Soil property variations in relation to exclosure and open grazing land use types in the Central Rift Valley area of Ethiopia
© Yimer et al. 2015
Received: 6 August 2015
Accepted: 10 September 2015
Published: 13 October 2015
Land degradation and unsustainable management practices have resulted in soil organic carbon and nutrient depletion, hydrological instability, reduced primary productivity, and low biological diversity. In response to these problems, communities in the Central Rift Valley area of Ethiopia have started to establish exclosures about three decades ago. This study has investigated the variations in selected soil properties (soil textural fractions, bulk density, soil moisture content, pH (H2O) and soil organic carbon content) under two land use types: open grazing land and exclosures in the Central Rift Valley of Ethiopia.
Results showed that soil organic carbon varied significantly with land use types (p = 0.040), soil depths (p = 0.010) and the interaction effect (p = 0.039). The soil moisture content showed significant variation (p < 0.0001) only with soil depth.
Exclosure land use type has shown an improvement in soil organic carbon against the findings by Mekuria et al. [International Conference on Advances in Agricultural, Biological & Environmental Sciences (AABES-2014), Dubai (UAE) 2014]. Thus, highly degraded open grazing should be designated as exclosure land management zone to restore and rehabilitate severely degraded landscape in the fragile environment of the rift valley area of Ethiopia.
Soil is a vital natural resource that is not capable of being renewed on the human time scale (Liu et al. 2006). It is a living and dynamic natural body that plays many key roles in terrestrial ecosystems, for instance, as sources of available nutrients to plants, maintenances in hydrological stability and biological diversity. Sustaining soil and environmental features are the most effective methods for ensuring sufficient food supply to support life, reduce soil degradation and improve soil health (Soares et al. 2005). They also play a role in the global carbon cycle and will have a positive impact if soils are managed. However, land use changes, mainly conversion of natural forest to agricultural and grazing lands are known to result in changes in soil chemical, physical and biological properties (Houghton et al. 2000), yet the sign and magnitude of these changes vary with land cover and land management (Celik 2005). Conversion of a natural ecosystem (for instance, natural forests) to an agricultural and unmanaged land use systems may cause some changes in the soil properties (Amusan et al. 2001). Conversion of natural landscapes into cultivated and grazing systems cause an abrupt decline in soil organic matter and reduces the nutrient content of soil through reduced litter production, increase erosion rates and decomposition of organic matter by oxidation (Chen and Xu 2010). Similarly, various reports (e.g. Solomon et al. 2002; Lal 2005; Yimer et al. 2007) indicate that conversion of natural forest into agricultural ecosystems negatively affects SOC concentration and stock by 20–50 %. Thus, mitigation strategies to reduce the impact of climate change (FAO 2006) by augmenting carbon sequestration and reducing CO2 emissions from soils include proper forest management and afforestation or reforestation programs and restoration of degraded landscapes through exclosure and integration of physical soil and water conservations with agronomic measures become crucial.
Livestock overgrazing is one of the most important factor that results in grassland degradation, soil erosion and nutrient losses (Wei et al. 2011). Due to overgrazing, the natural vegetation in the northern highlands of Ethiopia has virtually disappeared, leaving degraded communal grazing lands with irregularly spaced trees and shrubs and vast areas of bare lands devoid of vegetation (Nedessa et al. 2005). The effects of grazing on soil properties have shown to be dependent on the type of grazing management practices. For instance, communal grazing lands in northern Ethiopia have shown lower total soil nitrogen (N), available phosphorus (P), and cation exchange capacity compared to exclosures (Mekuria et al. 2011).
In the current study area, mixed farming system, in which livestock production is as important as crop production, forces farmers to allocate land for grazing (Dereje 2006). Recently, the problem in the study area is clearly observed as much of the extensively grazing lands on the sloping landscapes and flat bare lands were abandoned and are highly degraded. Hence, the area is exposed to serious land degradation, including soil erosion and heavy sediment deposition on the foot slope of the cultivated lands.
To control the land and soil degradation problems in the semi-arid and dry land areas of Ethiopia attention has been given to manage the degraded landscapes through conversion to exclosures. Exclosures are rehabilitation techniques (Mekuria et al. 2007) where areas are closed off from the interference of human and domestic animals with the goal of promoting natural regeneration of plants and reducing land degradation of formerly degraded communal grazing lands (Mekuria and Ayenekulu 2011) and to maintain economically productive and biologically diverse vegetation (Mengistu et al. 2005).
Exclosure reduce nutrient loss from a site by controlling runoff (vegetation acting as a physical barrier to soil erosion), this eventually improves the capability of the land to support diverse plant species, including exotic plantations (Mengistu et al. 2005). In Ethiopia, this has improved soil fertility by augmenting soil nutrients from decomposed plant remains and increase soil organic matter by 1.1 %, total nitrogen by 0.1 %, and available phosphorous by 1.8 mg kg−1 compared with communal grazing lands after exclosure establishment (Mekuria et al. 2007; Mengistu et al. 2005). Few case studies conducted in the highlands of Ethiopia have also shown that exclosures can be effective in restoring degraded lands and increase soil organic carbon (Mekuria et al. 2011; Mekuria 2013). For example, Mekuria (2013) reported an increase in soil carbon following the establishment of exclosures on grazing lands in the semi-arid lowlands of Northern Ethiopia, while Young-Zhong et al. (2005) demonstrated a decrease in soil carbon in Mongolia. Fusun et al. (2010) in Tibetan plateau livestock exclusion showed a significantly increased soil organic carbon and total N concentrations. On the contrary, Mekuria et al. (2014) concluded that in the “Woina Dega” agro-ecological zone of Ethiopia (ACZ) exclosures of 1–7 years had no effect on soil nutrients and soil carbon. All these examples suggest that studies on the impact of exclosures on soil properties and soil carbon are not consistent. These, together with the inconclusiveness of the evidence indicate that there is a need to study the changes in soil physical and chemical properties following the establishment of exclosures. Such understanding would help: (1) inform land managers working on the restoration of degraded ecosystem to improve ecosystem services, and (2) maximize carbon sequestration and other ecosystem services from existing exclosures established in degraded ecosystems (Mekuria et al. 2014).
Since exclosure is a new management option and a rapidly evolving complex ecosystem, it demands more investigations in the areas of its potential in maintaining vegetation diversity and thereby improves soil conditions. Thus, the study area was designated as “closed area” in 1967 with the aim of rehabilitation of highly degraded lands, and totally closed from interference of livestock and human being since 1998. Since then, gullies have become stabilized and their expansions have been blocked due to the improvements in vegetation cover through natural regenerations and enrichment plantations.
The rift valley area is the driest and most fragile part of Ethiopia where even limited disturbance of the land will have a profound effect on the soil, water resources and over all environment of the area. Though the problem is not unique to Ethiopia especially in the northern part of the country, but assessing the impacts of restoration practices in the rift valley would generate additional information representing the most environmental fragile part of the country where no detailed impact assessment studies have been conducted. Such information has a paramount importance in evaluating the impacts of restoration practices in improving soil properties and soil carbon sequestration potentials of degraded landscapes. Quantification and assessment of changes in C pool sizes and fluxes are fundamental to understanding the effects of changes in land use/land cover on ecosystem functioning and limiting greenhouse gas emissions (Jaramillo et al. 2003; Lemma et al. 2006; Tesfaye et al. 2016).
Based on these premises, however, this study has examined the long-term potential contribution of exclosure as alternative strategy for rehabilitation of degraded land and as a means to maintain soil property improvement. Thus, the objective of this study was to evaluate the possible effects of exclosure compared to adjacent open grazing land (hereafter referred to as “land use types”) on selected soil physical (particle fractions, bulk density and moisture) and chemical properties (pH and soil carbon) in the Central Rift Valley area of Ethiopia.
Description of the study area
The study area is characterized as dry tropical climate. The rainfall is bi-modal, with short rain season between April and June while the long rain season is between July and September. It receives a mean annual rainfall ranging from 500 to 700 mm. The mean monthly temperature is 25 °C with mean monthly maximum and minimum temperatures of 27 and 23 °C, respectively. Based on the data obtained from the District Agricultural Office (OPEDB 2000), the major land use types in the district include agricultural lands (17,744 ha), grazing land (5331 ha), forest coverage (6300 ha), water body (2020 ha), sugarcane plantation (2718 ha), park lands (2500 ha), irrigation (560 ha), investment lands (215 ha) and built up areas cover (7172 ha).
The major vegetation types of the study area are characterized by acacia woodland which includes species such as Acacia tortilis, Acacia oerfota, Acacia etbaica, Acacia Senegal and Acacia nilotica. Moreover, other tree species like Solanium Incanum, Solanium Somalensis, Calpurnia Aurea, Schinus Molle, Dichrostachys Cinera, Vernonia Natalensis and Croton Dichogamous (Dereje 2006) are common.
The study area falls within the Central Rift Valley area of Ethiopia. According to the OPEDB (2000), the major soil types in the district include Cambisols (45 %), Leptosols (35 %) and Andosol (20 %) with Leptosols as the dominant soil type in the study area mantling the sloping topography. These soils are characterized by whitish color and shallowness in their depth (less than 25 cm depth) overlying hard bed rocks.
A total of 48 composite soil samples were collected (4 blocks × 2 land use types × 3 replication of sample plots × 2 soil depths) forming a Randomized Complete Block Design (RCBD) for data analysis. The soil samples were air-dried at room temperature and sieved (<2 mm) prior to any laboratory analysis except for soil moisture content and bulk density determinations. Undisturbed soil samples were collected separately with a core sampler (10 cm height and 7.2 cm diameter) from each soil depth to determine the soil bulk density and thereby the soil moisture content (SMC, %). All samples were analyzed at Kulumsa Agricultural Research Center (KARC), Arsi Zone, Ethiopia.
The soil pH was measured potentiometrically with a digital pH meter in the supernatant suspension of 1: 2.5, soil: water suspension (Carter 1993). Soil organic carbon (SOC, %) content was determined following the Walkley and Black method (Walkley and Black 1934). The Kjeldahl digestion procedure was followed for the determination of total nitrogen (Bremner and Mulvancy 1982).
Statistical data analysis
The soil particle size class, bulk density, moisture content, soil pH, organic carbon, total nitrogen and carbon to nitrogen ratio obtained from the soil analyses were subject to two-way Analysis of Variance (ANOVA) to find out the variation with land use types and soil depths as well as their interaction effects using the General Linear Model (GLM) procedure of SPSS version 16.0 for windows (SPSSS Inc., Chicago, USA). Mean comparisons were made using the Tukey Honest Significant Difference (THSD) test (ρ < 0.05).
Results and discussions
Soil physical property variations
Soil particle size fractions (%)
Summary of two-way ANOVA results for sand, silt and clay fractions, soil moisture content and bulk density in relation to land use types and soil depths
Source of variation
LU × D
Soil textural fractions (%) in relation to the land use types and soil depths (mean ± SE)
The particle size class of the soil in the two land uses and soil depth is sandy loam, indicating no variations in the parent material the soil is derived from. The dominancy in the sand size fraction might have resulted from the selective transportation of the fine fractions down the slopes leaving behind sand size fractions. According to Sandor et al. (1986), soil erosion and selective removal of soil particles do affect the particle sizes. Although particle size fractions are inherent soil properties, their variation sometimes could be associated with the management practices, mainly through mismanagement of soil resources without appropriate conservation measures, resulting directly to the changes in particle size distribution through removals by sheet and rill erosions (Toy et al. 2002).
Soil bulk density (g cm−3) and Soil moisture content (%)
Soil bulk density and soil moisture content in relation to the land use types and soil depths (mean ± SE)
Soil chemical property variations
Soil pH (H2O)
Summary of two-way ANOVA results for soil pH, soil organic carbon, total nitrogen and carbon to nitrogen ratio in relation to land use types and soil depths
Source of variation
Land use (LU)
LU × D
Soil pH, soil organic carbon, total nitrogen and carbon to nitrogen ratios in relation to the land use types (mean ± SE)
Soil organic carbon (SOC, %)
Soil organic carbon (SOC, %) showed significant variations with land use types (p = 0.040), soil depths (p = 0.010) and the interaction effect (p = 0.039, Table 4). SOC was higher in soil under the exclosure (3.29 ± 0.20), in the upper surface soil (3.25 ± 0.19) than, respectively, in grazing land use type and lower sub surface soil layer (Table 5).
The soil under exclosure has a significantly higher soil organic carbon than open grazing land, which is a result of organic matter accumulation through liter fall from the trees/shrubs. The result is in agreement with Mekuria and Veldkamp (2005) in that grazing land and exclosure differ considerably in their soil organic carbon content reflecting the higher amount in exclosures than in open grazing land. The current exclosure site is dominated by Acacia species like Acacia etbaica, Acacia tortilis, Acacia oerfota, Acacia Senegal and Acacia nilotica which have increased the soil organic matter accumulation. The higher soil organic matter content in exclosure can potentially improve the soil physical properties such as soil structure and total porosity. This, in turn, increases accumulation of organic matter on the soil surface that may reduce the volume, velocity, and erosive capacity of surface run-off. Acacia etbaica tree in exclosure produces higher amount of woody litter, which has lower rate of decay (Descheemaeker et al. 2006). Conversely, the soil organic carbon content is significantly lower in the grazing land compared to the exclosure, probably the consequence of reduced amount of organic materials (litter fall) and exposure of micro-aggregate organic matter to microbial decomposition through weaker physical protection of organic matter in the soil. This is in line with Mikola et al. (2001) who justified a reduction of soil organic carbon as a result of lower biomass return in the grazing land due to very little (absence) of grass cover resulting from intensive grazing.
Soil organic carbon content decreases with soil depth. In agreement with this study, Hiederer (2009) explained the relationship between soil organic carbons with soil depth—as depth increases, soil organic carbon decreases in the soil profile. This can be justified with the presence of lower accumulation of organic matter resulting from lower below-ground root biomass in the sub-surface layer.
Total nitrogen (%) and carbon to nitrogen (C/N) ratio
The overall mean total nitrogen (%) content and the C/N ratios did not show any variation with land use types (p = 0.264 and p = 0.059, respectively) and soil depths (p = 0.300, and p = 0.170, respectively) (Tables 4). The higher total nitrogen content in the exclosure is the result of higher soil organic matter content and the presence of leguminous plants which have the capacity to fix nitrogen from the atmosphere through the roots’ nodules. The Pearson correlation result also showed a significant positive relationship with soil organic carbon (r = 0.819; p = 0.01). Carpenter et al. (2001) indicated that increasing amount of legumes in the exclosures improves nitrogen availability with exclosure age that contributes to the biomass accumulation. Mekuria and Veldkamp (2005) explained that total nitrogen is higher in exclosure than in free grazing land. Whereas, soils under grazing land have lower total nitrogen due to continuous overgrazing that results in the removal of grasses and organic matter from the soil. Islam and Weil (2000) explained the lower content of total nitrogen in open grazing land to be resulted from a combination of low organic matter inputs and greater carbon losses due to aggregate disruptions, microbial disturbance and continuous grazing.
The higher carbon to nitrogen ratio in soil of the study area can be related to high temperature and low precipitation that limits the decomposition or mineralization rate of organic matter. According to Landon (1991), the higher carbon to nitrogen ratio indicates that organic matter is not fully decomposed through microbial activities. However, the lower carbon to nitrogen ratio in grazing land relative to the exclosure could be due to the low amount of organic materials accumulation through the litter fall from the immature grasses/shrubs, and addition of cow dung, which is enriched with total nitrogen. The Pearson correlation result also showed a significant inverse relationship with total nitrogen (r = −0.860; p = 0.05). The lower carbon to nitrogen ratio in grazing land relative to exclosure was due to the breakdown of litter fall and the organic matter that enhance microorganisms to decompose into fine particles, which eventually decreases soil organic carbon in grazing land (Caravaca et al. 2002).
In the rift valley of Ethiopia, owing the rapid woodland conversion for expansion of agricultural and grazing lands there is a sharp decline of soil fertility due to low level of organic residues returned to the soil system and thereby high erosion processes. Besides the poor land management, there are no strict policy guidelines enforcing replanting of trees in the degraded landscapes. Hence, land degradation continues unabated resulting in loss of soil and soil organic carbon and ultimately deterioration the soil quality and poor agricultural performances.
Exclosure land use type has shown an improvement in soil organic carbon against the findings by Mekuria et al. (2014). The positive impacts of exclosure on selected soil properties (mainly on SOC and SMC) obtained in the present study witnessed that exclosure could be considered as an important practice in soil nutrient improvement. Thus, highly degraded grazing in the sloppy landscape should be designated as exclosure land management zone to improve soil organic matter (carbon) and other more sensitive soil physical and chemical properties. Besides improving the soil quality and productivity, such information is needed during the establishment of carbon projects and helps improve the livelihood of the local community through improved ecosystem services. Since the focus of this research was limited on few soil properties, further studies are needed to evaluate exclosure impacts on the other physical and chemical properties which are not considered in this study to have a complete understanding of the change as impacted by exclosure establishment.
Kebele is the lowest government administration structure in the country.
FY has substantially contributed in the design, drafting the manuscript, and critically revising the manuscript for important intellectual content. He has given also the final approval of the version to be published. GA has made substantial contributions in acquisition of data, data collection, entry, coding, and analysis, interpretation of results, writing and leading the overall activities of the research. AA has also been involved in sampling design, critically reviewing the manuscript and suggestions. All authors read and approved the final manuscript.
Fantaw Yimer (Ph.D.): Associate professor in soil science, Hawassa University, Wondo Genet College of Forestry and Natural Resources. He has published many articles in international journals. Email:firstname.lastname@example.org or email@example.com. Phone (Mobile): +251 (0) 911 340 986. Getachew Alemu: He is a Lecturer at Arsi University, Department of Natural Resource Management. Email: firstname.lastname@example.org; Phone (Mobile) + 251 (0) 910 264 249. Abdu Abdelkadir (PhD): Associate professor at Hawassa University, Wondo Genet College of Forestry and Natural Resources. He teaches and undertakes research on soil science and soil–water-plant relations. He has published many articles in international journals. Email: email@example.com; Phone (Mobile): +251 (0) 910 567 052
The authors are grateful to the Adama Science and Technology University, School of Agriculture for the financial support (Code: 6223). We thank also the two anonymous reviewers for their constructive comments.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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