Ref: C0130
Soil erosion, crusting and degradation in the South of AlJabal al Akhdar, Libya
M. M. Aburas, Omar Al-Mukhtar University, Faculty of Agriculture, Elbeida, Libya
Abstract:
Erosion-driven land degradation as a result of human impacts contributes to desertification
and forms a growing threat that could have severe repercussions for this fragile local
environment. Four sites were selected and investigated in order to assess erosion-related
soil degradation in the south of Al-Jabal al Akhdar, northeast of Libya. Surface soil samples
were collected to measure particle size distribution, organic matter content and aggregate
stability, while infiltration rate and soil resistance to penetration were measured in the field.
The field survey recorded clear indications of land degradation such as degraded vegetation;
shallow soils; soil crusting and different types of soil erosion stimulated by the removal of the
protective natural vegetation. The field observations were consistent with the results of the
investigation of soil properties. High sand and silt contents and relatively low content of clay
and low organic matter contributed to the deterioration of soil structure stability; hence
exposed soil aggregates could not resist disruptive forces of rain drop impact leading to
aggregate breakdown and particle detachment. The study area sometimes receives several
events of heavy rains during the winter, therefore exposed and unstable soils can develop
crusts. The present study showed that most soils under investigation characterised with low
infiltration rate, and this reduction of infiltration rate appears to be determined by soil
crusting, which would encourage further runoff and top soil loss. This investigation has
identified the main causes of soil degradation in the study area, accordingly appropriate
measures need to be taken to prevent more ecosystem deterioration.
Keywords: soil erosion, crusting, soil degradation
Introduction:
According to Lal (2001) “Soil degradation by accelerated erosion is a serious problem and
will remain so during the 21 st century, especially in developing countries of the tropics and
subtropics”. Human activities and climatic Variations are the main causes of soil degradation
in the arid and semi-arid regions. Accelerated soil erosion is the dominant physical process
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that results in soil degradation and this process is common on marginal steep lands that
have lost more than 60% of their vegetation cover (Thornes, 1988). It is generally agreed that
degree of soil degradation usually follows the clearance of vegetation by human action. Land
degradation sometimes takes place without the impact of humans, however, it would be in
balance with natural rehabilitation, whilst the human impact can cause accelerated and
irreversible land degradation (Stocking and Murnaghan, 2001).
Changes in soil properties that may reduce soil quality or productivity can be defined as soil
degradation. Therefore, to evaluate soil degradation, measuring soil property change and
observing soil degradation for a long term can be a useful technique. Several soil properties,
for instance clay dispersion, soil compaction and aggregate stability might be suitable to
evaluate soil susceptibility to water erosion (Soisungwan, 2005). In addition, soil crusting has
been increasingly regarded as a specific form of soil degradation. Soil surface crusts can
promote runoff, interrill soil erosion and reduce seedling emergence (Valentin and Bresson,
1997).
In the semi arid areas soil degradation is a fundamental problem. Therefore, the need to
baseline data on soil erosion and degradation and its relation to the change of soil properties
are essential. In this Libyan semi-arid region, erosion-related soil degradation has a
considerable impact on vulnerable soils. It is argued here that a better understanding of soil
degradation in this part of Libya is required in order to design appropriate soil conservation
measures and that such measures must be better related to more sustainable land
management practices.
Materials and Methods:
Four sites located in the south of Al-Jabal al Akhdar Mountain were chosen to investigate soil
degradation. Al Jabal al Akhdar is located at the northeast of Libya between the
Mediterranean coast and the Sahara desert (longitude 20o to 30o E and latitude 32o to 33o N).
The sites under investigation were Wadi Aldawai (22o 13ˉ 25˭ E, 32o 36ˉ 46˭ N), South
Almkhily (22o 18ˉ 08˭ E, 32o 07ˉ 04˭ N), Alaziat (22o 39ˉ 29˭ E, 32o 16ˉ 16˭ N), and Goot AlMaslgon (22o 37ˉ 20˭ E, 32o 32ˉ 50˭ N) (Figure 1).
The climate of the Mediterranean Libyan coast is different from the Sahara desert. Al Jabal al
Akhdar is the wettest part of Libya which is a consequence of its proximity to the Mediterranean and its upland character (Gebril, 1995). The climate can be divided into these zones:
Mediterranean sub-humid (annual rainfall > 500 mm) at the north east; Mediterranean semiarid (300-500 mm) at the west and the north west; and Mediterranean arid (< 300 mm) at the
south and represent the present study area. The average temperature is 10-30 cº. Rainfall is
concentrated during the cool winter season and there is a very marked summer drought
(Hamdi, 1978). All sites have slight to moderate slope ranges between 2 - 4 o, however suffers from water erosion caused by several events of heavy rains in the northern part of the
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area and floods coming from north to south. In addition, wind erosion is a dominant process
in the whole area. Soil erosion in this area is also encouraged by degraded plant vegetation
cover which often does not exceed 10%.
Soil samples were collected from the soil surface (0–15cm depth) of each site to achieve the
study objectives. The following laboratory and field measurements were applied:
1- Soil particle size analysis: Particle size distribution was determined by the pipette method
as described by Sheldrick and Wang (1993).
2 - Instability of soil aggregates: the index was determined using wet sieving as described by
Ekwue (1984) and based on the work of Adams et al (1958).
3- Final infiltration rate was measured in the field using the double range method as described by Parr and Bertrand (1960).
4- The soil organic matter content was determined by the modified Walkely-Black method
and as described by Nelson and Sommers (1996).
Mediterranean sea
Libya, North East
Figure1: The area of study in the northeast of Libya (Al Jabal al Akhdar region)
5- Soil resistance to penetration was measured in the field using hand penetrometer
Eijkelkamp, and using a cone with 2 cm2 base area. The following formula was applied: Cone
resistance = manometer reading / base area of cone. The method was applied by Gabriels et
al (1997).
6- Other field measurements were also carried out such as: soil depth, % rock fragments of
soil surface and % plant cover. In addition, the number of rills and gullies and sheet erosion
features were also recorded.
To analyze the relation between the values of soil resistance to penetration and soil properties, correlation and regression analyses were used. Best subsets regressions in the statisti-
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cal software Minitab 15 were used to find out the properties that affected most soil resistance
to penetration.
Table 1. Physiographical, vegetative and soil properties of the investigated sites.
Sites
Slope ⁰
Elevation
(meter)
% rock fragments of soil
surface
Plant cover
percentage
Soil depth
(cm)
Soil
crusts
Wadi Aldawai
2-3
520
30%
< 10%
21
Crusts of
2 - 3 mm
2-3
174
10-15%
< 10%
>25
None
2-3
176
> 50%
< 10%
10
Crusts <
4 mm
3-4
352
25%
20 %
20
Crusts of
2 - 4 mm
South
Almkhily
Alaziat
Goot Maslgon
Results and Discussions
Table 2. Physical and chemical properties of the investigated soils.
Sites
Aggregate
stability (%)
Infiltration
-1
rate (cm.hr )
Soil organic
matter (%)
Wadi
Aldawai
13.52
0.30
2.16
South
Almkhily
9.20
2.00
Alaziat
13.04
0.60
Soil resistance
to vertical
penetration
(MPa)*
Sand %
Silt %
1.8
29.5
50.8
0.09
2.0
86.2
8.00
05.8
0.32
1.5
55.5
26.7
17.8
2.0
40.0
44.2
15.8
Goot
5.04
0.20
0.09
Maslgon
2
2
MPa= mega pascal (100 N\cm = 1000 kN\m = 1 MPa).
Clay %
19.7
The results of the field survey (Table 1) show clear indications of land degradation such as
poor plant vegetation, shallow soil depth, noticeable amount of rock fragments on the soil
surface and the existence of soil crusts on the surface (Figures 2 and 4). The field-based
indications of sheet and rill erosion show also a serious level of degradation (Figure 2). In the
present study the overall field assessment pointed out the impact of land use practices on
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soil degradation levels. The results show the relative degradation is driven by accelerated
soil erosion stimulated by removal of protective natural vegetation and the introduction of
intensive land use practices such as barley cultivation and overgrazing (Figure 3).
Soil properties for the different sites are reported in Table 2. Aggregate stability, soil organic
matter and infiltration rate were generally low at all sites. The lower infiltration rates reflect
the associated effects of having lower organic matter content and unstable aggregates.
However site 2 (South Almkhily) has relatively higher infiltration rate compared to the other
sites; this is most likely related to its higher sand content. The loss of vegetation protection
could encourage aggregate breakdown under rain drop impact leading to crust formation and
increase the mechanical strength of soil surface (Figure 5). Moreover, this promotes surface
sealing and decrease infiltration which could reduce the amount of available soil moisture for
plant growth and explain the level of degradation reported in the area. Ben-Hur and Agassi
(1997) investigated interrill erodibility in Mediterranean soils and used final infiltration rate to
indicate soil erodibility. They suggested that final infiltration rate is related to erodibility under
surface sealed conditions, as both are influenced by aggregate breakdown. In comparison,
the northern part of Al-Jabal al Akhdar which is characterised with higher average of annual
rainfall and better plant vegetation, soils are characterised with relatively higher organic matter content, higher clay percentage, more stable aggregates and higher infiltration rate,
(Aburas, 2009).
The results in Table 2 show also values of soil resistance to vertical penetration which can
indicate the mechanical strength of soil surface. Values were ranged between 1.5 – 2.0 MPa.
According to the literature such values could reduce seedling emergence, plant and root
growth (Gabriels et al, 1997). Significant impact on seedling emergence of different plants
were recorded with crust strength ranged between 1.2 – 1.8 MPa (Taylor et al, 1966; Parker
and Taylor, 1965). The results of the present study show evident mechanical strength due to
the crust formation on the soil surface at most sites; this has resulted in considerable
infiltration rate reduction (Figure 6), which could contribute to the increase of runoff and soil
erosion that has been reported in the field survey.
Figure 2: Soil erosion and degradation features in the area.
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Figure 3: Barley cultivation on shallow soils after the removal of the disturbed natural plant cover.
Figure 4: Soil crust formation is a common process in the study area
2.5
2
Soil
resistance to 1.5
penetration
1
(MPa)
R² = 0.555
0.5
0
0
5
10
15
Aggregate stability %
Figure 5: The relation between aggregate stability
and crust strength
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0.7
0.6
0.5
Infiltration 0.4
rate cm.hr-1 0.3
0.2
0.1
0
R² = 0.9727
0
0.5
1
1.5
2
2.5
Soil resistance to penetration (MPa)
Figure 6: the impact of the mechanical strength of
the soil surface on infiltration rate
1
Conclusions:
Exposed and unstable soils can develop crusts; this demonstrates the importance of the
cover factor and the consequences of removing the protective natural vegetation for the introduction of cultivation when minimal soil conservation measures are taken. It is clear that
vegetation is the most important single factor that regulates soil erosion in an environment in
which rain drop impact is the dominant cause in the soil erosion process. However, after
clearance of natural vegetation by human activities, soil response to erosion will be influenced by its inherent and dynamic properties. Therefore severe erosion is likely to occur if
these erodible soils in this fragile local environment lose the protection of the natural plant
cover.
References:
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Use in Mediterranean Libya. PhD thesis. University of Newcastle upon Tyne. UK.
Adams, J. E., Kirkham, D. & Scholtes, W. H. (1958) 'Soil erodibility and other physical properties of some Iowa soils.' Iowa State College Journal of Science, 32, (4), pp. 485540.
Ben-Hur, M. & Aggasi, M. (1997) 'Predicting interrill erodibility factor from measured infiltration rate', Water Resources Research, 33, (10), pp. 2409-2415.
Ekwue, E. I. (1984) Experimental investigation on the effect of preparation of soil samples on
measured values of soil erodibility. M.Sc Thesis. Cranfield Institute of Technology,
Silso College, UK.
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