0.32752 0.00288 (soil 2),

0.30639 0.00034 (soil 3), and 0.19890 0.00071 cm2.g

-1 (water). The calibration of

GCT presented a high correlation coefficient of 0.999.

Figure 1 shows the tomographic images of soil 1 (Xanthic Ferralsol) submitted

to different numbers of W-D cycles. These images reveal a clear tendency of

modifications in soil structure after these cycles. Between the treatments T0 and T1 the

alteration in soil structure does not present variations, but comparing T0 and T2 the

modifications become more evident. The W-D cycles probably caused variations in pore

distribution for this soil.

6

Figure 1. Changes in soil structure induced by wetting/drying cycles for soil 1 (Xanthic

Ferralsol). T0 no W-D cycles; T1 3 cycles; and T2 9 cycles. S refers to soil sample. Color scale

represents the Tomographic Unit (TU) distribution.

Figure 2 shows a quantitative analysis of soil porosity in the slice of analysis

after application of the cycles. Analyzing the graphs in Fig. 2 an increase in soil

porosity can be observed after the application of the cycles. All soil samples, with

exception to sample 4, presented increases in soil porosity with the increase of the

number of cycles. For sample 4 the same soil porosity was found for all treatments. The

7

increase in soil porosity probably occurred due to modifications in soil volume during

wetting and drying. During wetting an increase in the height of the soil column was

observed and after drying a decrease. Therefore, during wetting and after drying the

process of friction between soil column and cylinder wall can occur influencing the

adequate settlement of the sample. For soil 1 an increase was observed after the W-D

cycles in the height of the soil column near the cylinder wall, for samples 1, 2, 5, and 6.

This increase was about 1 mm and occurred only after T1. For sample 1 after T2 it is

possible to observe (Figs. 1 and 2) one hole (macropore) with an area of 6.44 mm2. For

sample 2 the area of the observed hole is 3.41 mm2 for T1 and it increased to 4.31 mm

2

for T2. For sample 5 the size of the hole increased from 15.38 to 23.70 mm2 between T0

and T1 and decreased to 21.89 mm2 after 9 W-D cycles. These increases in hole areas

could be due to the influence of the friction between the soil column and the cylinder

wall. This behavior could explain the increase in soil porosity after W-D cycles.

Hussein and Adey [23] found a progressive increase in soil porosity with successive W-

D cycles. These authors observed modifications in soil structure changing from massive

in plowed (disturbed) soil to complex crumb, block and platy structures after successive

cycles. In this work [23] the proportion of voids found in the disturbed soil was lower

than 11.3 %, and after a fast capillary wetting the observable porosity increased to 23.9

% after only one cycle. The authors called the observable porosity that in which large

external pores, cracks, and small voids were excluded and would, therefore, be less than

the true soil porosity. Pardini et al. [4] also indicated possible variations of surface

roughness, porosity, and pore size distribution during W-D cycles. They found after

three W-D cycles an increase in soil porosity induced by the formation of big cracks and

fissures. The presence of cracks was not observed in our soil samples, although W-D

cycles could promote the formation of microfissures. Another process influenced by W-

D cycles is soil aggregation, because hydration and dehydration action produces cycles

of contact and retraction of clay particles and aggregates. Li et al. [24] observed that W-

D cycles can change aggregate sizes affecting pore size distribution. In this experiment

[24] the authors observed the formation of small, shallow cracks at the soil surface after

W-D cycles. Our soil 1 is composed mostly by sand, and because of this, swelling and

shrinking were not present. Generally this kind of behavior occurs in some clayey soils

in which it is common to observe soil cracks or fissures [8].

8

Figure 2. Distribution of soil porosity after wetting/drying cycle applications for the three

different treatments for soil 1.

Figures 3 to 6 show the tomographic images for soils 2 (Eutric Nitosol) and 3

(Rhodic Ferralsol), and the respective analysis of the soil porosity distribution after W-

D cycles. Some loamy and clayey soils suffer an increase in volume when wet and a

volume reduction when dry, and this can cause modifications in soil structure like

fissures. These fissures play an important role in hydrological process since they affect

soil water movement and evaporation rates. For soils 2 and 3 although having higher

contents of clay than sand and silt, it was not possible to observe fissures or cracks.

Maybe for these soils the W-D cycles could have induced microfissure formation, but

due to the resolution limitation of our GCT it was not possible to observe them.

9

Figure 3. Changes in soil structure induced by wetting/drying cycles for soil 2 (Eutric Nitosol).

Figures 3 and 4 exhibit similar results in comparison to those of soil 1, with

exception of samples 3, 5 and 6. These practically do not present differences in soil

porosity between T1 and T2. Alterations induced after 3 cycles of W-D became constant

with time until the application of 9 cycles. The process of W-D disperses and re-

organizes clay particles causing modifications in soil structure. This process could have

occurred with soil 2 (Fig. 3) affecting soil pore distributions, and increasing soil

10

porosity (Fig. 4). The same phenomenon of an increase in the height of soil column near

the cylinder wall after W-D cycles was observed for soil 2.

Figure 4. Distribution of soil porosity after wetting/drying cycle applications for the three

different treatments for soil 2.

For samples 1, 4, and 6 this increase was of 1 mm and occurred only for T2 in

comparison to T0 and T1. Samples 2 and 3 exhibited differences in the height of the soil

column of 1 mm only between T0 and the other treatments. Sample 5 did not present

differences in height among treatments. In Fig. 4 it is possible to observe the variation

of the hole (macropore) area for samples 2, 4, and 6. For sample 2 the hole area

increased as follows T0 (8.57 mm2), T1 (9.91 mm

2), and T2 (15.23 mm

2) with W-D

cycles. Sample 4 exhibited the same behavior as sample 2 with an increase of 6.47 (T0),

11

8.68 (T1), and 10.96 mm2 (T2). For sample 6 only after successive wetting and drying

the existence of a hole with decreasing area from 9.26 to 4.83 mm2 was observed. For

soil 2 the increase in the height of the soil column with W-D cycles probably caused the

increase in soil porosity.

Figure 5. Changes in soil structure induced by wetting/drying cycles for soil 3 (Rhodic

Ferralsol).

Figures 5 and 6 present results for soil 3. Analyzing Fig. 5 it is possible to

observe through image analysis that W-D cycles induced alterations in soil structure,

12

but the modifications for some soil samples were smaller when compared with soils 1

and 2.

Figure 6. Distribution of soil porosity after wetting/drying cycle applications for the three

different treatments for soil 3.

In Fig. 6 samples 2 and 3 practically do not present differences in soil porosity

between treatments and samples 1, 5, and 6 have almost the same value of porosity

between T0 and T1. For soil 3 samples 1, 4, 5, and 6 presented increases in the height of

the soil column among treatments of 1.5 mm (sample 1) and 1 mm (samples 4, 5, and

6), and occurred only for T2 in comparison to the other treatments. All soil samples,

with exception of samples 2 and 4, presented holes (macropores) in their structure.

Samples 1, 5, and 6 exhibit the evolution in the size of the holes with the increase in the

13

number of W-D cycles. For sample 1 it is possible to observe one hole for T0 and T1

with areas of 18.54 and 23.66 mm2, respectively, and two holes with areas of 6.47 and

34.67 mm2 for T2. For sample 3 the size of the hole remained the same between T0 and

T1 (10.21 mm2) and presented a small increase for T2 (12.89 mm

2). For sample 5 the

size of the hole decreased between T0 (9.69 mm2) and T1 (4.31 mm

2), and for T2 the

area of the same hole was of 6.81 mm2. It is possible to observe a new hole for T2 with

an area of 7.61 mm2 for sample 5. Sample 6 presents two holes for each treatment with

areas of 23.60 and 15.01 mm2 (T0), 35.45 and 17.29 mm

2 (T1), and 38.81 and 30.57

mm2 (T2). The evolution in the size of the holes with the increase of the number of W-D

cycles can explain the increase in soil porosity for soil 3. The oscillation in soil porosity

values for samples 2 and 5 can be explained because soil 3 has not a well-defined

structure, and possible alterations in soil structure due to W-D cycles could be a random

process. Therefore, soil wetting or drying can allow increases or decreases in soil

porosity.

Table 2 shows the values of soil porosity obtained for the three soils used in the

experiment.

Table 2 reveals a tendency of an increase in soil porosity for soil 1 with

exception of sample 4 that did not present statistical differences between treatments. All

samples of this soil exhibit statistical differences at the 5 % probability level, through

the Duncan test. These results confirm and quantify the alterations in soil structure

induced by W-D cycles. The increase in soil porosity between T0 and T2, for soil 1, was

approximately 6 %. In the case of soil 2 the increase in soil porosity was of

approximately 7 % between T0 and T2. For soil 3 samples 2 and 3 did not present

statistical differences between treatments, and samples 2 and 5 demonstrated a decrease

in soil porosity after 3 W-D cycles indicating a random behavior of modifications in soil

structure when this soil was wetted and dried. The increase in soil porosity for soil 3

was approximately 4 %, and between T0 and T1 the soil porosity remained the same.

The results presented in Table 2 ratify that W-D cycles, dominated by the soil friction

between soil column and core cylinder wall, influence the inner structure of the soil and

can affect soil water distribution or retention (Figure 7).

14

Table 2. Soil porosity for the different treatments for the soils submitted to

wetting/drying cycles.

Soil samples

Xanthic Ferralsol

T0

Soil porosity (%)

T1

T2

1 33 4 (12)a

34 3 (9)a 36 4 (11)

b

2 30 2 (7)a

30 3 (10)a 33 3 (9)

b

3 33 2 (6)a

33 3 (9)a 35 2 (6)

b

4 35 4 (11)a

35 4 (11)a 36 5 (14)

a

5 35 4 (11)a

36 5 (14)a,b

37 6 (16)b

6 34 2 (6)a

34 3 (9)a 36 4 (11)

b

Average 33 2 (6)

34 2 (6) 35 2 (6)

Eutric Nitosol

1 38 5 (13)a

37 4 (11)a 40 3 (8)

b

2 39 4 (10)a

39 4 (10)a 43 4 (9)

b

3 38 3 (8)a

41 2 (5)b 42 2 (5)

b

4 39 3 (8)a

39 2 (5)a 41 2 (5)

b

5 40 3 (8)a

41 3 (7)a,b

42 3 (7)b

6 40 3 (8)a

43 5 (12)b 43 5 (12)

b

Average 39 1 (3)

40 2 (5) 42 1 (2)

Rhodic Ferralsol

1 51 5 (10)a

51 5 (10)a 55 6 (11)

b

2 45 4 (9)a

44 3 (7)a 45 4 (9)

a

3 48 4 (8)a

48 5 (10)a 49 4 (8)

a

4 47 4 (9)a

49 4 (8)b 50 4 (8)

b

5 50 5 (10)a

49 4 (8)a 53 4 (8)

b

6 46 4 (9)a

47 4 (9)a 51 4 (8)

b

Average 48 2 (4)

48 3 (6) 50 3 (6) Different letters are statistically different (P=0.05) through the Duncan test. The test only was applied between samples of same soil, but it was not applied between soils. Values between parentheses represent the coefficient of variation (%).

It is very important have in mind that soil sampling procedures can affect soil

structure causing compaction near the cylinder wall. Pires et al. [25] evaluated the

influence of different cylinder sizes in soil structure, showing a density gradient from

the center to the edge of the samples indicating a clear tendency of compaction near the

edge. In this work [25] the authors indicated that cylinders with volume of 100 cm3

caused the lowest impact in soil structure in comparison to other of dimensions of 25,

50 and 55 cm3. Having this influence of sampling in mind, to calculate soil porosity we

used areas smaller than the real sample size inside the cylinder, eliminating the

compacted borders. These areas were selected at least 7 mm away from the edges of

cylinder and 5 mm away from the bottom and the top of the samples. Cautious

15

procedures were also adopted during sampling to avoid friction between soil core and

the inner wall of cylinder. Rogasik et al. [26] and Baveye et al. [27] present two

important reports about the effect of sampling volume on some soil physical properties.

These authors [26, 27] try to determine the smallest instrumental volume of influence

that leads to low variance values when measurements are performed at different regions

in a certain area of soil samples.

Table 3 and Figure 7 ratifies the hypothesis that alterations in soil structure cause

modifications in soil water retention. This result has important consequences in

agricultural applications because these modifications in soil structure can affect plant

and root development, and the evaluation of soil water retention curves [21]. For soil 1

it was observed that after the application of 3 and 9 W-D cycles a higher amount of

water remained inside the soil samples. This result is important because this water can

be used by the plant and the root. Fig. 7A shows that for high matric potentials there is

no difference between T1 and T2 for soil 1 (Table 3), however after 60 cm of water

column (mesopore region) statistical differences (Student test) occur between

treatments. Soils 2 and 3 (Figs. 7B and 7C) exhibit the same behavior between T1 and

T2, however, for the former, only after 800 cm of water column (small mesopore region)

there are statistical differences between treatments and, for the latest, it is possible to

identify statistical differences for all matric potentials (Table 3). This difference in soil

water retention was probably caused by modifications in soil structure induced by W-D

cycles. Sarmah et al. [28] found a decrease in water infiltration rate after W-D cycles

probably due to the slaking and collapse of microaggregates. This collapse alters the

inner structure of the soil affecting its porosity. Modifications in the pore shape and in

the ratio between micro and mesoporosities are probably induced after W-D cycles

having an impact on water retention characteristics. Phogat and Aylmore [29] showed

changes in soil macroporosity distribution on wetting and subsequent drying process,

with the macroporosity decreasing after W-D cycles. This result indicates instability in

the system upon wetting. In our case an increase in the number of micropores and

mesopores with subsequent W-D cycles probably occurred, because a larger quantity of

water remained inside the mesopores and micropores for T2 in soils 1, 2, and 3. Sartori

et al. [30] and Pagliai et al. [31] discussed the influence of W-D cycles in soil porosity,

pore shape, and pore size distribution. Their results [30, 31] demonstrate an increase in

16

soil porosity for pores in the range of 30 m to 1 mm and strong modifications in the

shape and pore size distribution after W-D cycles.

(A)

(B)

(C)

Figure 7. Soil water retention curves obtained for treatments T1 (3 cycles) and T2 (9 cycles of

wetting/drying) for: (A) Xanthic Ferralsol, (B) Eutric Nitosol, and (C) Rhodic Ferralsol.

Xanthic Ferralsol

0.1

1

10

100

1000

0.1 0.15 0.2 0.25 0.3 0.35 0.4

m3.m

-3

m

m o

f H

2O

T0

T1

T2

Eutric Nitosol

0.1

1

10

100

1000

0.2 0.25 0.3 0.35 0.4 0.45

m3.m

-3

m (

m o

f H

2O

)

T0

T1

T2

Rhodic Ferralsol

0.1

1

10

100

1000

0.2 0.25 0.3 0.35 0.4 0.45 0.5

m3.m

-3

m (

m o

f H

2O

)

T0

T1

T2

17

Table 3. Matric potential (m) and soil water content () for each applied pressure for

the Xanthic Ferralsol, Eutric Nitosol, and Rhodic Ferralsol.

Xanthic Ferralsol Eutric Nitosol Rhodic Ferralsol

T1 T2 T1 T2 T1 T2

m

(m of H2O)

(m3.m

-3)

(m

3.m

-3)

(m

3.m

-3)

(m

3.m

-3)

(m

3.m

-3)

(m

3.m

-3)

0.0 0.3617a

0.3704a

0.4309a

0.4325a

0.4873a

0.5124b

0.1 0.3380a

0.3442a

0.3959a

0.3970a

0.4343a

0.4635b

0.2 0.3099a

0.3147a

0.3761a

0.3780a

0.4138a

0.4394b

0.6 0.2441a

0.2521a

0.3448a

0.3469a

0.3810a

0.3988b

1.2 0.2060a

0.2188b

0.3277a

0.3292a

0.3619a

0.3754b

2.0 0.1837a

0.2002b

0.3161a

0.3176a

0.3490a

0.3599b

3.3 0.1666a

0.1862b

0.3056a

0.3074a

0.3372a

0.3462b

4.3 0.1591a

0.1802b

0.3004a

0.3024a

0.3314a

0.3395b

6.0 0.1511a

0.1739b

0.2941a

0.2966a

0.3244a

0.3316b

8.0 0.1452a

0.1693b

0.2890a

0.2919a

0.3187a

0.3252b

15.0 0.1351a

0.1616b

0.2786a

0.2827b

0.3070a

0.3126b

40.0 0.1248a

0.1539b

0.2645a

0.2707b

0.2912a

0.2962b

80.0 0.1202a

0.1506b

0.2559a

0.2637b

0.2814a

0.2865b

120.0 0.1182a

0.1492b

0.2513a

0.2601b

0.2762a

0.2816b

150.0 0.1173a

0.1486b

0.2489a

0.2583b

0.2735a

0.2790b

Different letters are statistically different (P=0.05) through the Student test. The test only was applied between

samples of same soil and treatments, but it was not applied between soils.

Bresson and Moran [32] evaluated structural changes induced by wetting and

drying showing the changes in porosity for coarse and fine seedbeds after W-D cycles

along the height of samples. These authors [32] show, for coarse seedbed after drying, a

structural degradation of the bottom of the sample mainly due to coalescence and

formation of many 20-50 m pores due to slaking (or physical dispersion) in fine and

few coarse aggregates. For fine seedbed the main process involved in the structural

degradation was the agglomeration of the particles and/or microfragments of the dust

fraction changing soil porosity. Unfortunately the GCT scanner used by us has a

resolution not sufficient to investigate micrometric scales and to show possible

modifications in pore shape or number of micropore and mesopores. In spite of this

limitation, the results obtained in this study confirm the effect of W-D cycles on soil

porosity and its impact in soil structure.

Acknowledgements

18

To FAPESP (grant no. 02/05066-5) for financial support.

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