Effects of enclosure management on carbon sequestration, soil properties and vegetation attributes in East African rangelands

Catena 159 (2017) 9–19

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Effects of enclosure management on carbon sequestration, soil properties and vegetation attributes in East African rangelands

MARK

Kenea Feyisaa, Sheleme Beyenea, Ayana Angassaa,e,⁎, Mohammed Y. Saidb, Jan de Leeuwd, Aster Abebea, Bekele Megersac a

College of Agriculture, Hawassa University, P.O. Box 05, Hawassa, Ethiopia International Livestock Research Institute (ILRI), P.O. Box 30709, 00100 Nairobi, Kenya c School of Veterinary Medicine, Hawassa University, P.O. Box 05, Hawassa, Ethiopia d World Agroforestry Centre (ICRAF), P.O. Box 30677, 00100 Nairobi, Kenya e Botswana University of Agriculture and Natural Resources, Department of Animal Science and Production, Private bag 0027, Gaborone, Botswana b

A R T I C L E I N F O

A B S T R A C T

Keywords: Soil carbon stock Total nitrogen stock Rangelands Herbaceous biomass Borana

The use of enclosures has globally gained popularity as an effective strategy to enhance soil carbon sequestration, but empirical evidence is lacking particularly in arid and semi-arid rangelands of Africa. This study addressed the effectiveness of long-term (15–37 years old) enclosures in enhancing soil carbon sequestration in a semi-arid rangeland of Southern Ethiopia. We tested for differences in soil properties and vegetation characteristics between enclosures and adjacent open-grazed areas, while accounting for effects of age of enclosures and soil depths. Three enclosures age categories (< 20, 20–30 and > 30 years) each paired with adjacent opengrazed areas were selected. We collected soil samples at three soil depths (0–5 cm, 5–15 cm and 15–30 cm), and vegetation attributes from 90 plots within 9 enclosures and adjacent open grazing sites. The results showed that soil properties did not differ significantly (P > 0.05) between the two management systems across the three soil depths. However, relatively higher soil organic carbon content and stock was recorded in the enclosures than open-grazed lands. We recorded an overall mean of soil organic carbon stock of 39.6 ± 3.5 Mg ha− 1 in enclosures of < 20 years old, 40.8 ± 3.4 Mg ha− 1 in enclosures of 20–30 years old and 51.0 ± 4.4 Mg ha− 1 in enclosures of > 30 years old. The soil organic carbon stock for the adjacent open-grazed areas ranged from 34.4 ± 2.5 to 47.9 ± 5.1 Mg ha− 1. The age of enclosures did not show any significant effect on soil organic carbon stocks. However, enclosure management had a significant (P ≤ 0.05) effect on vegetation attributes. We concluded that enclosure had a significant role in terms of soil carbon sequestration and adaptation to climate change.

1. Introduction

resources is a major problem in arid and semi-arid ecosystems with significant impact on the environment and livelihoods of the pastoral communities (Angassa, 2014; Oba et al., 2000). Previous studies (Bikila et al., 2016; Tessema et al., 2011) have shown that increased grazing pressure has resulted in the losses of soil carbon (C) and nitrogen (N). According to Piñeiro et al. (2010), the effect of grazing modifies the structure and function of ecosystems, affecting biomass and soil organic storage. Management practices such as community enclosures and rotational grazing can help to restore rangeland ecosystems (Nosetto et al., 2006). Similarly, others (Howden et al., 1991; Glenn et al., 1993; Walker and Steffen, 1993) have also indicated that rehabilitation of degraded rangelands by means of grazing exclusion is a key strategy in reducing the loss of carbon (C) from terrestrial ecosystem. Although several studies

Rangelands constitute the largest and most diverse land resources in the world (Reeder and Schuman, 2002), and hold great potential for carbon sequestration (Lal, 2004). According to Campbell et al. (2008), rangelands can store enormous amount of terrestrial carbon stocks both globally (36%) and in Africa (59%), and help to mitigate the impact of climate change (Neely et al., 2009). Sequestration of carbon in the soil system is essential for the improvement of soil quality, nutrient retention and water holding capacity to increase the net primary productivity for more carbon assimilation (Lal, 2015). Despite the potential of rangelands for carbon sequestration, heavy grazing pressure has contributed to the rapid losses of soil carbon (C) and nitrogen (N) (Reid et al., 2004). The degradation of rangeland

⁎

Corresponding author at: Botswana University of Agriculture and Natural Resources, Department of Animal Science and Production, Private bag 0027, Gaborone, Botswana. E-mail address: [email protected] (A. Angassa).

http://dx.doi.org/10.1016/j.catena.2017.08.002 Received 12 February 2017; Received in revised form 21 July 2017; Accepted 3 August 2017 0341-8162/ © 2017 Elsevier B.V. All rights reserved.

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The Borana rangelands are characterized by arid to semi-arid climate with most areas receiving between 238 mm and 896 mm annual precipitation, with a high coefficient of variability (18% to 69%) (Angassa and Oba, 2007). The rainfall in Borana is bimodal with the long rainy season occurs between March and May, while the short rainy season occurs between October and November. The major dry season is from December to February. The mean daily temperature in the Borana rangelands is 24 °C, with a mean maximum and minimum daily temperatures of 28 °C and 17 °C, respectively (Coppock, 1994). Soils in the Borana rangelands are developed from granitic and volcanic parent materials and their mixtures (Coppock, 1994). The upland soil is mainly red in color, while the bottom land soils are dominated by dark vertisols (Oba et al., 2000). Upland soils elsewhere in the Borana rangelands are well drained and usually have equitable proportion of sand (53%), clay (30%) and silts (17%) (Coppock, 1994). Vegetation of the study area is dominated by tropical savannah with varying proportions of perennial grasses and woody vegetation (Angassa and Oba, 2010). In the enclosures management, disturbance due to grazing and browsing was minimal. The vegetation within enclosures was dominated by tall grasses (Cenchrus ciliaris, Chrysopogon aucheri, Cynodon dactylon, Sporobolus, Digitaria milanjiana and Panicum repens) with moderately closed canopy of the upper storey trees (Acacia tortilis, Acacia seyal and Balanites aegyptiaca). On the other hand, the open-grazed areas adjacent to enclosures were unfenced and the impact of grazing was severe due to increased grazing pressure (Homann et al., 2008). In the past, the Borana pastoralists used the communal rangelands for seasonal grazing that involved livestock movements between the wet and dry seasons grazing (Coppock, 1994). Before the 1970s, DidaHara was an open perennial grassland, which was used for grazing during the rainy season. After the 1980s, range enclosures were established by the community following the development of perennial ponds that attracted permanent settlements. The creation of settlements had substantially affected the patterns of seasonal grazing to a yearround grazing (Angassa and Oba, 2010). The pastoral population in Dida-Hara area is settled in semi-permanent settlements locally known as Olla. The mean livestock holding of household was estimated at 12.63 cattle, 11.13 small ruminants (goat and sheep) and 2.38 camels (Solomon et al., 2007). Most of the communal rangelands are degraded due to the high stocking rate, which was estimated at 0.235 Tropical Livestock Units ha− 1(1 TLU ~ 250 kg) (Homann et al., 2008). Although the stocking density of enclosures was not regulated, overgrazing is not a threat as enclosures are seasonally grazed by calves only during the dry season and rested during the growth season (Angassa and Oba, 2010).

(Mekuria et al., 2007; Pei et al., 2008; Steffens et al., 2008; Su et al., 2005) have documented an increase in soil C accumulation in exclusions, others (Nosetto et al., 2006; Reeder and Schuman, 2002; Shrestha and Stahl, 2008) have reported no improvement in soil carbon. On the other hand, earlier studies (Mekuria et al., 2011; Mureithi et al., 2014; Verdoodt et al., 2009) have confirmed that soil properties and vegetation characteristics have improved following grazing exclusion. Generally, differences among various studies and regions are apparent because of variations in climate, soil properties and soil depth, landscape position, plant community composition and management practices (Derner and Schuman, 2007; McSherry and Ritchie, 2013). Similarly, the complexity of vegetation dynamics, land use practices and soil characteristics are additional inadequacies to fully understand the driving factors (Angassa et al., 2012). This suggests the need to investigate the effect of grazing management on soil carbon stocks. In East Africa, community enclosures are extensively practiced by pastoralists to conserve standing pasture for dry season grazing (Angassa and Oba, 2010; Oba et al., 2001; Verdoodt et al., 2010). Enclosures are defined as area of rangeland, which is enclosed by a fence with branches of thorny Acacia trees as well as traditional rules to protect vegetation from grazing and/or browsing with the exception of calves and sick animals (Coppock, 1994; Oba, 1998). In some cultures, such enclosed areas may be called “exclosures” (Aerts et al., 2009). According to Angassa and Oba (2010), calves are allowed grazing inside enclosures for 3 to 4 months depending on the length of the dry season. The use of communities' enclosures that are over 30 years old in Southern Ethiopia may improve restoration of rangeland vegetation with significant implication on the potential of carbon sequestration. However, this potential has rarely been investigated. Thus, understanding the potential of rangeland restoration for carbon sequestration may help to inform policies for diversification of local livelihood options through carbon finance. Previous studies (Angassa and Oba, 2010; Dalle et al., 2006; Oba et al., 2000) that have so far conducted in the rangelands of Borana in Souther Ethiopia only focused on the effects of grazing on vegetation ecology and range condition assessment. On the other hand, few studies (Belay and Kebede, 2010; Bikila et al., 2016; Yusuf et al., 2015) have reported the influence of land use and vegetation types on carbon stocks and these studies are limited and sites specific. Furthermore, these studies did not show the effects of enclosures management along age chronosequence on soil carbon stock. We expected that age of enclosures may reflect the spatial separation of carbon sequestration in terms of time of restoration of vegetation states. Therefore, the objectives of the study were: (i) to investigate the effects of management systems on soil properties, SOC, TN stocks and vegetation characteristics; (ii) to investigate how the age of enclosures influence SOC, TN dynamics, and vegetation characteristics. We hypothesized that (1) There is no difference in soil C and N, and herbaceous biomass between enclosure management and the adjacent open-grazed areas; (2) enclosure management would favor more accumulation of soil C and N, and herbaceous biomass than the adjacent open-grazed areas; (3) there is no difference between the older and younger ages of enclosures in soil nutrients and herbaceous biomass accumulation; (4) The older age of enclosures would accumulate more soil nutrients and herbaceous biomass than the younger enclosures.

2.2. Experimental design Before starting the actual field sampling, field survey and mapping of enclosures were carried out in Did-Hara between May and June 2012 with technical support from the International Livestock Research Institute (ILRI, Kenya). In the identification and mapping processes, knowledgeable community members, especially those managing the enclosures and experts working in the area were participated. A high resolution satellite from Google earth® and Geographic Position System (GPS) were used to delineate the boundary of enclosures following line features (fence). Hence, a total of 16 semi-private enclosures were identified and mapped (Fig. 2). Information collected during the preliminary field survey was used as a guide in selecting the study sites for the final data collection. Following the initial step, we selected 9 semi-private enclosures associated with settlements (Olla) using a systematic random sampling method. The selected enclosures ranged from 60 to 620 ha in size, while ages of the enclosures were between 15 and 37 years old at the time of data collection (May–June 2013). We grouped the enclosures into three age chronosequence: [ < 20 years old (younger), 20–30 years old

2. Materials and methods 2.1. Study area The study was carried out in Dida-Hara area (04°47.318′N and 038°20.017′E), which is located at about 30 km North-East of Yabello town in Borana rangelands, Southern Ethiopia. Dida-Hara covers an area of about 985 km2 (Fig. 1). The altitude of the Borana Plateau ranges from 1000 to 1500 m above sea level (m.a.s.l.) with a few peaks up to 2000 m (Coppock, 1994). Dida-Hara is located between an altitude range of 1200 and 1600 m.a.s.l. (Angassa and Oba, 2010). 10

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Fig. 1. Location of the study area (Did Hara) in Yabello district of Borana rangelands, southern Ethiopia.

were collected from similar depths using soil auger (Eijkelkamp, inside diameter of 7.6 cm) for bulk density determination (Aynekulu et al., 2011). A cumulative sampling plate was used as guide to prevent the collapse of the auger near the surface, and to aid full recovery of the mass soil samples (Fig. 3b). The same auger was used throughout soil sampling to avoid change in estimating the volume of soil. The mass of soil samples were collected in zip lock bags for further processing. Hence, a total of 270 composite soil samples and 270 mass of soil samples (for bulk density) were collected from the study sites of enclosure and open-grazed lands. Soil samples were analyzed at the Soil and Plant Laboratory, College of Agriculture, Hawassa University. In the laboratory, cumulative mass soil samples were air-dried and the sub-samples were oven-dried at 105 °C for about 48 h to a constant weight and used to determine the total oven-dried soil weight for each depth. The remaining air-dried mass soil samples were sieved to fine mass soil (< 2 mm) and coarse fragments (> 2 mm) and weighed separately. The composite soil samples were air-dried in the laboratory and sieved through a 2 mm screen prior to the analysis. Soil organic carbon (SOC) was determined using the dichromate oxidation method (Walkley and Black, 1934). Total Nitrogen was analyzed by Kjedhal method (Bremner and Mulvaney, 1982). The cation exchange capacity (CEC) was determined using the ammonium acetate method and soil pH in 1:2.5 (soil to water suspension) following Sertsu and Bekele (2000) using pH meter (UK made). The available phosphorus was determined by Mehlich-3 procedure (Mehlich, 1984) using inductively coupled spectrophotometer (ICP) at the Soil and Water Testing Laboratory of Horticoop Private Limited Company at Debrezeit, Ethiopia. The particle size was analyzed after dispersion in a mixer with hexametaphosphate to determine the amount of sand, silt and clay using the hydrometer method as described by Sertsu and Bekele (2000). Bulk density was determined from the

(medium) and > 30 years old (older)] each with three replicates (N = 3), whereas each enclosure age had a corresponding adjacent open-grazed site as control. Therefore, age in this paper is used as an independent variable whereas site served as replicates. These selected enclosures were distributed within 20 km radius in Dida-Hara area and all enclosures were selected from the uplands (> 1510 m.a.s.l.). The experimental design consisted of control-impact pairs across boundaries between enclosure and the open-grazed management intervention areas (Fig. 3a). At each paired site, a 500 m long line transect was established in the enclosures and adjacent open-grazed sites. We checked whether the selected paired sites had similar soils and landscape position. The transect position was chosen at about 20–30 m away from the fence border between enclosure and the adjacent opengrazed areas to avoid any edge effect. At each paired site, five plots of (30 m × 30 m) were established along each transect. The plots were well spaced (separated by at least 100 m) to account for spatial variability and avoid pseudo-replication (Fig. 3a). Ninety plots were used for field data collection from 9 enclosures and the control adjacent opengrazing sites. Data collection on soil and vegetation was carried out at the end of the main rainy season between May and June in 2013. 2.3. Soil sampling, analysis and estimating SOC stock We collected soil samples at three soil depths (0–5 cm, 5–15 cm and 15–30 cm) from 10 sub-plots (auger points) using auger (Eijkelkamp Agrisearch Netherlands Equipment BV) (Fig. 3b). The sub-plots were established in a zigzag sampling strategy with equal distance of 15 m (minimum) from each other (Fig. 3b) depending on the variability of soils and vegetation of the sampling sites. The collected soil samples from the sub-plots were mixed to form a composite sample for each depth and plot. In each plot of 30 m × 30 m, three mass soil samples 11

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Fig. 2. Map of enclosures and sampling sites in Dida Hara area in Yabello district of Borana rangelands, southern Ethiopia.

oven-dry of soil weight (g) divided soil volume (cm3) estimated from soil area of auger (Eijkelkamp, internal diameter 7.6 cm) and the thickness of the soil layer (cm) (Aynekulu et al., 2011). The SOC stock for each depth interval was calculated following Aynekulu et al. (2015) as: SOC stock (Mg ha− 1) = OC / 100 × BD (i) × (1 − frag) × D × 100, where OC is the soil organic carbon content (%) in fine soil (fraction < 2 mm) determined in the laboratory (%), BD = bulk density (i), frag = % volume of coarse fragment / 100, D = depth of the soil layer (cm) and 100 used to convert the unit to Mg ha− 1 (i) Bd (g cm− 3) = M/V, where M = the oven-dry weight of soil, (g). Here, auger (Eijkelkamp), internal diameter of 7.6 cm (radius 3.8 cm) was used to calculate soil volume as V (cm3) = π × (3.8)2 × depth of the soil layer (cm). The TN stock was calculated in an analogous to SOC stock by substituting the soil organic carbon content (% OC) by the total nitrogen content (%TN). The SOC and TN stocks to the depth of 30 cm were calculated as the sum of SOC and TN stock in each soil depth intervals (0–5, 5–15 and 15–30 cm).

which was hand-harvested from each sub-plot, was weighed for fresh weight immediately in the field and oven dried at 65 °C for 48 h to a constant weight for dry matter estimation. Samples from the three subplots were pooled together and reported as dry weight (g m2). The number of herbaceous species rooted in each sub-plot was counted and recorded to estimate the total number of species occurring per unit area (1 m2) (Oba et al., 2001). Herbaceous cover (grass and forbs) was estimated visually based on the area (soil part) covered by herbaceous base compared to bare ground in each quadrat. In each 10 m × 10 m quadrat, individual woody plant species (mature, saplings and seedlings) were identified and counted, and then multiplied 10,000 m2 area to estimate the number of stems per hectare (Angassa and Oba, 2010). 2.5. Data analysis Before data analysis, we tested our data for normality and homogeneity of tests. We also checked the data for outliers. The effects of management and age of enclosures across the soil depths on soil properties and nutrient stocks were analyzed using mixed model procedure (proc mixed) in SAS. The categorical fixed variables in the model for soil parameters included: [management (dummy variable: enclosure vs. open-grazed areas), age of enclosures (three categories)] at each soil depths (three levels) and to the 30 cm depth. For vegetation attributes, the model included effects of management along enclosures

2.4. Vegetation survey After establishing the 30 m × 30 m plot in paired enclosure and the adjacent open-grazed lands, samples for herbaceous biomass was determined by hand cutting to the ground level from three sub-plots of 0.5 m × 0.5 m quadrat size per plot (Fig. 3b). The herbaceous biomass, 12

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Fig. 3. (a) Sample plot distribution along the transect line in each enclosure and adjacent open-grazed site (see image picture of the paired site as demarcated by thorny Acacia tree), (b) sub-plots layout strategy used during soil and vegetation sampling. The sub-plots are not drawn to scale.

showed a significant difference (P < 0.05) between the older (> 30 years) and younger (< 20 years) ages of enclosures (Table 1). Our results showed a relatively higher available phosphorus (P) value in the medium and older ages of enclosures as compared to the adjacent open-grazed areas in the 0–5 cm depth of the soil (Table 1). The present results confirmed that grazing management had no significant (P > 0.05) effect on SOC content and SOC stock. However, enclosures showed a relatively higher SOC content (%) and SOC stock than the adjacent open-grazed areas. On the other hand, our results showed variability in terms of SOC content (%) and SOC stock for the different age categories of enclosures vs. adjacent open-grazed areas across the three depths of the soil (Table 1). There were an increase of SOC content from 8 to 10%, 11 to 30% and 13 to 17% in the younger, medium and older ages of enclosure, respectively as compared to the corresponding adjacent open-grazed areas across the three depths of the soil. There was greater variability between the medium age of enclosure and its adjacent open-grazed areas in terms of SOC content across the three depths of the soil. Similarly, SOC stock showed an increase from − 0.5 to 2.8, 1.2 to 2.9, and 0.3 to 1.4 Mg ha− 1, respectively in the younger, medium and older ages of enclosure management as compared to their adjacent open-grazed areas across the three depths of the soil (Table 1). The values for SOC stock showed both decline (− 3.8%) and increase (51%) in the younger age of enclosure management. However, there were an increase from 10.3 to 26.0%, and 3.2 to 7.6% in the medium and older age of enclosures management compared to their corresponding open-grazed areas across the three depths of the soil (Table 1). In the current study, we did not find any significant difference between the younger and medium age of enclosures in terms of TN content (%), while in the older age of enclosures (> 30 years) TN content (%) was increased by 29.0% as compared to the adjacent open-grazed areas across the three depths of the soil (Table 1). The TN stock values showed both decrease (− 6.7%) and increase (33.0%) in the younger age of enclosures. We also recorded an increase in TN stock from 4.5% to 33.0% and 12.0% to 25.0% in the medium and older ages of enclosure, respectively as compared to their adjacent open-grazed areas (Table 1). The effects of enclosure management vs. open-grazed areas across each and the combined soil depths on the average contents and storages of SOC and TN are presented in Fig. 4.1(a–d). Generally, enclosure

age categories. The sampling plot was included in the model as a random effect to control for repeated measures. Mean comparisons were made using Tukey's test (adjusted) at 0.05 probability level. All values were reported as mean ± standard error (SE). All statistical analyses were performed using SAS version 9.3 software package (SAS Institute, 2012). 3. Results 3.1. Effects of grazing management on soil properties The distribution of soil particle size fractions (sand, silt and clay) did not show any significant (P > 0.05) variation between the two management systems across the three depths of the soil. However, the proportions of silt and clay fractions were relatively higher in enclosures than adjacent open-grazed lands (Table 1). We also found that the soil textural fractions was not uniform along enclosure's age chronosequence as sand fraction was relatively higher in the medium age of enclosures than the younger and older ages of enclosures. On the other hand, there were a significantly higher clay and silt fractions in the older and younger ages of enclosures than the medium age of enclosures (Table 1). Overall, the proportion of sand content was dominant in terms of textural fractions under both management systems (Table 1). In the present study, soil bulk density did not show any significant (P > 0.05) difference between enclosures and open-grazed areas. However, the open-grazed areas had a relatively higher soil bulk density (1.1 to 1.5 g cm− 3) than enclosure management (0.90 to 1.4 g cm− 3) across the three depths of the soil. Generally, soil bulk density increased with the age of enclosures (Table 1). Soil pH values varied with the age chronosequence of enclosures vs. open-grazed areas. The medium age of enclosures showed a significantly higher (P < 0.05) pH value than adjacent open-grazed areas in the 5–15 cm and 15–30 cm depths of the soil layers (Table 1). Overall, the soil pH was relatively higher in enclosures than opengrazed areas across the three depths of the soil (Table 1). We also recorded a relatively higher CEC value in enclosure management than adjacent open-grazed areas. However, the CEC value showed a significant (P < 0.05) difference between the medium age of enclosure and its adjacent open-grazed areas. There was a linear relationship between the values for CEC and age of enclosures. The values for CEC 13

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Table 1 Comparisons of selected soil properties and nutrient stocks (mean ± standard error, N = 3) in the enclosure and adjacent open-grazed lands across three soil depths in Dida-Hara area of Borana rangelands, Southern Ethiopia. Soil parameters

Depth (cm)

Land management across enclosure age sequence < 20 years

20–30 years

Enclosure Sand (%)

Silt (%)

Clay (%)

BD (g cm− 3)

pH (H2O)

CEC (cmolc kg− 1)

AvP (mg kg− 1)

SOC content (%)

TN content (%)

C:N ratio

SOC stock (Mg ha− 1)

TN stock (Mg ha− 1)

0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30 0–5 5–15 15–30

Open-grazed a

44.1 ± 1.4 43.1 ± 1.3a 41.8 ± 1.4a 25.2 ± 1.6a 25.6 ± 1.5a 26.0 ± 1.5a 30.8 ± 1.1a 31.3 ± 1.0a 32.2 ± 0.9a 0.9 ± 0.1a 1.0 ± 0.1a 1.1 ± 0.1a 6.2 ± 0.1a 6.1 ± 0.1a 5.9 ± 0.1a 15.6 ± 0.6a 17.3 ± 0.6a 18.3 ± 0.8a 3.9 ± 0.3a 3.4 ± 0.2a 2.6 ± 0.2a 1.4 ± 0.1a 1.3 ± 0.1a 1.1 ± 0.1a 0.2 ± 0.0a 0.1 ± 0.0a 0.1 ± 0.0a 9.9 ± 1.1a 9.9 ± 1.1a 9.2 ± 1.1a 7.7 ± 0.0a 12.8 ± 1.1a 19.2 ± 2.0a 0.8 ± 1.0a 1.4 ± 1.1a 2.2 ± 0.1a

> 30 years

Enclosure

Open-grazed a

a

47.3 ± 1.4 45.4 ± 1.6a 42.6 ± 1.7a 19.3 ± 1.4a 19.9 ± 1.6a 21.9 ± 1.5a 33.5 ± 1.0a 34.7 ± 0.9a 35.7 ± 1.1a 1.1 ± 0.1a 1.2 ± 0.1a 1.4 ± 0.1 6.3 ± 0.2a 6.4 ± 0.2a 6.6 ± 0.2a 18.4 ± 1.4a 18.6 ± 0.8a 18.8 ± 0.7a 6.9 ± 2.0a 4.9 ± 1.2a 3.5 ± 0.5a 1.5 ± 0.1a 1.3 ± 0.1a 1.0 ± 0.1a 0.1 ± 0.0a 0.1 ± 0.0a 0.1 ± 0.0a 10.3 ± 0.9a 9.5 ± 0.8a 9.0 ± 1.1a 7.5 ± 1.0a 14.1 ± 1.1a 19.2 ± 2.0a 0.8 ± 0.1a 1.6 ± 0.1a 2.3 ± 0.2a

44.7 ± 1.3 44.2 ± 1.3a 44.6 ± 1.8a 24.9 ± 1.6a 24.3 ± 1.4a 24.4 ± 2.0a 30.4 ± 1.1a 31.5 ± 1.2a 31.5 ± 1.2a 1.1 ± 0.1a 1.1 ± 0.1a 1.2 ± 0.1a 5.9 ± 0.1a 5.9 ± 0.1a 5.9 ± 0.1a 15.5 ± 0.7a 16.3 ± 1.0a 17.8 ± 1.1a 4.4 ± 0.3a 3.1 ± 0.2a 2.8 ± 0.2a 1.3 ± 0.1a 1.2 ± 0.1a 1.0 ± 0.1a 0.2 ± 0.0a 0.1 ± 0.0a 0.1 ± 0.0a 9.1 ± 0.8a 9.6 ± 1.0.0a 9.0 ± 1.2a 5.1 ± 0.5a 13.3 ± 1.7a 16.4 ± 1.1a 0.6 ± 0.1a 1.5 ± 0.2a 2.1 ± 0.2a

a

49.3 ± 2.2 47.5 ± 2.4a 46.2 ± 2.2a 19.8 ± 1.6a 20.9 ± 1.9a 21.0 ± 1.8a 30.9 ± 1.2a 31.6 ± 1.2a 32.8 ± 0.8b 1.0 ± 0.1a 1.2 ± 0.1a 1.5 ± 0.1a 6.2 ± 0.2a 6.0 ± 0.2b 6.2 ± 0.2b 14.4 ± 0.7b 14.3 ± 0.1.2b 12.7 ± 1.1b 5.7 ± 0.6a 3.1 ± 0.2a 2.8 ± 0.2a 1.3 ± 0.1a 1.0 ± 0.1a 0.9 ± 0.1a 0.1 ± 0.0a 0.1 ± 0.0a 0.1 ± 0.0a 10.6 ± 0.7a 7.9 ± 0.6a 8.5 ± 0.8a 6.3 ± 0.8a 11.2 ± 1.2a 17.4 ± 2a 0.6 ± 0.1a 1.5 ± 0.2a 2.2 ± 0.1a

Enclosure

Open-grazed a

46.9 ± 1.7 44.9 ± 1.6a 42.9 ± 2.0a 18.1 ± 1.5a 20.9 ± 1.7a 21.3 ± 1.3a 35.0 ± 1.0a 34.2 ± 0.9a 35.8 ± 1.0a 1.2 ± 0.2a 1.1 ± 0.1a 1.3 ± 0.1a 6.2 ± 0.1a 6.2 ± 0.1a 6.1 ± 0.1a 22.6 ± 2.9a 21.6 ± 0.6a 23.0 ± 2.7a 5.4 ± 1.0a 3.9 ± 1.1a 2.7 ± 0.4a 1.7 ± 0.2a 1.6 ± 0.1a 1.4 ± 0.1a 0.2 ± 0.0a 0.2 ± 0.0a 0.2 ± 0.0a 9.4 ± 0.6a 8.8 ± 0.6a 8.9 ± 0.9a 9.7 ± 1.6a 16.9 ± 1.6a 24.3 ± 2.0a 1.1 ± 0.1a a 2.0 ± 0.2a 2.8 ± 0.2a

45.5 ± 2.4a 42.5 ± 2.1a 43.4 ± 2.3a 20.7 ± 2.1a 21.7 ± 2.1a 22.7 ± 2.1a 33.8 ± 1.0a 35.9 ± 0.7a 33.9 ± 0.8a 1.3 ± 0.2a 1.1 ± 0.1a 1.3 ± 0.1a 5.9 ± 0.1a 5.9 ± 0.1a 5.8 ± 0.1a 19.8 ± 1.1a 21.7 ± 0.7a 22.9 ± 1.3a 4.9 ± 0.3a 3.2 ± 0.3a 3.0 ± 0.5a 1.5 ± 0.2a 1.4 ± 0.1a 1.2 ± 0.1a 0.2 ± 0.0a 0.2 ± 0.0a 0.1 ± 0.0b 9.4 ± 0.9a 9.8 ± 1.0a 9.7 ± 1.4a 9.4 ± 1.5a 15.7 ± 1.7a 22.9 ± 2.5a 0.9 ± 0.1a 1.6 ± 0.1b 2.5 ± 0.2a

Note: BD - Bulk density, CEC - Cation Exchange Capacity, AvP - Available Phosphorus, SOC - Soil Organic Carbon, TN - Total Nitrogen, C:N - Carbon to Nitrogen ratio, n = 3 - replication of management systems. Means ( ± SE) followed by the same letters are not statistically different at P < 05 between enclosure and the adjacent open-grazed lands, while means ( ± SE) in bold showed significant difference between enclosure age categories, after Tukey's test (adjusted) at P < 0.05.

relatively greater SOC and TN contents and stocks when compared to their corresponding adjacent open-grazed areas (Fig. 4.2(a–d)). Hence, SOC content in the enclosure management had increased by 8.3% (in the younger age), 9.1% (medium age), and 14.3% (older age) as compared to the adjacent open-grazed areas. The result showed a relatively higher SOC content with 14.3% times increase in the older age of enclosures (> 30 years) than its adjacent open-grazed areas (Fig. 4.2(a)). Overall, enclosures had accumulated more SOC content than the adjacent open-grazed areas. Similarly, the results also showed that enclosures had more SOC stocks than the adjacent open-grazed areas. On the other hand, relatively more accumulation of SOC stock was recorded in the medium and younger age of enclosures than their adjacent open-grazed areas when compared with the older age of enclosures categories (Fig. 4.2(b)). When accounted for the age effects, the older age of enclosure had accumulated more SOC stock than the younger and medium ages of enclosures, although significant difference was observed only between the older and younger ages of enclosures. The mean SOC sequestration rates along the age of enclosures were 0.2 Mg ha− 1 yr− 1 for the age range between 16 and 22 years old (younger to medium age), 0.6 Mg ha− 1 yr− 1 between the of 22 and 35 years old (medium to older age), and 0.8 Mg ha− 1 yr− 1 between the age of 16 and 35 years old (younger to older age) (Table 1). We found no significant difference for the proportion of TN content

management had a higher SOC content across the three depths of the soil than the open-grazed areas (Fig. 4.1(a–d)). There was a relatively higher increase (16.7%) in terms of SOC content for enclosures than open-grazed areas in the 5–15 cm (Fig. 4.1(a)). Likewise, enclosure had 1.3, 1.2 and 2.0 Mg ha− 1 more SOC stock accumulation in the 0–5 cm, 5–15 cm and 15–30 cm, respectively than the adjacent open-grazed areas. Overall, there was an increase by 9.0 to 18.6% for SOC stock in enclosures as compared to the adjacent open-grazed areas (Fig. 4.1(b)). On average, enclosures had higher accumulation of SOC sock (43.8 ± 2.3 Mg ha− 1) than the open-grazed areas (39.2 ± 0.2 Mg ha− 1) in the 0–30 cm depth intervals. Total N content (%) showed a similar trend with SOC content as there was more accumulation of N content (%) and 16.7% times increased in enclosures than open-grazed areas in the 15–30 cm (Fig. 4.1(c)). Furthermore, there was higher accumulation of TN stock for enclosures (0.2 Mg ha− 1, i.e., 31.4% times increased) than opengrazed areas in the 0–5 cm (Fig. 4.1(d)). On average, enclosure had accumulated a higher TN stock (5.0 ± 0.2 Mg ha− 1) than the adjacent open-grazed areas (4.5 ± 0.2 Mg ha− 1) in the 0–30 cm. The effects of enclosure along age chronosequence vs. adjacent open-grazed areas for the values of SOC and TN content sand stocks are presented in Fig. 4.2(a–d). In the 0–30 cm intervals of the soil, enclosures management along the age sequence had accumulated 14

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Fig. 4.1. Difference between enclosure and the adjacent open-grazed lands in: (a) soil organic carbon (SOC) content, (b), SOC stock, (c) Total nitrogen (TN) content and (d) TN stock. Means (N = 9, SE bars) followed by same letter within a given soil depth did not differ significantly (P < 0.05), after Tukey's test (adjusted). Error bars indicate standard error of the mean (SE). N-replication of paired enclosure and open grazed lands across all sites.

Fig. 4.2. Differences between enclosures and the adjacent open-grazed lands along the age sequence in: (a) soil organic carbon (SOC) content, (b) SOC stock, (c) Total nitrogen (TN) content and (d) TN stock in 30 cm soil depth. Means (N = 3, SE bars) followed by similar letters within a given enclosure age category did not differ significantly (P < 0.05), after Tukey's test (adjusted). Error bars indicate standard error of the mean (SE). N-replication of paired enclosure and open grazed lands across the age sequence.

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basal cover was increased with increasing age of enclosures with a significant variation between the older and younger age of enclosures (Table 2). The results of the current study showed no significant (P > 0.05) difference between enclosures and open-grazed areas in terms of woody density. However, woody density was varied along age chronosequence of enclosure. Generally, lower density of woody plants was recorded in the younger age of enclosures than in the adjacent open-grazed areas. On average, enclosures had relatively higher woody density (1309 stems ha− 1) than open-grazed areas (1238 number of stems ha− 1) (Table 2).

between the younger and medium ages of enclosures. Generally, TN content was higher for the older age of enclosures (20%) than the adjacent open-grazed areas for the full depth (0–30 cm) of the soil (Fig. 4.2(c)). On average, the older age of enclosures had accumulated more TN stock (0.9 Mg ha− 1 (18%)) than the adjacent open-grazed areas (Fig. 4.2(d)). The results showed that the accumulation of TN stock was higher for enclosures than the adjacent open-grazed areas in the 0–30 cm intervals of the soil. The results along age chronosequence of enclosures showed that the average annual TN sequestration rates were 0.02 Mg ha− 1 yr− 1 (for the younger age of enclosures), 0.01 Mg ha− 1 yr− 1 (for the medium age of enclosures), and 0.03 Mg ha− 1 yr− 1 (for the older ages of enclosures) in the 0–30 cm intervals. When accounted for the age effect, the older age of enclosure had accumulated more TN stock than the younger and medium ages of enclosures with a significant difference between the older and younger ages of enclosures. In the present study, we also found a higher C:N in enclosures management than in the open-grazed areas. Generally, there was a variation in C:N along the age chronosequence of enclosures and soil depths (Table 1).

4. Discussions 4.1. Effects of grazing management on soil properties The lack of significant difference between enclosures and opengrazed areas in terms of soil particle distribution confirms that soil particle size distribution and soil texture are inherent characteristics of the soil that cannot be modified by grazing management practices. Angassa et al. (2012) have also reported the lack of significant difference between grazed and ungrazed areas in terms of soil particle size distribution, suggesting that grazing management might have minimal effect on soil texture. Although particle size fractions are inherent soil properties, their variation along the age chronosequence of enclosures could be associated with differences in management practices by different users at the settlement level. Furthermore, the higher sand fraction, low silt and clay contents in the open-grazed areas might have resulted from the process of accelerated soil erosion that facilitated by the poor ground cover (Oba et al., 2000). Generally, the results of the current finding in terms of sand proportion, silt and clay contents, and sandy-clay loam textural classes agree with the report by Coppock (1994), who indicated that most parts of the Borana rangelands are characterized by sandy and shallow soils (53% sand, 17% silt and 30% clay). The average bulk density of the soil in the study sites did not show any significant difference between enclosures and open-grazed areas. However, a slightly higher soil bulk density in the open-grazed areas could be attributed to the effect of soil compaction as a result of continues grazing. Similarly, Mureithi et al. (2014) recorded a significantly lower soil bulk density in enclosures than in the open-grazing lands in Baringo district of Kenya. The observed increase in soil bulk density with the advanced age of enclosures could be associated with the level of management practices by the individual settlements where the enclosures be affiliated with. A study conducted by Yusuf et al. (2015) in Borana rangelands of Southern Ethiopia has also indicated variation in soil bulk density across different intensity of bush encroachment. A slightly higher pH in enclosures than in the adjacent open-grazed areas might be attributed to the effect more soil organic carbon within

3.2. Effects of grazing management on vegetation characteristics The results showed that enclosures management had more accumulation of the above-ground herbaceous biomass, species richness, and herbaceous basal cover than the open-grazed areas. Our results also showed that enclosure had higher herbaceous biomass (difference ± SE: 71.8 ± 4.8 g m− 2) than the open-grazed areas. However, the results varied along age chronosequence of enclosures. Generally, herbaceous biomass showed an increase of 2.7 times in the younger age of enclosures, 0.9 times in the medium age of enclosures and 2 times in the older age of enclosures as compared to their corresponding adjacent open-grazed areas (Table 2). In terms of age effect, the older age of enclosures had 30.0 g m− 2 (28.5% times increase) more herbaceous biomass accumulation than the medium and younger ages of enclosures. The results of the current study showed that enclosures had significantly higher (P < 0.05) species richness than the open-grazed areas. Age wise, the results showed that the younger age of enclosures had a significantly higher number of species richness than the adjacent open-grazed areas. Generally, our results showed a non-linear relationship between herbaceous species richness and age of enclosures, which was initially increased and then showed a decline with the advanced age of enclosures (Table 2). Likewise, enclosures had more herbaceous cover (difference ± SE: 20.9 ± 3.4%) than the opengrazed areas. On the other hand, our results showed variability in terms of herbaceous basal cover along age chronosequence of enclosures where basal cover was significantly higher in the older age of enclosure (28.0%) than in the adjacent open-grazed areas. Overall, herbaceous

Table 2 Comparisons of means ( ± standard error, N = 3) of vegetation characteristics in enclosure and adjacent open-grazed lands along the age chronosequence in Dida-Hara area of Borana rangelands, Southern Ethiopia. Enclosures' age group

< 20 years 20–30 years > 30 years Overall

Management systems

Enclosure Open-grazed Enclosure Open-grazed Enclosure Open-grazed Enclosure Open-grazed

N

3 3 3 9 9

Vegetation attributes Herbaceous biomass (g m− 2)

Herbaceous richness (number of species m− 2

105.2 (11.4)a 28.6 (6.4)b 105.4 (19.7)a 56.1 (8.9)b 135.5 (24.7)a 43.6 (4.81)b 115.4 (11.2)a 43.6 (4.8)b

5.0 3.0 5.0 4.0 5.0 4.0 5.0 3.0

(0.2)a (0.5)b (0.4) (0.5) (0.17) (0.5) (0.15)a (0.30)b

Basal cover (%)

Woody density (stems ha− 1)

25.04 (5.2)a 7.5 (2.23)b 31.5 (5.0)a 14.8 (3.3)b 45.0 (4.98)a 16.7 (3.13)b 33.9 (3.11)a 13.0 (1.75)b

1167 1573 1307 1067 1453 1073 1309 1238

(174) (222) (282) (166) (393) (90) (169) (102)

Means ( ± SE) followed by different superscript letters are significantly different between enclosure and open-grazed lands, after Tukey's test (adjusted) at P < 0.05. N - Replication of management systems.

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species composition in influencing the distribution of soil organic carbon. In review of the vertical distribution of SOC, Jobbágy and Jackson (2000) have argued that loses of SOC is more sensitive in the uppermost of the soils due to the impact of land management systems. Furthermore, Fultz et al. (2013) have also highlighted that the depth interval as small as 0–5 cm has a very important implication in terms of capturing the very minimal changes in SOC and TN due to the impacts of management and land use changes. Hence, our sampling up to depth of 30 cm is recommended to detect for any significant changes in SOC that might occur under the existing grazing management practices. However, our results did not support this argument as the difference for SOC and TN contents and stocks between enclosures and open-grazed areas was non-significant across the three depths of the soil. In spite of this, we observed differences between enclosures and open-grazed areas in terms of SOC and TN stocks in the upper depth of 0–5 cm, and SOC and TN contents in the lower depth of 15–30 cm. Hence, our results for SOC and TN stocks agree with the findings of Shrestha and Stahl (2008), who found a highly significant change in terms of SOC accumulation in the 0–5 cm depth after 40 years of grazing exclusion in a semi-arid sagebrush steppe of Wyoming. Jobbágy and Jackson (2000) have also stated that the vertical distribution of SOC in different ecosystems might be synchronized by the type of vegetation through the root-shoot allocations along the soil profile. The different ages of enclosures management could play a significant role in regulating ecosystem services within rangelands through enhancement of vegetation restoration, soil erosion control and carbon storage (Mekuria, 2013). However, our results in terms of the rate of SOC sequestration indicate a declining trend as the age of enclosures advances. Overall, the mean difference in the rate of SOC sequestration indicates an increase in carbon by 0.3 Mg ha− 1 yr− 1 in the younger and medium ages of enclosures as compared to the opengrazed areas in the 0–30 cm depth. On the other hand, more rate of TN (0.03 Mg ha− 1 yr− 1) sequestration was attained with the older age of enclosures category. These reflects that long-term enclosures in semi-arid ecosystems might not help in terms of achieving maximum SOC sequestration potential most likely due to the loss of SOC associated with other confounding factors. Dean et al. (2012) also indicate that the potential of rangelands for soil carbon sequestration may be hindered by uncertainly on the direction and magnitude of changes in soil organic carbon associated with shifts in woody vegetation cover. Conant et al. (2001) have also shown that the response of SOC to grazing is highly variable across various ecosystems. Generally, our results in terms of SOC and TN stocks are in contrast with previous study (Mekuria and Aynekulu, 2011), who found an increase in soil carbon (26.0 to 53.7 Mg ha− 1) and TN stocks (2.4 to 6.9 Mg ha− 1) after 20 years of grazing exclusion. In the semi-arid rangelands of Kenya, Verdoodt et al. (2009) have recorded an increase in SOC by 6.6, 9.6, and 10.6 Mg ha− 1 after 15, 18 and 23 years of grazing exclusion in a 0–15 cm depth, respectively. Others (Pei et al., 2008; Su et al., 2005; Yong-Zhong et al., 2005) have also reported a significant increase in SOC and TN following grazing exclusion in a desert steppe of the Alxa region, sandy grassland in Inner Mongolia, China. Our results indicate that the overall mean of SOC stock, across the studied ages of enclosures (15–37 years old), falls within the range of 39.6 to 51 Mg ha− 1 in the 0–30 cm depth. This agrees with the report by Yusuf et al. (2015). Similarly, Alam et al. (2013) have also reported SOC within the range of 20 and 80 Mg ha− 1 for tropical woodland and savanna ecosystems. Generally, the low accumulation of SOC and TN stocks in the current study might be attributed to the impacts of low rainfall, change in the timing of rain and frequent droughts on vegetation growth and productivity.

enclosures that in turn trap base cations. In a semi-arid savanna of Ethiopia, Tessema et al. (2011) have reported that grazing pressure did not significantly affect soil pH, although to some extent higher pH value was found in the light grazed areas. The inconsistent variations in soil pH along the age chronosequence of enclosures is most likely related to past management practices (range burning and cultivation). In this study, the overall mean pH values in enclosure (5.9 to 6.6) and opengrazed lands (5.8 to 6.2) across the three soil depths fall within the moderately acidic soils according to the ratings of Landon (1991), which is in contract with the findings of Angassa et al. (2012), who have reported that soils of the Borana rangelands are moderately alkaline with higher values of pH for grazed (7.56) vs. ungrazed (7.38) areas. The results of the present study indicate that the CEC values are higher in enclosures than in the open-grazed areas, which agree with a study conducted in a semi-arid rangeland of Kenya (Mureithi et al., 2014), suggesting that enclosures can accumulate more CEC values than open-grazed areas. The same authors have also indicated that soils with high organic matter content might have higher cation exchange capacity due to more accumulation of litter from excessive aboveground biomass in enclosures. Generally, the results for CEC values in the current study fall within the range of 15–25 cmolc kg− 1 (i.e., medium) according to the rating and classification by Landon (1991). Angassa et al. (2012) have also found similar values for CEC in Borana rangelands of Southern Ethiopia, indicating the low level of soil fertility in the study areas. In the present study, the two management systems did not show any significant variation in terms of available phosphorus, although results varied along age chronosequence and soil depths. As oppose to the present finding, Angassa et al. (2012) have reported a significantly higher P value in the ungrazed sites in the rangelands of Southern Ethiopia. Similarly, our results are in contrast with the findings of Mekuria and Aynekulu (2011), who reported a significant increase in terms of available phosphorus stock in enclosures (in the range of 17 to 39 kg ha− 1) in semi-arid areas of Tigray in Northern Ethiopia. Generally, our results for the values of P under both grazing management systems correspond to a very low range according to the ratings by Landon (1991). The probable explanation for the very low values may be due to the low status of P in the soil of the parent material in the study areas. Thus, the lack of significant difference between enclosures and adjacent open-grazed areas in terms SOC and TN contents and stocks does not support our hypothesis that states that enclosures enhance more accumulation of SOC and TN in semi-arid rangelands. However, the relatively more accumulation of SOC and TN content and stocks in enclosures management than open-grazed areas might be attributed to the higher accumulation of herbaceous biomass, litter covers of grasses and woody plants. In the rangelands of Southern Ethiopia, Angassa et al. (2012) have also reported a similar result, suggesting that enclosures had accumulated more SOC content (0.89%) than the opengrazed areas (0.86%). On the other hand, Rathjen (2012) have found a significantly higher below-ground carbon allocation in the open-grazed areas than in the enclosures in the rangelands of Southern Ethiopia. Reeder et al. (2004) have argued that plant root residues are the primary source of soil organic matter that help to improve more accumulation of below-ground biomass that in succession enhances soil organic carbon. With regard to age effects, the results of the present findings indicate a non-liner relationship in terms of SOC and TN contents and stocks along the age chronosequence of enclosures and across the three depths of the soil. This inconsistency is most likely explained in terms of differences in site characteristics before the establishment of enclosures in addition to the existing management practices such as stocking densities of calves for each enclosure. Others (McSherry and Ritchie, 2013; Reeder and Schuman, 2002; Piñeiro et al., 2010) have also indicated the impact of grazing intensity, and vegetation types and plant 17

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Economic Cooperation and Development (BMZ) (GIZ) GmbHA [10.7860.9-001.00]. We thank Mr. Shem Kifugo, GIS specialist at ILRI, Kenya for his support in survey and mapping of the enclosures for this study. We are grateful for the insightful comments given by two anonymous reviewers for the improvement of the quality of our manuscript.

4.2. Effects of grazing management on vegetation characteristics The observed higher accumulation of herbaceous biomass in enclosures than the adjacent open-grazed areas in the current study agrees with the findings of Angassa and Oba (2010), who reported that enclosures can accumulate more above-ground herbaceous biomass than open-grazed in Southern Ethiopia. The decline in herbaceous biomass in the open-grazed areas might be due to year round continuous grazing (Wairore et al., 2015). The results of our findings indicate that the age of enclosures had no significant effect on herbaceous biomass. Similar observation was also reported by Angassa and Oba (2010) in Borana rangelands of Southern Ethiopia. It seems that the accumulation of herbaceous biomass in enclosures (115.4 g m− 2) and open-grazed plots (44.0 g m− 2) in the current study is very low as compared to the finding of Oba et al. (2001), who reported high accumulation of herbaceous biomass (440 g m− 2) for enclosure and continuously grazed areas (172 g m− 2) in northern Kenya. This suggests that the variations in sites and climatic conditions are most likely contributed to the low accumulation of biomass. Low primary productivity and poor condition of semi-arid ecosystems has also noted elsewhere in the world (Abebe et al., 2006; Milchunas and Lauenroth, 1993). Our findings in terms of herbaceous species richness were comparable with the findings of previous studies (Aerts et al., 2006; Angassa and Oba, 2010; Verdoodt et al., 2010), who recorded higher herbaceous species within enclosures most probably because of reduced disturbance in enclosures. The same authors have also pointed out that an increase in species richness to some extent, suggesting that short-term grazing exclusion could promote herbaceous species richness, while long-term grazing exclusion may not contribute to species diversity. Overall, our results indicate that the cover of herbaceous plants in the open-grazed areas was very low, indicating that the impacts of heavy grazing pressure on rangeland vegetation and conversely the role of enclosures in restoration of rangeland vegetation. A similar study by Zhao et al. (2011) in the Inner Mongolia in northern China have also found that heavy grazing is followed by severe damage on vegetation cover. The current study also indicates that woody plant density was relatively lower in the open-grazed areas than in the enclosures. This may support the notion that heavy grazing is not the only factor in driving bush encroachment in arid and semi-arid grazing areas (Angassa and Oba, 2008). On the other hand, others (Mekuria and Yami, 2013; Wairore et al., 2015) have reported the reduction in woody plant density, which could be linked to heavy grazing pressure in the communal areas.

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5. Conclusion We found that enclosures management did not show any significant impacts on soil properties, and SOC and TN within 0–30 cm depth. On the other hand, we found a significant improvement in terms of vegetation characteristics in the enclosure management. We conclude that the data generated in the current study can inform policies, policy makers and local land managers about the role of enclosure management in terms of forage preservation, potential for carbon sequestration, livelihood diversification and ecosystems services. We suggest that there is a need for a comprehensive assessment of the long-term dynamics of carbon in relation to climate for better understanding of the benefits of enclosures for adapting to climate change/variability. Acknowledgments This study was performed under collaborative research agreement “Livelihood diversifying potential of livestock based carbon sequestration options in pastoral and agro-pastoral systems in Africa” between the International Livestock Research Institute (ILRI, Kenya) and Hawassa University, financed by the German Federal Ministry for 18

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