Background

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.

Methods

Description of the study area

The study was conducted in Dehra Kebele Footnote 1, Dodota Sire District, Arsi Zone, which is about 125 km southeast of Addis Ababa and 50 km North of Assela, the Zonal capital (Fig. 1). Geographically, it is located between 80 16′N–8° 20′ 31″N latitude and 39° 19′E–39° 20′ 47″E longitude, lying between 1635 and 1795 m a.s.l. altitudes, and covering a total area of about 2436 hectares (OPEDB 2000) with a gently sloping and hilly landscapes.

Fig. 1
figure 1

Location map of the study area

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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.

Soil sampling

The elevation range (1635–1795 m a.s.l.) was divided into four blocks with 40 meters contour vertical intervals to control the variability in soil properties due to micro-topographic differences. Then the presence of the two land use types (open grazing land and exclosure) were checked followed by locating sample plots in each land use types. In each elevation range (block) and land use types three sample plots (20 m × 20 m) were established randomly and five small soil pits (from four corners and at the center) were opened in an ‘X’ design. From each small and shallow pit, soil samples from two fixed depths (0–10 and 10–20 cm) were collected and samples from the same depth were thoroughly mixed in a large bucket to form composite soil samples for the plot (Fig. 2).

Fig. 2
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Soil sampling layout/sketch at exclosure (a) and grazing (b) land use types

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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.

Soil analysis

Soil particle sizes for the <2 mm fractions were determined following the Boycouos hydrometric method (Bouyoucos 1962) after destroying organic matter using hydrogen peroxide (H2O2) and dispersing the soils with sodium hexameta phosphate (NaPO3). The soil bulk density (Bd) was determined using the core method and calculated as the mass of oven-dried soil (105 °C) divided by its volume (Chen et al. 2010).

$$\rho {\text{s}}\left( {\frac{\text{g}}{{ {\text{cm}}3}}} \right) = \frac{\text{Ms}}{\text{Vb}}$$

where ρs = soil bulk density (g cm−3), Ms = mass of soil after oven dry (g), Vb = bulk volume of the soil (cm−3)

The gravimetric soil moisture content (SMC, %) was determined following the method described by (Cuenca 1989). Before the soil was oven dried, the initial weights were measured followed by oven drying for 24 h at 105 °C, and weighing the oven dried soil. Gravimetric soil moisture content was determined using the following formula.

$${\text{MC}}\left( {\% } \right) = \frac{{{\text{Wwet}} - {\text{Wdry}}}}{\text{Wdry}} \times 100$$

where MC = soil water content on mass basis (%), Wwet = the weight of the wet soil sample (g), and Wdry = the weight of the dried soil sample (g)

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 (%)

The soil particle size fraction (%) of sand, silt and clay did not significantly varied with land use types and soil depths (Table 1). The overall mean sand fraction was found to be high (over 70 %) in both land use types and depths (Table 2). The silt and clay particle size fractions were found to be slightly higher in the exclosure (20.0 ± 0.52, 8.2 ± 0.35, respectively), than in the grazing land (19.4 ± 0.52, 7.50 ± 0.33, respectively) (Table 2).

Table 1 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
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Table 2 Soil textural fractions (%) in relation to the land use types and soil depths (mean ± SE)
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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 (%)

Both soil bulk density and SMC did not vary with land use types. However, variation in soil moisture content was significant with soil depth (p < 0.0001, Table 1): higher (4.77 ± 0.19) in the subsurface soil layer (10–20) than in the top surface soil (Table 3).

Table 3 Soil bulk density and soil moisture content in relation to the land use types and soil depths (mean ± SE)
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The higher soil bulk density under grazing land use is attributed to the lower SOC and the effect of soil compaction due to livestock trampling. The correlation also showed a strong association with SOC contents (Fig. 3). Bewket and Stroosnijder (2003) also indicated higher soil bulk density under open grazing land than exclosure.

Fig. 3
figure 3

Relationships between mean SOC and Bd at exclosure (a) and grazing land (b) land use

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Although variation was not significant, soil moisture content was found to be higher under exclosure than in open grazing land use due to higher soil organic carbon and decreased soil bulk density (Fig. 4). This is consistent with the study by Kevin et al. (2011) that higher organic carbon increases the soil moisture content through improvements in soil structure. The soil moisture content also showed significant difference with soil depths; higher in subsurface soil under due to the relatively higher fine particle fractions (silt + clay) in the subsurface soil giving a better moisture holding capacity. In addition, the presence of less evaporation from the sub-soil coupled with increased downward water movement through gravity could have contributed to the increased amounts of soil moisture with depth. Fua (2004) also indicated that the sub-surface soil had higher soil moisture content as compared with the layer above.

Fig. 4
figure 4

Relationships between mean SOC and SMC at exclosure (a) and grazing land (b) land

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Soil chemical property variations

Soil pH (H2O)

No significant variation was observed in soil pH with land use types (p = 0.400) and soil depth (p = 0.305, Table 4). Though not significant, soil pH was slightly higher in the exclosure (7.16) than in the grazing land (7.08, Table 5). The absence of significant variation in soil pH with land use types might be due to less leaching of base cations. According to Mekuria et al. (2007), exclosures and grazing lands showed no significant variation in soil pH. However, the overall mean soil pH was slightly higher in the exclosure due to the effects of organic matter that trap base cations as compared to the grazing land. Therefore, organic matter accumulation might reduce soil erosion resulting in higher soluble base cations (Ca2+ and Mg2+) that reduce H+ responsible for acidity, which in turn increases soil pH in the soil. In the study, the overall mean pH value (7.07–7.17) is within the preferred range (6.6–7.3) (Landon 1991), indicating that there are more base cations rather than aluminum, manganese and hydrogen. In addition, the neutral nature of soil of the study area could be attributed to the low rainfall which is inadequate to remove basic cations out of the surface horizons of the soils.

Table 4 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
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Table 5 Soil pH, soil organic carbon, total nitrogen and carbon to nitrogen ratios in relation to the land use types (mean ± SE)
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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).

Conclusions

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.