Exclosure effects on soil physicochemical properties and woody species diversity in the south Rift valley basin of Ethiopia

Establishing exclosures has become common rehabilitation and restoration of degraded lands in Ethiopia. This study examined the effects of exclosure on identified soil physical and chemical properties, and woody diversity at the Wamole sub-watershed. Representative soil samples were taken from the open grazing land and eight-year-old exclosure. Six transects and 18 plots, with an area of 20 m × 20 m and containing nine from each open grazing ground and nine from the exclosure, were constructed alongside to sample the vegetation. From every plot, by ‘X’ design, composite soil samples were collected for investigation of total nitrogen (TN), available phosphorus, cation exchange capacity (CEC), soil pH and soil organic carbon content (SOC), and the bulk density (BD) of the soil, 18 undisturbed soil samples were taken from 0 to 20 cm deep. 34 and 28 woody species belonging to 25 and 20 families were registered between exclosure and adjacent open grazing land respectively. Significantly (P < 0.05) higher Index Shannon-Wiener (3.36) in the exclosure indicated better species diversity in the exclosure than in the open grazing land (3.13). Soil properties such as available phosphorus, pH, OC, TN, and CEC showed significant differences (p < 0.05) across different land uses. Exclosures facilitated to reclaim of degraded lands by re-establishing vegetation and improving soil nutrient status in a comparatively short period. It suggests that further research on socio-economic aspects of exclosures has to bring livelihood improvement in the locality to establish additional degraded open grazing lands in the research sub-watershed.


Introduction
It is commonly acknowledged that land degradation is a severe global issue, especially in developing countries [1]; influencing the economy of the lower-and middle classes [2].About 3.2 billion human population are impacted by land deterioration, which affects around 29 percent of the global land surface [3].Soil erosion, which accounts for about 80% of the present global deterioration of farming land [4].
The key soil depletion processes include accelerated soil erosion, decreased soil organic carbon, reduction of biodiversity, soil fertility minimization, acidification, and salinization [5].Water erosion, the main cause of soil degradation, predominantly affects cropland and has yearly soil loss rates the productive surface soil is lost at about 42 t/ha on cultivated fields, yet it can be as high as 300 t/ha in separate farms [6].It had a negative influence on the ecosystem because it significantly reduced crop yield, slowed vegetation growth, decreased soil depth, eroded the top fertile soil, and depleted the soil of a significant amount of clay and humus [7].
Restoration initiatives were undertaken in the 1970s with an emphasis on the rapidly degrading highlands of Ethiopia to address the country's problem with soil degradation [8].In this context, the exclosure initiative has emerged as a fruitful endeavor in several Ethiopian regions, including Tigray, Wello, Shewa, and Southern Ethiopia [9,10].
Due to overpopulation, agriculture has been expanding, especially on steeper slopes, which has intensified soil erosion and land degradation [11,12].Reforestation of degraded areas is frequently viewed as the most efficient rehabilitation strategy to reduce the issue owing to soil degradation and loss of biodiversity, in Sub-Saharan African countries including Ethiopia [13].
Exclosures are land management practices to recover the ecological situations of degraded land [14,15], and additional farming practices are firmly prohibited to encourage the natural regeneration of degraded lands [16].It increases landscape productivity and income-generating activities like livestock fattening [17], and it is thought to significantly contribute to soil fertility restoration, nutrient loss, loss restriction, and soil erosion reduction [18,19].
Exclosure of communal lands has been shown in various studies to significantly increase the diversity of wood species [20][21][22].Increased levels of diversity were achieved in exclosure compared to the surrounding open grazing area [20,22,23].Currently, exclosure is a popular and effective method for restoring degraded lands, particularly in the Wamole sub-watershed, Rift Valley Basin of Ethiopia.When the community takes an active part while receiving technical assistance from the local government, exclosure administration and safeguarding are effective [11,24].
Recovering degraded lands can be a useful strategy for enhancing the composition of the flora, storing carbon in the soil and vegetation, and enhancing the hydrological cycles and microclimate [25][26][27].The main advantages of exclosures were an increase in plant cover, and biodiversity [20,28,29] and an improved quantity of carbon in the biosphere [25,[30][31][32].
Similarly, among other soil conservation practices, exclosures in the Wamole sub-watershed have been widely used to reverse degraded land to productive because of their ability to trap deposits, tolerate damage, and support the regeneration of disappeared flora and fauna.The understanding of the changes brought in soil properties and vegetation with the application of exclosure practices in the southern Rift Valley areas, particularly in the study sub-watershed is scanty.Hence, this study was initiated to investigate the influence of exclosure on soil physical and chemical properties and woody diversity in the Wamole sub-watershed, Rift Valley Basin.

Description of study area
This investigation was carried out at the Wamole sub-watershed, which is located in the Great Rift Valley, Shebedino Woreda, Sidama region, and south of Hawassa, capital city of Sidama regional state, and apart at a distance of 21.4 km.Hawassa, the distance from Addis Ababa, the capital city of Ethiopia, to the Wamole subwatershed district is 288 km.It receives an annual rainfall that ranges between 900 to 1500 mm, with a mean annual temperature between 16 and 25 °C.The study site was located at 6°49′30″ to 6°54′30″ latitude and 38°24′ 26″ to 38°27′15″ E longitude (figure 1).The sub-watershed was situated 1840 meters above sea level; its area is 2490 hectares (ha).

Sampling strategy of vegetation data
One exclosure was specifically chosen for this study because it was an 8-year-old exclosure that was next to open grazing land, warranting that the soil and topography situations were as similar as feasible, the open grazing land was used as a control and for comparative.To gather information about the vegetation, systematic sampling was performed to study the woody vegetation [39].
Depending on the site circumstances in an east-west orientation, plots were built along each of the parallel line transects that were randomly formed on the nearby open grazing pasture and the exclosure.The number of transects at each site was determined by the density, spatial variability, and size of the site's vegetation [20].Each transect was built with three slope locations (top, medium, and bottom slope), with sampling plots of 20 m × 20 m size spaced out at intervals of 50 m.Of these, nine were from the exclosure, and nine were from open grazing lands close to the exclosure site.According to their local and scientific names, as well as helpful nomenclatures of woody species, useful trees, and useful shrubs, the number of distinct wood species in each of the quadrants was identified and recorded [40].

Vegetation data analysis 2.3.1. Species diversity
The Shannon-Wiener index (H) and Simpson's Diversity Index were used to calculate the species diversities, richness, and evenness in exclosure and open grazing lands [41].The Simpson's Diversity Index was established by Simpson [42] and is explained in equation (1).
Where D is the Simpson's diversity index ranges from 0 to 1; ni refers to individual woody species in the two land use systems, N is the total number of woody species found in exclosure and open grazing land.The Shannon-Weiner index is calculated using the following equation (2): Where H refers to the Shannon-Wiener index, R refers to the number of species, and Ci is the fraction of entities of the ith species expressed as a proportion of a total cover in the sample.Shannon diversity can range from zero for communities with just one kind to a considerable amount for communities with several species, each with a small number of individuals.Indicators of Shannon variety include strong > 3.0, medium (2.0-3.0),low (1.0-2.0), and very low (<1.0)[43].
The Taramessa restoration site and open grazing land sample plots were used to quantify the plant variety, richness, evenness, and abundance at five different locations within each site.All woody species smaller than 2 cm were counted and identified in each plot.

Species evenness
Species evenness (J), often known as a diversity index in mathematics, is a measure of biodiversity that expresses how numerically equal the community is, and calculated as equation (3): refers to species evenness, H refers Shannon index, and H max refers to the total quantity of species observed.

Species richness
Species richness is the quantity of species present in an ecological community or area(D), the species richness was estimated following the standard procedures outlined by Whittaker [44] in equation (4).Where D refers richness of species, S stands for the number of various species that were identified in the sample, and N stands for the number of distinct species in the sample as a whole.
In equation (5), Sorensen's Similarity Index (SC) was used to calculate how similar the composition of woody species was in the enclosure and the enclosure surrounding open grazing areas [45].The term 'c' refers to a number of species that are present in the second land use but not in the first land use.

Soil sample data collection and analysis
Exclosure established by government and non-government organizations 8 years and above for rehabilitatedegraded land and open grazing land on the adjacent side of sub-watershed.The sampling sites were selected based on their landscape position (upper slope, middle slope, and foot slope).A 10 m × 10 m plot's top 20 cm depth soil was sampled using an 'X' sampling design, with sharp-edged and closed circular augers manually pushed down the soil profile from the three sites' upper slope, middle slope, and bottom slope [46].There were 18 composite soil samples obtained in all (2 types of land use, 3 slopes, and 3 replications) (figure 2).To determine bulk densities, undisturbed soil samples from both land uses were also collected using a core sampler.
Standard soil analysis procedures were used to examine the soil's physical and chemical characteristics at the soil labs of Hawassa University, the College of Agriculture, and the Southern Region Agricultural [47].Following established protocols, the material was tagged, thoroughly mixed, air-dried, crushed, sieved with a 2 mm sieve, and kept for laboratory examination.
The hydrometer methodology was used to determine the soil texture [48].Soil samples were sieved using a 0.5 mm sieve to analyze the total nitrogen amount of the soil.The pH of a supernatant suspension of 1:2.5 soils to water is determined using a pH meter [49].The Walkley and Black wet digestion technique [50] is used to measure soil organic carbon.By dividing the proportion of organic carbon value by 1.724 conversion factors, the amount of organic matter in soils is determined [47].The ammonium acetate method was used for assessing the Cation Exchange Capacity (CEC).Olsen's method and the Kjeldahl process were used to test available phosphorus and total nitrogen, respectively [51].Using the core technique, soil bulk density (Bd) was computed as the mass of oven-dried soil (105 °C) divided by its volume [52], as described in equation (6).
Where Bd refers soil bulk density (g/cm 3 ), Ma = mass of soil After oven dry (g), Vt = volume of the bulk soil (cm 3 ).
The bulk and particle densities were used to compute the soil's total porosity using equation (7), as stated in Sertsu and Bekele [47].

= -Ẃhere
B d denotes the soil's bulk density, f denotes its overall porosity, and P d denotes the particle density which is predicted to have taken on a mean value of 2.65 g cm −3 .

Data analysis
Following the general linear model (GLM) approach, soil data were organized to one-way ANOVA at a significance level of (P < 0.05) to evaluate the variation with different land use types on soils and vegetation.Using the least significance difference (LSD) test, the significance of the treatment means that were substantially different were divided.

Results and discussions
3.1.Plant diversity, richness, and evenness 3.1.1.Species composition There were a total of 28 plant species representing 20 families on open grazing land and 34 plant species representing 25 families in the exclosure, including trees and shrubs (table 1).The continual anthropogenic disturbances like tree cutting for fuel wood, severe browsing, and over-grazing, had a significant impact on the decline of the configuration of woody species in grazing lands [53].This is in line with other parts of Ethiopia, particularly in northern Ethiopia [22,23,54], southern Ethiopia [55], and northwestern Ethiopia [19].
The families with the greatest species diversity were Lauraceae (2), Asteraceae (2), Rutaceae (3), and Moraceae (1) in the open grazing land, compared to Rutaceae (4) and Moraceae (2) in the exclosure.. Additionally, three woody species in total were identified, 12 of which were common to both the exclosure and the open grazing land (table 2).In contrast, to open grazing land, this demonstrated that exclosure had richer trees and bushes.This is consistent with Yayneshet et al [23], which showed that exclosures had improved the arrangement, diversity, and quantity of flora and fauna compared with open grazing lands.In the exclosure and open grazing land, trees and shrubs established 73.5%, 32.3 and 64.2%, and 28.5% of the woody species, respectively (table 1).
Protective measures against illegal woodcutting and a ban on interfering with animals have been appropriately implemented in the exclosure.This administration may be raising the fraction of woodland species and permitting the growth of grain storage in exclosures [56].The data analysis findings demonstrated that exclosures could be successful in restoring deteriorated sites given suitable safeguards after interloping was taken properly [20].

Vegetation similarity
According to the calculated Sorensen, similarity score of 68.5%, there was a 31.5% divergence in the configuration of woody species between the two land use groups.As a result, it was thought to be the greatest representative of the pattern of species turnover across consecutive plant communities [45].Once more, the variation in species composition is attributable to the re-emergence of species that had previously disappeared over time from the nearby open grazing land.The degree of species compositional disruption between the two sites may be correlated with the degree of species resemblances and differences in the two land uses [20].

Woody species' diversity richness, and evenness
The Taramessa exclosure's woody vegetation and open grazing land both had their Shannon diversity indices assessed, as a result, the exclosure had higher diversity than the open grazing land.The species richness was greater in the exclosure than in open grazing land (table 2).This proved that more species were negatively impacted by interventions in open grazing land than in exclosure, but that when interferences are reduced, the species can be restored.
In the region under exclosure and open grazing land, the Shannon index values for all the woody species discovered were 3.36 and 3.13, respectively (table 2).The greater Shannon diversity indices in the exclosure imply that the exclosure has greater species richness than the adjacent open grazing land [57].The exclosure had a higher evenness (0.95) than the open grazing land, indicating the presence of fewer (12) but more equally distributed species (table 3).This study agreed with Mekuria et al [58], the evenness of the species that recovered spontaneously on the site suggests that restoration in the exclosure could be improved.The exclosure has a higher species richness, but the evenness is low due to the dominance of a few species.Exclosure practices enhanced soil moisture and fertility while enhancing the diversity and regeneration capabilities of woody species [59].
Plant species diversity restoration is a vital conservation strategy for restoring degraded that have lost canopy cover [60].When compared to open grazing areas, the exclosures had a higher composition, diversity, and density of woody species [61].Similarly, investigations on exclosures in Wello by Kibret [43] found that the implementation of exclosure improved the structure, density, and richness of woody species in comparison to nearby grazing sites.
Woody species were significantly more abundant in exclosures than in open grazing fields, emphasizing the relevance of exclosures in biological diversity management [20].The key beneficial changes noted after the establishment of exclosure were the recovery of grasses and woody, the management of biodiversity, and the decrease of soil erosion [41].
Similar to what has been documented in Ethiopia, exclosure has a wider array of woody plants and more diversity than surrounding open grazing fields [20,55,[62][63][64].According to Mengistu et al [20], the wood species variety discovered in exclosure is three times greater than that of open grazing areas after 22 years of exclosure formation.

Soil physicochemical properties 3.2.1. Soil physical properties 3.2.1.1. Soil texture
The most frequent way to describe a soil's physical makeup is by its texture, which has an effect on soil characteristics including nutrient levels and decomposition, aeration, infiltration rate, drainage, and/or permeability [65].
Sand showed a significant difference between open grazing land and exclosure (table 4).The open grazing land had greater average values of sand (78.2%), whereas the exclosure had lower mean values (62.9%).A similar result was reported by Umer and Sinore [66] who found higher sand content in the Wera Sub-watershed of southern Ethiopia.This is due to the discerning removal of clay fractions while departure coarse soils in the grazing field [67].
The exclosure had a greater clay concentration (26.4%) than open grazing land, which had a lower clay level (18.2%).The larger clay percentage in the enclosure showed that soil erosion in the area was relatively low, whereas the lower clay content in the open grazing areas revealed that soil erosion was substantially higher [68].
Table 4 showed that there was a significant difference in silt content between the exclosure and the open grazing land.In comparison to open grazing land, which had lower average silt levels (3.5%), exclosures had higher average silt values (10.7%).Nevertheless, sandy loam is the soil's particle size class in the two land use categories.Most notably the mismanagement of soil resources, sheet and rill erosions, and the absence of appropriate conservation measures, have resulted in changes in particle size distribution [69].However, it is generally believed that management measures do not quickly change the textural groups of soil [70].

Soil bulk density
Soil bulk density was one of the significant indicators used in this study to evaluate the condition and quality of soil in terms of its physical characteristics.It is an important physical attribute that changes as a result of disturbance or soil management activities [66].There was a significant difference in bulk densities between the open grazing land (1.16 g cm −3 ) and the exclosure (1.06 g/cm 3 ) fields (table 4).Reduced soil organic carbon levels and the effect of cattle trampling aided soil compaction are linked to increased soil bulk density under open grazing land [71].The association also revealed a robust relationship with OC concentrations [67], indicating that soil bulk density was higher in open grazing land than under exclosure.
This could be due to grazing compaction and erosion of the topsoil as a result of the absence of vegetation cover [70,71].Fine textured soils often have a lower bulk density than sandy soils because they are more porous [72].Bewket and Stroosnijder [67] showed lower soil bulk density attained in exclosure than open grazing land.A negative association was discovered among soil bulk density, organic carbon, and clay content (table 5).

Total porosity
The total porosity of the different land uses showed a significant difference (P < 0.05).The total porosity of exclosure is higher than open grazing land (table 4).This might be due to the comparatively low bulk density and high organic matter content [71].The outcome was also consistent with findings made by Gesesse et al [73], Challa et al [74], and Sirna and Hailu [75] who reported that higher porosity from the exclosure resulted from conservation techniques that decreased runoff.The highest total porosity reported for exclosure land could be attributed to a greater accumulation of fine particles, and organic matter content, favoring improving soil structural stability [76][77][78].

Soil pH
Soil pH was chosen as a chemical criterion for measuring the enhancements achieved due to the physical implementations since it decides numerous soil reactions.Nevertheless, the exclosure had the greatest average pH value (7.0), whereas open gazing land had the lowest (6.0) (table 6).
The lower average pH value on open grazing land might be attributable to cation loss due to erosion and leaching [70], the comparatively smaller base saturation, and the reduced quantity of organic matter (OM).According to Hazelton and Murphy [80], soils under exclosure and open grazing lands can be categorized as slightly acidic and moderately acidic respectively.This finding disagreed with Umer and Sinore [66]; Fantaw et al [71]; and Mekuria et al [79] who discovered no substantial differences in soil pH between exclosure and grazing fields.

Organic carbon
The results of the analysis of variance showed a difference in soil organic content (SOC) between exclosure and open grazing lands that were statistically significant (P < 0.05) (table 6).Organic carbon was higher in soil under the exclosure (2.03%) than in open grazing land (1.17%) respectively (table 6).The findings by Girmay et al [33], stated that exclosure had higher organic carbon levels than open grazing land use types.Similarly, the study conducted by Mekuria et al [79] in northern Ethiopia showed in the exclosure, there were knowingly greater organic carbon values than in adjacent grazing land.
This output is consistent with that of Fantaw et al [71], who reported that more biomass found the exclosure occasioned in greater litter input and consequently higher organic matter buildup in the soil.Correspondingly, Descheemaeker et al [11] found that exclosures improved water retention ability, increased soil depth, and improved soil organic matter content Contrarily, Mekuria et al [81] and Sinore and Doboch [82] reported that open grazing areas had much more soil organic carbon than exclosure, contradicting this finding.

Available phosphorous
The analysis of variance revealed significant changes in the soil's available phosphorous between exclosure and open grazing lands (table 6).For exclosure and open grazing land, respectively, the average available phosphorus levels were 17.4 and 12.7 (mg/kg).This variation might be recognized in the greater amounts of both OM and clay particles in exclosure than in open grazing land.Similar findings were made by Mekuria [22] and Bedada et al [70] who found that available phosphorus levels in the exclosure soils were frequently higher than in nearby grazing pastures.
Organic carbon was positively related to total nitrogen (r = 0.85 * ) and negatively related to available phosphorus and clay concentration (r = 0.88) & r = 0.75) (table 5).This accomplishment supported the findings of Hadda and Sur [83], who found that the amount of available phosphorus in the exclosure was higher than in the grazing land.Mekuria [22] suggested that eluvations and phosphorus surface transfer via erosion may be to blame for the low levels of available phosphorous content in grazing pastures.

Total nitrogen
Total nitrogen (TN) levels between exclosure and open grazing lands differed noticeably (table 6).Exclosure had the greatest mean (0.38%), whereas open grazing land had the lowest mean (0.15%)).This might be greater OM, the existence of leguminous plants to fix atmospheric nitrogen, and lower soil erosion rates [70,71].This output agreed with Mekuria et al [81], who investigated there was a significant variation in TN between two land uses grazing land and exclosure.This implied that the land management practices used in exclosure had contributed to the replenishment of soil nutrients.
Descheemaeker [84] discovered that TN in exclosure was better than grazing fields and degraded land in several parts of Ethiopia, moreover, Mekuria and Ayenekulu [34] in the Tigray region, showed there was greater TN than neighboring communal grazing land.Mekuria et al [85] on the other hand, found that communal grazing pasture had considerably greater total nitrogen than exclosures in the Nile Basin of Northern Ethiopia.
Pearson correlation results revealed a substantial positive link between soil organic carbon (table 5; r = 0.82).Carpenter et al [86] discovered growing the number of beans in exclosures recovered nitrogen, contributing to increased biomass buildup.Regular overgrazing removes organic matter and grasses from the soil, lowering the total nitrogen levels in the soils on grazing lands [87].6).This could be due to an exclosure accumulating high organic carbon and having a higher ability to hold cations, which would increase soil fertility.Moreover, it was reported that there was a strong relationship between CEC and OM [65].
Hailu et al [77] who investigated exclosure fields had much higher cation exchange capacity than neighboring open grazing lands, this could be due to the variations of clay and OM between the land use types.This finding is supported by Sinore and Doboch [82] and Kibret [43], who found that organic matter, in particular, improves soil CEC and that clay and organic colloids are related to soil CEC.This outcome is supported by Mekuria et al [81] assessment that the CEC was much higher in the enclosure than in the grazing field.The quantity and nature of clay, and the extent of OM present in the soil, had a significant effect on the CEC of soil [58,71,82].This finding is consistent with Kibret [43] finding that the presence of OM and clay improved the soil.
The commencement of exclosure on degraded land in the Wamole sub-watershed improved soil physicochemical properties by encouraging natural vegetation regeneration.Except for silt and sand fractions, the soil pH, total nitrogen, accessible phosphorus, soil organic carbon, cation exchange capacity, and clay content were all considerably higher in the exclosure.

Conclusion
The species richness which also higher in the exclosure than in open grazing land, the woody species found in the exclosure had a Shannon diversity value of 3.36 and 3.13 in open grazing land, respectively.The exclosure had higher species richness than the open grazing land.The exclosure had a substantially higher Index Shannon-Wiener than open grazing land, indicating greater species diversity.
Between different land uses, there were noticeable changes in the soil's chemical attributes such as total nitrogen, available phosphorus, pH, soil organic carbon, and cation exchange capacity.Exclosure showed higher sand particles, while open grazing land showed high values of clay, total porosity, and bulk density.It may be concluded that protecting degraded grazing areas from unmanaged human and domestic animal interaction for a longer period can positively influence soil qualities such as OC, TN, and available phosphorus.This study demonstrated that exclosures contributed to reclaiming deteriorated lands by recovering vegetation, decreasing erosion, and increasing soil nutrient levels in a relatively short period, better contributing to enhanced livelihoods of smallholders.This data is crucial for assessing the efficacy of exclosure restoration of degraded areas and supporting making sound judgments of policymakers.

Figure 1 .
Figure 1.Location map of the wamole sub-watershed.
's similarity index, a depicts a variety of plant species found in both land uses (exclosure and open grazing land); Species present in the first land use but lacking from the second is indicated by the number b;

Table 1 .
The variety of species and their families in the exclosure and open grazing land.

Table 2 .
Kinds of woody species configuration in the exclosure and open grazing land.

Table 4 .
Soil physical properties (mean and SEM) are influenced by various land uses (n = 18).

Table 3 .
Diversity indices analysis in taramessa exclosure and open grazing land.

Table 5 .
Pearson correlation of soil physical and chemical properties.

Table 6 .
Mean and SEM values of soil chemical properties for the two land uses (n = 18).The highest mean value of cation exchange capacity (27.7 cmol kg −1 ) was recorded in exclosure, however, the lowest mean value (21.28 cmol kg −1 ) was registered on open grazing land (table