Tractor Wheel Compaction Effect on Soil Water Infiltration ...

INTRODUCTIONSoil compaction induced by agricultural machinery sometimes extends to the plow layer. This may alter the pore system and consequently reduce water infiltration that eventually results in yield reduction. Compaction is one of the major threats to soil quality as it reduces pore volume and modifies pore geometry (Tolon-Becerra et al. 2012). Soil compaction is not only associated with agriculture but also with forest harvesting, amenity land use, pipeline installation, land restoration and wildlife trampling (Batey 2009). Military training exercises using heavy-tracked vehicles is an intensive land-use activity that may result in vegetation disturbances and soil compaction (Lindsey et al, 2012). Also agricultural lands are recently under pressure of heavy machinery (weighing as much as 30 to 60 Mg) used for constructional projects and activities (Berli et al. 2004).

The compaction of soil affects nearly all properties of the soils: physical, chemical and biological. Soil compaction alters its structure by crushing aggregates or combining them into larger units, increasing its bulk density and decreasing

ISSN: 1394-7990Malaysian Journal of Soil Science Vol. 21: 47- 61 (2017) Malaysian Society of Soil Science

Tractor Wheel Compaction Effect on Soil Water Infiltration, Hydraulic Conductivity and Bulk Density

Nooshin Ramezani,* G. Abbas Sayyad and A. Rahman Barzegar

Department of Soil Science, College of Agriculture, Shahid Chamran University of Ahwaz, Ahwaz, Iran.

ABSTRACTSoil compaction alters the soil pore system, and may adversely affect the availability of water and air to plants and microorganisms. This study was conducted on a loamy soil to investigate water flow path using dye patterns. Five treatments were compared: control (no traffic), single, two, four and eight passages in three replications in the field. A dye tracer of Brilliant Blue FCF solution was uniformly added to each treatment for eight hours at a rate of 5 mm/h using a rain simulator. Flow paths were photographed with a digital camera. The images were processed by digital image analysis in order to analyse the spatial distribution of the stained area. Results indicated that induced compaction significantly altered the hydraulic properties of the soil. Highest impact was observed at 0-20 cm soil depth; no visible changes were observed in soil physical properties for subsoil. Results also showed that stained area as index of water infiltration was reduced by 77.5% in eight times passages treatment compared to control. Dye infiltration was uniform in control treatment while in the four and eight times tractor traffic treatments, dye infiltration was low on the surface and preferential flow of dye was observed in deeper parts of the profile.

Keywords: Dye tracer, image processing, rainfall simulator.

___________________*Corresponding author : E-mail: [email protected]

Malaysian Journal of Soil Science Vol. 21, 201748

the number of coarser pores leading to reduced permeability of water and air. It also increases surface runoff, erosion, flooding and reduces groundwater recharge (Batey 2009 ).

Soil compaction may be assessed in terms of parameters such as bulk density and porosity. Although these parameters may be used to easily evaluate soil compaction, they do not evaluate changes in the pore size and continuity due to soil compaction. It is practically difficult to predict the effect of soil compaction on water movement only by the above measured indicators. Therefore, techniques have been developed to directly or indirectly measure the pore volume and the changes in soil structure (Sander and Gerke 2007). Tracing experiment is a method to determine the effect of soil compaction on water movement. Mooney and Nippatsuk (2003) quantified the effects of soil compaction on water flow using dye tracers and image analysis and reported that visual techniques of dye tracing and image analysis could enable improved understanding of flow pathways of soil water associated with soil compaction.

Dye tracing experiment is a suitable technique has been used by researchers in recent years for assessing flow patterns (Flury and Fluhler 1994). Spatial distribution of a dye tracer may be used as an effective means to study compacted soil layer. Kulli et al. (2003) investigated the flow patterns of stained areas for the soils of similar plots before and after compaction, and reported that the traffic of heavy machinery significantly modified infiltration patterns.

Image processing seems to be a suitable technique to show stained paths and quantify flow patterns in soils (Markus and Flühler 2004). Dye tracing infiltration methods have been used together with image processing to evaluate the role of soil structure as well as preferential flow (Flury and Flühler 1994; Ghodrati and Jury 1990). Various tracers were used in different experiments to show water movement. The more common type is Brilliant Blue FCF (Sander and Gerke 2007), because it is not toxic, can easily be seen in the soil, and is not adsorbed by soil particles (Ohrstrom et al. 2002). Researchers (Germa´n Heis and Flury 2000; Turpin et al., 2007) tested Brilliant blue FCF for toxicity, adaptation and movement. They reported that this tracer was one of the best tracers to study water flow in soils.

A few studies have quantitatively shown the effect of soil compaction on water flow. Low organic matter of soils and frequent agricultural machinery traffic in different stages of crop production are considered important factors in agricultural compaction.

The aim of this study was to quantify the effect of compaction on water flow through the soil, especially by examining water preferential flow using a dye tracer and to compare it with other indicators such as bulk density and porosity.

MATERIALS AND METHODSThe experiment was conducted in Ahwaz, southwest of Iran (31o20’N; 48o40’E) at an elevation of 20 m above sea level. A factorial design experiment was used in the frame of complete randomized blocks with five treatments and six depths and three replications. Some physical properties of soil are shown in Table 1.

Malaysian Journal of Soil Science Vol. 21, 2017 49

Plots were irrigated prior to implementing the treatments. Therefore, at the time of the experiment (tractor passage), soil surface moisture was adjusted to near field capacity. Soil moisture contents at depths of 10, 20, 30, 40, 50 and 60 cm were measured. After irrigation, treatments including 0 (no tractor passage), 1-, 2-, 4- and 8- time tractor passages (T0, T1, T2, T4 and T8 respectively) were implemented. A tractor model of MF399 (Table 2) was used.

One day after tractor passage, Brilliant Blue FCF solution with concentration of 4 g L-1 (Flury and Flühler 1994) was added to each treatment for a period of 8 hours and intensity of 5 mm hr-1 using a rainfall simulator. The simulation specifications were as follow (Fig. 1): rain intensity of 5 mm/hr, outlet flow of 330 mLhr-1, and tank volume of 200 L circle with diameter of 160 cm covered by a rain simulator.

Considering the tiny droplets produced by the simulator, the perimeter of the device was covered by a tarp sheet to minimise the effect of wind and other external factors.

A profile was dug in each treatment and the flow paths photographed by a digital camera of high resolution, 24 h after dye application. Pictures were analysed and areas of water flow paths determined. The flow path areas were compared to distinguish the differences of compacted treatments.

TABLE 1Some physical properties of the studied soil

5

A few studies have quantitatively shown the effect of soil compaction on water flow. Low 1

organic matter of soils and frequent agricultural machinery traffic in different stages of crop 2

production are considered important factors in agricultural compaction. 3

4

The aim of this study was to quantify the effect of compaction on water flow through the soil, 5

especially by examining water preferential flow using a dye tracer and to compare it with other 6

indicators such as bulk density and porosity. 7

8

MATERIALS AND METHODS 9

The experiment was conducted in Ahwaz, southwest of Iran ( ) at an elevation 10

of 20 m above sea level. A factorial design experiment was used in the frame of complete 11

randomized blocks with five treatments and six depths and three replications. Some physical 12

properties of soil are shown in Table 1. 13

TABLE 1 14

Some physical properties of the studied soil 15 Depth Sand Silt Clay Soil

texture Bulk density

Porosity Organic matter

cm --------------------%---------------- (Mg m-3) -------------%----------- 0-10 40.5 30.5 29.0 Loam 1.3 50.1 0.67 10-20 47.5 34.0 18.5 Loam 1.3 49.4 0.37 20-30 43.5 34.0 22.5 Loam 1.3 49.4 0.30 30-40 45.0 28.0 27.0 Loam 1.4 49.0 0.22 40-50 51.0 20.0 29.0 Sandy

clay Loam

1.3 49.4 0.17

50-60 65.0 20.0 15.0 Sandy loam

1.4 48.6 0.13

16

Plots were irrigated prior to implementing the treatments. Therefore, at the time of the 17

experiment (tractor passage), soil surface moisture was adjusted to near field capacity. Soil 18

TABLE 2Characteristics of tractor used for treatments

6

moisture contents at depths of 10, 20, 30, 40, 50 and 60 cm were measured. After irrigation, 1

treatments including 0 (no tractor passage), 1-, 2-, 4- and 8- time tractor passages (T0,T1,T2, T4 2

and T8 respectively) were implemented. A tractor model of MF399 (Table 2) was used. 3

TABLE 2 4 Characteristics of tractor used for treatments 5

Specification Unit Model MF399 Tractor width 2m Tractor length 4.3m Rear tire pressure 1.1 Bar Front tire pressure 1.4 Bar Tractor weight (with full tank)

3677 kg

Maximum static load of front axial 49.4 kg Velocity 5.7 km h-1

6

One day after tractor passage, Brilliant Blue FCF solution with concentration of 4 g L-1 (Flury 7

and Flühler 1994) was added to each treatment for a period of 8 hours and intensity of 5 mm/hr 8

using a rainfall simulator. The simulation specifications were as follow (Fig. 1): rain intensity of 9

5 mm/hr, outlet flow of 330 mLhr-1, and tank volume of 200 L circle with diameter of 160 cm 10

covered by a rain simulator. 11

12

Figure 1. View of the rain simulator in field and laboratory 13


MATLAB software was used for image processing. Dye tracer paths were translated into flow patterns by image processing techniques. Then, the ratio of stained surface area to total surface, stained area of each layer to total surface, and stained area of each layer to area of that layer were quantitatively measured for different depths of soil profile of each treatment.

One day after treatments application, bulk density was determined using the sample holders of core sampler and total porosity of the soil was calculated (Sultani et al. 2007). Aggregate stability was determined using the wet sieving methods and mean weight diameter (MWD index) calculated (Barzegar et al. 2004). Soil moisture characteristic curve of each treatment was measured using pressure plate apparatus (Sultani et al., 2007(2016 in ref list). Measurement of soil hydraulic properties is important to improve the understanding of soil physical behaviour (Soracco et al. 2011). Hydraulic properties including number of pores (Watson and Luxmoore 1986), hydraulic conductivity and the percent of water flowing through the pores were measured using disc infiltrometer at four different tensions (h = 0, -3, -5, -15 cm H2O) for each treatment (Ankeny et al. 1991). Analysis of data was performed by MSTATC, version 2.1 developed by Russel (1994) and data significance levels were distingushed by LSD tests at p≤0.05.

RESULTS AND DISCUSSION

Physical and Hydraulic Properties of the SoilSoil moisture before applying treatments was 18.5 ± 1%. Soil textures were similar in all treatments; loam from 0-40 cm, sandy clay loam from 40-50 cm and sandy loam from 50-60 cm. Hydraulic and physical properties of the soil changed notably with increasing the compaction levels. Increasing compaction levels resulted in higher soil bulk density and lower total porosity (P≤0.01 (Table 3).

Fig. 1: View of the rain simulator in field and laboratory


With 8-time tractor passages, average bulk density increased by 5.3% compared to control, and the average total pore space decreased by 6%. Major effect of compaction was observed from surface down to a depth of 20 cm (Fig. 2). The highest bulk density and lowest total porosity were at 10-20 cm depth from the surface. Kuht and Reintame, (2004) reported that compaction affects surface and near soil surface characteristics.

(a) (b)Fig. 2: (a) Changes of bulk density and (b) totalporosity of different tractor passages; T

subscript numbers indicates the number of tractor passes

Control treatment had the highest MWD (an index of soil structural stability) and 8-time tractor passage had the lowest. Maximum MWD index was obtained at depths of 20-30 cm. and lowest at depths of 10-20 cm. The surface layer of 0-10 cm depth was the least affected due to presence of plant cover and higher organic matter content (Fig. 3). Boizard et al. (2002) reported that compaction changes physical properties of upper soil layers. Barzegar et al. (2004) indicated that higher passages of agricultural machinery may result in the formation of smaller aggregates especially when the soil is dry.

TABLE 3Statistical analysis of compaction effect on bulk density and percent pore space

8

(Ankeny et al. 1991). Analysis of data was performed by MSTATC, version 2.1 developed by 1

Russel (1994) and data significance levels were distingushed by LSD tests at p≤0.05. 2

3

RESULTS AND DISCUSSION 4

Physical and Hydraulic Properties of the Soil 5

Soil moisture before applying treatments was 18.5 ± 1%. Soil textures were similar in all 6

treatments; loam from 0-40 cm, sandy clay loam from 40-50 cm and sandy loam from 50-60 cm. 7

Hydraulic and physical properties of the soil changed notably with increasing the compaction 8

levels. Increasing compaction levels resulted in higher soil bulk density and lower total porosity 9

(P≤0.01 (Table 3). 10

TABLE 3 11 Statistical analysis of compaction effect on bulk density and percent pore space 12

Source of variations

df F-value

Total porosity (%) Bulk density Compaction (c) 4 **79.64 **55.79 Depth (d) 5 **76.11 **175.93

d *c 20 **30.35 **30.33 p≤0.01** 13

14

With 8-time tractor passages, average bulk density increased by 5.3% compared to control, and 15

the average total pore space decreased by 6%. Major effect of compaction was observed from 16

surface down to a depth of 20 cm (Fig. 2). The highest bulk density and lowest total porosity 17

were at 10-20 cm depth from the surface. Kuht and Reintame, (2004) reported that compaction 18

affects surface and near soil surface characteristics. 19

9

1

(a) (b) 2

Figure 2. (a) Changes of bulk density and (b) totalporosity of different tractor passages; T subscript 3 numbers indicates the number of tractor passes 4

5 Control treatment had the highest MWD (an index of soil structural stability) and 8-time tractor 6

passage had the lowest. Maximum MWD index was obtained at depths of 20-30 cm. and lowest 7

at depths of 10-20 cm. The surface layer of 0-10 cm depth was the least affected due to presence 8

of plant cover and higher organic matter content (Fig. 3). Boizard et al. (2002) reported that 9

compaction changes physical properties of upper soil layers. Barzegar et al. (2004) indicated that 10

higher passages of agricultural machinery may result in the formation of smaller aggregates 11

especially when the soil is dry. 12

13

-70

-60

-50

-40

-30

-20

-10

01.3 1.4 1.5 1.6

(cm)

Dep

th

Bulk density (Mg m-3)

T0

T1

T2

T4

T8-70

-60

-50

-40

-30

-20

-10

00 20 40 60

Dep

th (c

m)

Total porosity (%)

T0

T1

T2

T4

T8


The soil water holding capacity was significantly affected by soil compaction levels (P≤0.01) (Table 4). Control treatment had the maximum soil water holding capacity. The lowest amount of water stored in the soil was observed in the 8-time tractor passage treatment (Fig. 4).

Effect of water potential on water holding capacity was significant (P≤0.01). According to capillary principal, water outflow occurs in pores with different radius at variable tensions.

Number of pores decrease with increasing compaction levels. The highest number of pores (large and medium) was measured in control, while the lowest number occurred in the treatment with 8-time tractor passages (Table 5).

Fig. 3: Changes of MWD with depth of different treatments; T subscript numbers indicate the number of tractor passes

10

1

Figure 3. Changes of MWD with depth of different treatments; T subscript numbers indicate the number 2 of tractor passes 3

4

The soil water holding capacity was significantly affected by soil compaction levels (P≤0.01) 5

(Table 4). Control treatment had the maximum soil water holding capacity. The lowest amount 6

of water stored in the soil was observed in the 8-time tractor passage treatment (Fig. 4). 7

TABLE 4 8 Statistical analysis of compaction effect on soil water holding capacity at different water 9

potentials 10 Source of variations

df F-value

Compaction (c) 4 **88.4531 water potential (w) 5 **133022.26 c×w 20 2071.88**

p≤0.01** 11 12

-30

-25

-20

-15

-10

-5

00 0.1 0.2 0.3 0.4

Dep

th (c

m)

T0

T1

T2

T4

T8

10

1

Figure 3. Changes of MWD with depth of different treatments; T subscript numbers indicate the number 2 of tractor passes 3

4

The soil water holding capacity was significantly affected by soil compaction levels (P≤0.01) 5

(Table 4). Control treatment had the maximum soil water holding capacity. The lowest amount 6

of water stored in the soil was observed in the 8-time tractor passage treatment (Fig. 4). 7

TABLE 4 8 Statistical analysis of compaction effect on soil water holding capacity at different water 9

potentials 10 Source of variations

df F-value

Compaction (c) 4 **88.4531 water potential (w) 5 **133022.26 c×w 20 2071.88**

p≤0.01** 11 12

-30

-25

-20

-15

-10

-5

00 0.1 0.2 0.3 0.4

Dep

th (c

m)

T0

T1

T2

T4

T8

TABLE 4Statistical analysis of compaction effect on soil water holding capacity at different

water potentials


In each row the capital letters show the difference in the number of pores between the two classes of pores in each treatment, and lowercase letters in each column indicates differences between treatments in the number of pores (significant level was 0.01).

Both macro- and meso-pores were reduced as the compaction levels increased. However, the number of medium size pores was more than the macro-pores in all treatments. With just one passage of tractor, the number of large and medium pores decreased 16.5 and 13%, respectively, relative to control treatment. These changes for the 8-time tractor passages were 85 and 62.5%. Results clearly indicated that the macro-pores were more affected by compaction levels than the meso-pores (Table 5).

By increasing the compaction levels, the percentage of effective porosity decreased significantly for both pore size classes (Fig 5). In all of the treatments, the difference in effective porosity between large and medium size pores was significant. The highest water flow for both pore sizes was in control treatment and the lowest was for the 8-time tractor passages. For all of the treatments, the maximum flow occurred in large pores and minimum in the medium pores. Although large pores consist of a small portion of soil total porosity, they were the principal path for water movement.

11

1

Figure 4. Soil moisture characteristic curve of different treatments; T subscript numbers indicate the 2 number of tractor passes 3

4

Effect of water potential on water holding capacity was significant (P≤0.01). According to 5

capillary principal, water outflow occurs in pores with different radius at variable tensions. 6

Number of pores decrease with increasing compaction levels. The highest number of pores (large 7

and medium) was measured in control, while the lowest number occurred in the treatment with 8

8-time tractor passages (Table 5). 9

TABLE 5 10 Number of pores per square meter (N) in different treatments 11 Treatments

Large pores Medium pores no traffic (TNN 12Ff 1905Aa single passage (T1) 10Ff 1663Bb two times passage (T2) 7Ff 1350Cc four times passage (T4) 3Ff 1070Dd eight times passage (T8) 2Ff 716Ee 12 In each row the capital letters show the difference in the number of pores between the two classes of pores in each 13 treatment, and lowercase letters in each column indicates differences between treatments in the number of pores 14 (significant level was 0.01). 15 16

Both macro- and meso-pores were reduced as the compaction levels increased. However, the 17

number of medium size pores was more than the macro-pores in all treatments. With just one 18

0

10

20

30

40

50

-1500-1000-5000

(%(

θv

(kPa( water potential

T0

T1

T2

T4

T8

Fig. 4: Soil moisture characteristic curve of different treatments; T subscript numbers indicate the number of tractor passes

TABLE 5Number of pores per square meter (N) in different treatments

11

1

Figure 4. Soil moisture characteristic curve of different treatments; T subscript numbers indicate the 2 number of tractor passes 3

4

Effect of water potential on water holding capacity was significant (P≤0.01). According to 5

capillary principal, water outflow occurs in pores with different radius at variable tensions. 6

Number of pores decrease with increasing compaction levels. The highest number of pores (large 7

and medium) was measured in control, while the lowest number occurred in the treatment with 8

8-time tractor passages (Table 5). 9

TABLE 5 10 Number of pores per square meter (N) in different treatments 11 Treatments

Large pores Medium pores no traffic (TNN 12Ff 1905Aa single passage (T1) 10Ff 1663Bb two times passage (T2) 7Ff 1350Cc four times passage (T4) 3Ff 1070Dd eight times passage (T8) 2Ff 716Ee 12 In each row the capital letters show the difference in the number of pores between the two classes of pores in each 13 treatment, and lowercase letters in each column indicates differences between treatments in the number of pores 14 (significant level was 0.01). 15 16

Both macro- and meso-pores were reduced as the compaction levels increased. However, the 17

number of medium size pores was more than the macro-pores in all treatments. With just one 18

0

10

20

30

40

50

-1500-1000-5000

(%(

θv

(kPa( water potential

T0

T1

T2

T4

T8


(b) (a)Fig. 5: (a) Flow distribution and (b) effective porosity percent of each category of the soil pores in different treatments; the capital letters indicate the effect of compaction on flow distribution and effective porosity percent and lower case letters show the

difference between the two classes of pores. (P≤0.01); T subscript numbers indicate the number of tractor passes.

Hydraulic conductivity also decreased as the compaction levels increased. The maximum rate of saturated hydraulic conductivity was in control treatment and the lowest rate was observed in the 8-time tractor passes (Fig. 6).

12

passage of tractor, the number of large and medium pores decreased 16.5 and 13%, respectively, 1

relative to control treatment. These changes for the 8-time tractor passages were 85 and 62.5%. 2

Results clearly indicated that the macro-pores were more affected by compaction levels than the 3

meso-pores (Table 5). 4

5

By increasing the compaction levels, the percentage of effective porosity decreased significantly 6

for both pore size classes (Fig 5). In all of the treatments, the difference in effective porosity 7

between large and medium size pores was significant. The highest water flow for both pore sizes 8

was in control treatment and the lowest was for the 8-time tractor passages. For all of the 9

treatments, the maximum flow occurred in large pores and minimum in the medium pores. 10

Although large pores consist of a small portion of soil total porosity, they were the principal path 11

for water movement. 12

13

(b )14 (a) 15

Figure 5. (a) Flow distribution and (b) effective porosity percent of each category of the soil pores in 16 different treatments; the capital letters indicate the effect of compaction on flow distribution and effective 17 porosity percent and lower case letters show the difference between the two classes of pores. (P≤0.01); T 18

subscript numbers indicate the number of tractor passes. 19 20

Jj Ii Hh Gg Ff

Aa Bb

Cc

Dd Ee

0

10

20

30

40

50

60

70

T0 T1 T2 T4 T8

Flow

dis

tribu

tion

(%)

MediumPores

Largepores

Aa

ABb

BCc

Cd

CDe

Df Df Df Df Df 0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

T0 T1 T2 T4 T8Ef

fect

ive

poro

sity

(%

)

Mediumpores

Largepores

Fig. 6. Hydraulic conductivity of different compaction levels measured at different soil water potential; T subscript numbers indicate the number of tractor passes

13

Hydraulic conductivity also decreased as the compaction levels increased. The maximum rate of 1

saturated hydraulic conductivity was in control treatment and the lowest rate was observed in the 2

8-time tractor passes (Fig. 6). 3

4 5

Figure 6. Hydraulic conductivity of different compaction levels measured at different soil water potential; 6 T subscript numbers indicate the number of tractor passes 7

8 9

In the single tractor passing treatment, the saturated hydraulic conductivity decreased 17.3% 10

compared to the control. However, this reached 71% for 8-time tractor passages. The same trend 11

was also observed for unsaturated hydraulic conductivity. The differences between one and eight 12

times tractor passages with respect to control were 17.5 and 66.5%, respectively. Since 13

compaction has more effect on large pores, it is conceivable that larger pores diminish by 14

compaction and thus the hydraulic conductivity decreases. Zhang et al. (2006) reported that 15

hydraulic conductivity is lower for the soil with several tractor passages compared with no 16

passage. Mossadeghi-Björklund et al. (2016) reported that compaction is the process by which 17

0

5

10

15

20

25

30

35

-16-14-12-10-8-6-4-20

Hyd

raul

ic c

ondu

ctiv

ity (

cm/d

ay)

water potential (water cm)

T0

T1

T2

T4

T8


In the single tractor passing treatment, the saturated hydraulic conductivity decreased 17.3% compared to the control. However, this reached 71% for 8-time tractor passages. The same trend was also observed for unsaturated hydraulic conductivity. The differences between one and eight times tractor passages with respect to control were 17.5 and 66.5%, respectively. Since compaction has more effect on large pores, it is conceivable that larger pores diminish by compaction and thus the hydraulic conductivity decreases. Zhang et al. (2006) reported that hydraulic conductivity is lower for the soil with several tractor passages compared with no passage. Mossadeghi-Björklund et al. (2016) reported that compaction is the process by which soil bulk density increases and porosity decreases. Compaction not only reduces total pore volume but also modifies the pore size distribution and reduces the saturated and near-saturated hydraulic conductivity of soil.

Flow patternThe rate of dye movement into the soil decreased when compaction levels increased (p ≤ 0.01). The highest dye area was observed in control treatment while the lowest area was measured in the 8-time tractor passage. The stained area in the treatment with 8-time tractor passages showed a 77.5% decline compared to control treatment. Dye infiltration (stained area) decreased with increasing depth, with the difference between 0-10 cm and 50-60 cm being 97%. When dye tracer was added to the soil, it infiltrated directly into the soil of blank treatment. But for the compacted treatment (especially 4x and 8x passage), a kind of water logging developed on the surface. There were few flow paths exactly beneath the wheel, but for the sections under its right or left side, flow patterns were similar to control (Fig. 7). Kulli et al. (2003) also concluded that the difference in flow paths beneath the wheel and on either side of compacted treatments was due to both compression and lower permeability. Weiler and Naef (2000) showed that the preferential flow on the soil surface starts in the saturated zone. Yasuda et al. (2001) reported that because of water accumulation on the soil surface and expansion of clay particles a significant portion of soil water is bypassing the soil matrix and occurring in the preferential flow.


The T0 of Fig. 8 and P0 of Fig. 9 depict the stained area and flow pattern of the control treatment. As it is evident in the image, the maximum stained area was observed in the control compared to other treatments.

Fig. 7: Comparison of infiltration rate under wheel and on either side in the treatment with 8x passage. ׀ indicates flow path beneath the wheel and ׀׀shows the side of

compacted treatment.

Fig. 8. Images of dye tracer penetration of different treatments; T subscript numbers indicate the number of tractor passages.


Fig. 9: Flow patterns (P) of different treatments; subscripts represent the number of tractor passages

The distribution of available pores for solution infiltration in control was more suitable??? and uniform than other treatments. The highest stained area was observed in 0-10 cm of soil surface. About 93% of this layer was stained by tracer infiltration (Table 6). In this treatment, the stained area decreased with increasing depth. Dye tracer infiltrated down to a depth of 40 cm and from that point downward, no dye was observed. Table 6 shows the percent stained area for different treatments.

In the treatment of one tractor passage, the uniformity of pores and percent stained area was less relative to the control treatment (T1 of Fig. 8 and P1 of Fig. 9). Stained area with one passage reduced 36.8% relative to the control treatment. The higher bulk density and lower porosity reduced the infiltration rate of dye tracer in the treatment with one tractor passage relative to control.

Strong decline of dye infiltration was observed in the treatment with eight times tractor passages (T8 of Fig. 8 and P8 of Fig. 9). The difference in dye tracer infiltration between 8- and 4-time passages was 25%. Because of the lowest pore numbers and soil total porosity, hydraulic conductivity, aggregate stability and consequently the highest bulk density resulted in a decline of the stained area and percent stained area.

17

1 P1P0 2

3 P8P4P2 4

5 6

Figure 9. Flow patterns (P) of different treatments; subscripts represent the number of tractor passages 7 8 The distribution of available pores for solution infiltration in control was more suitable??? and 9

uniform than other treatments. The highest stained area was observed in 0-10 cm of soil surface. 10

About 93% of this layer was stained by tracer infiltration (Table 6). In this treatment, the stained 11

area decreased with increasing depth. Dye tracer infiltrated down to a depth of 40 cm and from 12

that point downward, no dye was observed. Table 6 shows the percent stained area for different 13

treatments. 14


Comparison of Dye Tracer Infiltration Rate at Different DepthsThe highest percent of stained area was observed at 0-10 cm depth of control treatment. At this depth, infiltration rate decreased significantly with increasing compaction level. The lowest rate at this depth was in the treatment with 8-time passages. In single time passage, dye infiltration rate decreased 14% relative to control, while it reached 70% for 8-time tractor passages.

For the 10-20 cm depth, the stained area rate also decreased significantly with increasing compaction level. The maximum stained area percentage was detected in control and the minimum was noticed in the treatment with 8-time passages. The difference in stained area percentages between once and 8-time passages relative to control at this depth were 39.5 and 96.5%, respectively. This indicates compaction has more effect at this depth compared to the above layer. As mentioned earlier, the main compaction effect on bulk density and total porosity occurred at 10-20 cm. At this depth, infiltration rate for the treatment with 4-time tractor passes was 32% more than 2-time passages. Compaction produced by 4-time treatments might be the reason for developing preferential flow and an increase in infiltration for this treatment in comparison to 2-time tractor passages.

At a depth of 20-30 cm, a significant decrease in stained area was also evident. No infiltration was observed at this depth with 2-time passage treatment. For 30-40 cm depth, the highest stained area was measured in the control treatment followed by the 8-time passages due to preferential flow at which macro-pore continuity was still maintained despite reductions in macroporosity (Mossadeghi-Björklund et al. 2016). Kulli et al. (2003) showed that the above postulated decrease in permeability caused by the vehicle traffic led to local ponding, leading to enhanced preferential flow in the compacted plots. No infiltration of dye tracer was observed for the 1 and 2- time passage treatments at 40-50 cm depth; infiltration occurred

TABLE 6Percent stained area at different depths of different treatments

T subscripts represent the number of tractor passage; A1 is the ratio of stained areato the area of each layer (%), and A2 is the ratio of stained area to the total area (%).

18

TABLE 6 1 Percent stained area at different depths of different treatments 2

Percent stained

area

Depth (cm)

0-10 10-20 20-30 30-40 40-50 50-60

treatment A1 T0

93.00 78.10 42.50 24.90 0.00 0.00 A2 36.59 31.09 17.89 14.11 0.00 0.00 A1 T1 80.00 47.10 29.70 0.00 0.00 0.00 A2 51.00 29.50 19.30 0.00 0.00 0.00 A1 T2 75.24 14.83 0.00 0.00 0.00 0.00 A2 82.68 16.30 0.00 0.00 0.00 0.00 A1 T4 42.00 21.80 13.90 2.80 0.00 0.00 A2 52.00 27.60 17.27 3.49 0.00 0.00 A1 T8 30.00 2.40 7.80 16.00 4.60 0.20 A2 49.50 3.38 12.79 26.36 3.79 0.37

T subscripts represent the number of tractor passage; A1 is the ratio of stained area to the area of each 3 layer (%), and A2 is the ratio of stained area to the total area (%). 4 5 6

In the treatment of one tractor passage, the uniformity of pores and percent stained area was less 7

relative to the control treatment (T1 of Fig. 8 and P1 of Fig. 9). Stained area with one passage 8

reduced 36.8% relative to the control treatment. The higher bulk density and lower porosity 9

reduced the infiltration rate of dye tracer in the treatment with one tractor passage relative to 10

control. 11

Strong decline of dye infiltration was observed in the treatment with eight times tractor passages 12

(T8 of Fig. 8 and P8 of Fig. 9). The difference in dye tracer infiltration between 8- and 4-time 13

passages was 25%. Because of the lowest pore numbers and soil total porosity, hydraulic 14

conductivity, aggregate stability and consequently the highest bulk density resulted in a decline 15

of the stained area and percent stained area. 16

17

Comparison of Dye Tracer Infiltration Rate at Different Depths 18


only in the treatment with 8-time passages. The situation at 50-60 cm depth was similar to the layer above.

CONCLUSIONSThe compaction changed the continuity and pore size distribution resulting in reduced infiltration, hydraulic conductivity, total porosity, aggregate stability, soil water retention and increased bulk density. The results of this study showed that infiltration rate of dye tracer reduced significantly with induced compaction. Our results showed that the compaction effect by tractor is only limited to 0-20 cm soil depth. Reduction of large pores in the soil surface and uniformity of infiltration paths were shown by flow patterns. Despite very little surface infiltration in the treatment with 4-time and 8-time tractor passages, dye tracer was observed in the lower depths because of the preferential flow. Reduced infiltration of the dye tracer due to machinery traffic caused a local water-logged condition followed by an increase in preferential flows in the compacted treatments. Results of this study indicate that laboratory measurements provide an insight on the relationship between compaction depth and infiltration of the dye tracer. Flow patterns not only provide information about compaction effect, but indicate how compaction may change the flow of water. Dye-tracing experiments complement the classic methods for evaluation of compaction impacts on water infiltration and transfer.

ACKNOWLEDGEMENTSThe authors would like to thank Dr. Yaghoub Mansouri and Engineer Salman Taherizadeh for their suggestions and the helpful comments.

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Tractor Wheel Compaction Effect on Soil Water Infiltration ...

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