Carbon sequestration and soil restoration potential of grazing lands under exclosure management in a semi‐arid environment of northern Ethiopia

Abstract Exclosures are used to regenerate native vegetation as a way to reduce soil erosion, increase rain water infiltration and provide fodder and woody biomass in degraded grazing lands. Therefore, this study assessed the impact of grazing exclosure on carbon sequestration and soil nutrients under 5 and 10 years of grazing exclosures and freely grazed areas in Tigray, northern Ethiopia. Carbon stocks and soil nutrients increased with increasing grazing exclusion. However, open grazing lands and 5 years of grazing exclosure did not differ in above‐ and belowground carbon stocks. Moreover, 10 years of grazing exclosure had a higher (p < 0.01) grass, herb and litter carbon stocks compared to 5 years exclosure and open grazing lands. The total carbon stock was higher for 10 years exclosure (75.65 t C ha‐1) than the 5 years exclosure (55.06 t C ha‐1) and in open grazing areas (51.98 t C ha‐1). Grazing lands closed for 10 years had a higher SOC, organic matter, total N, available P, and exchangeable K + and Na + compared to 5 year's exclosure and open grazing lands. Therefore, establishment of grazing exclosures had a positive effect in restoring degraded grazing lands, thus improving carbon sequestration potentials and soil nutrients.

Similarly, Conant, Cerri, Osborne, and Paustian (2017) in a new synthesis stated improved grazing management increases soil carbon. Thus, uncontrolled (open) grazing could result in severe degradation of both native vegetation and soil fertility in communal grazing lands in arid and semi-arid environments (Yayneshet et al., 2009).
However, a global review of Mcsherry and Ritchie (2013) showed that increasing grazing intensity increases soil organic carbon (SOC) in C4-dominated and C4-C3 mixed grassland, but decreased in C3dominated grasslands. Therefore, the effect of grazing intensity on SOC is highly context-specific and depends on types of grasslands.
Restoration of degraded lands in arid and semi-arid environments often involves excluding livestock from degraded sites (Mekuria, Veldkamp, Corre, & Haile, 2011;Mekuria et al., 2007;Mengistu et al., 2005;Yayneshet et al., 2009). According to Aerts, Nyssen, and Haile (2009) and Seyoum et al. (2015), exclosures are areas protected from human and domestic animal disturbances with the purpose of regenerating native vegetation and reducing land degradation of the formerly degraded communal grazing lands. Yayneshet et al. (2009) reported that exclosures can be effective in enhancing the composition, diversity, and density of vegetation on degraded grazing lands.
Moreover, exclosures can be effective in restoring degraded soils and increasing soil carbon in the highlands of Tigray (Mekuria et al. (2011). Accordingly, rehabilitation of degraded communal grazing lands through establishing exclosures has become increasingly important in Tigray region, northern Ethiopia. Hence, approximately 1.5 million hectares of land have been excluded from grazing in the last three decades in the highlands of Tigray region (Seyoum et al., 2015). However, information on carbon sequestration and soil restoration potentials of degraded grazing lands after grazing exclusion in semi-arid environments of Tigray region of Ethiopia is lacking. In the study area, the grazing exclosures were established in 2005 and 2010 in the lowlands of northern Ethiopia. Therefore, the objective of this study was (a) to assess the effect of grazing exclusion on biomass and soil carbon stocks and (b) to evaluate the impact of grazing exclusion on selected soil physicochemical properties.

| Study area
The study site was located in the semi-arid areas of Tselemti district in the northwestern Tigray region of Ethiopia (Figure 1) (Gebregerges et al., 2017). The mean woody species density including seedlings encountered in open grazing land, 5-year exclosure, and 10-year exclosure were 391, 1,449, and 2,431 stems/ha, respectively (Gebregerges et al., 2017). In the rainy season, 16 different grass species were recorded and the area was dominated by grass species and Amaranthus spp were the common herbaceous species observed in moderate number in the study area (Gebregerges et al., 2017).

| Study site selection and field layout
A field observation was made throughout the areas to be sampled prior to the field layout for vegetation and soil sampling. There were three grazing exclosures for each age class from which human and domestic animals interference was excluded. The grazing exclosures were well protected by guards who had been appointed by the community and were being paid by the govern-

| Sampling of woody vegetation and carbon stock determination
Data on woody vegetation were collected during September to October 2015. In each plot, every tree and shrub having a diameter of ≥2.5 cm at stump height (30 cm from the ground) and breast height (130 cm above the ground) circumference were measured with a meter tape and converted to diameter at breast height (DBH). The diameter was measured separately and considered as individual trees when the bole was branched at breast height or below. Moreover, in cases where tree/ shrub boles buttressed, DBH was measured from the point just 5 cm above the buttresses. The diameters of multi-stemmed shrubs were measured the same way as single-stemmed trees according to Eshete and Ståhl (1998). The height of woody species was measured using calibrated bamboo stick having 7 m height graduated with 10 cm markings.
Trees greater than 7 m in height were measured using clinometers.
The biomass and carbon stock of dominant tree/shrub was estimated using allometric equations developed for each tree/shrub species according to previous studies (Brown, 1997;Henry et al., 2011). The general allometric equation developed by WBISPP (2000) for all woody species were also used for estimating the aboveground woody biomass carbon stocks when species-specific allometric equations were absent. Then, the aboveground woody biomass carbon is calculated from the aboveground biomass using a biomass carbon conversion factor of 0.5 (Liu et al., 2014). Moreover, the belowground biomass for trees and shrubs was estimated from root-shoot ratios by taking in to account the 27% of aboveground biomass of woody species (Penman et al., 2003).

| Sampling of herbaceous vegetation and carbon stock determination
The aboveground biomass of herbaceous vegetation was measured in a 1 m 2 quadrat from September to October 2015. Destructive sampling method was used for measuring the biomass of grasses and herbs by harvesting the whole fresh vegetation within each quadrat using hand shears. Clipped fresh samples together with litters were wellmixed and weighed in the field using sensitive balance. Subsample of the total weight was separated and placed in a marked bag and taken to the laboratory to determine an oven-dry-to-wet mass ratio that is used to convert the total wet mass to oven dry mass. The subsample was air dried and latter oven-dried at Mekelle Soil Laboratory at 80°C for 24 hr according to Rau, Johnson, Blank, and Chambers (2009) until constant weight was obtained and finally re-weighed for their dry weight using a sensitive balance with a precision of 0.1 g. Herbaceous vegetation carbon stocks were calculated as 50% of oven-dried herbaceous biomass (Pearson, Walker, & Brown, 2005).

| Sampling of soil parameters and laboratory analyses
Soil profile pits of 30 cm length and 50 cm width were opened in the center of the smaller (1 m 2 ) plots. Soil samples were collected in each plot at three soil depths (0-10, 10-20, and 20-30 cm) from the four sides of the profile pits. Undisturbed soil samples were taken from each soil depth from the soil profile walls using a core sampler of 100 cm 3 volume for soil bulk density (BD) determination. Equal volume of each sample from a given transect line were pooled and mixed together according to their depth, air dried and passed through a 2 mm sieve to separate debris and gravel. Finally, composite samples were divided into four equal parts, of which one was randomly chosen and stored in plastic bags, labeled, sealed, and transported to the soil laboratory for physical and chemical analyses. In the laboratory, soil samples were dried in an oven at 105°C for 24 hr for bulk density analysis. Bulk density was measured using the core method (Klute, 1986), and SOC was determined by Walkley-black method (Walkley & Black, 1934). Soil texture was analyzed by hydrometer method, pH using a pH-meter in a 1:2.5 soils:water ratio. The percent soil organic matter (SOM) was calculated by multiplying the percent organic carbon by a factor of 1.724 (Brady & Weil, 1990). Total nitrogen was determined by the Kjeldahl method (Bremner & Mulvaney, 1982), available K and P were analyzed using ammonium acetate method and Olsen method (Olsen & Phosphorus, 1982), respectively. Mg and Ca were determined using atomic absorption spectrophotometer and flame photometer was used for K and Na (Jackson, 1958). EC was determined using the sodium saturation ratio (Reeuwijk, 1992), and cation exchange capacity (CEC) was determined using ammonium acetate method (Chapman & Norman, 1965).

| Soil organic carbon stock assessment
Soil organic carbon was calculated using Pearson, Brown, and Birdsey (2007).
where, %Carbon = carbon concentration (%) determined in the laboratory following Walkley and Black (1934) method.

| Estimation of total carbon stocks
The total carbon stock (C t ) was calculated by summing the carbon stock values of the individual carbon pools of the land cover type using the following formula.

| Statistical analysis
One-way analysis of variance (ANOVA) using a general linear model (GLM) was applied to test for mean differences of biomass and carbon stock across grazing land management practices. Two-way ANOVA was performed to test for mean differences of soil physicochemical properties across grazing land management and depth. Tukey HSD test was employed to investigate differences between means at p ≤ 0.05. Data were analyzed using SAS Software (SAS Inc., 2002).

| Carbon stocks across grazing land management practices
Significantly higher aboveground carbon stock was recorded in

| Soil texture and bulk density
Clay and sand contents of the soil were not significantly (p > 0.05) affected by grazing land management, soil depth, and the interaction effects of grazing land management and soil depth. However, silt content of the soil was significantly (p < 0.05) affected by grazing land management (Table 1)  A reduction in grazing intensity did not show a difference in soil pH and electrical conductivity (EC). A reduction in grazing intensity improved available phosphorus (Av. P) concentration (p < 0.05).

| Soil chemical properties
Significantly highest Av. P was recorded in 10 years grazing exclosure, while the lowest was observed in open grazing land ( Figure 6).
In the 5 and 10 years grazing exclosure, significantly highest Av. P was recorded in the topsoil (0-10 cm). Overall, significantly higher    was similar to result of Alemu (2012). However, the woody biomass carbon stocks found in this study was two times lower compared to the woody biomass carbon stocks reported from rangelands exclosed for about 20 years in the southern parts of Ethiopia (Bikila et al., 2016). Moreover, the mean carbon stock in our study was four, five, three times higher than the carbon stocks reported in the Nile basin (Mekuria et al., 2015), in highlands of Northern Ethiopia , and in the shrublands of northern Kenya (Dabasso et al., 2014) (Bikila et al., 2016;Solomon et al., 2017;Xiong et al., 2016)  Similarly, Li, Zhao, Chen, Luo, and Wang (2012) reported grazing exclusion is a positive way to restore desertified ecosystems and has a high potential for sequestering soil carbon in the semi-arid Horqin Sandy Land. A meta-analysis by Dlamini et al. (2016) reported that grassland degradation significantly reduced SOC stocks by 16% in dry climates (<600 mm) compared to 8% in wet climates (>1,000 mm).

| Carbon Stocks
Improved grazing management, fertilization, sowing legumes and improved grass species, irrigation, and conversion from cultivation all contributed to grassland improvement. A new synthesis by Conant et al. (2017) also stated that improved grazing management in combination with other factors tends to lead to increased soil C, at rates ranging from 0.105 to more than 1 Mg C ha -1 year -1 . In contrary to our result, soil organic carbon showed no significant differences between grazed and nongrazed conditions in NW Patagonia, Argentina (Nosetto, Jobbágy, & Paruelo, 2006) and in southern Ethiopian rangelands (Aynekulu et al., 2017). A global review by Mcsherry and Ritchie (Mcsherry & Ritchie, 2013) showed that increasing grazing intensity increased SOC by 6%-7% on C4-dominated and C4-C3 mixed grasslands, but decreased SOC by an average of 18% in C3-dominated grasslands. Carbon stocks in the soil layers 0-5 and 5-15 cm under grazed grassland were significantly larger than in the ungrazed grassland Tibetan montane pasture (Hafner et al., 2011). Shrestha and Stahl (2008) found no variation in soil organic carbon due to grazing exclusion at three of their four study sites, where exclosures had been established more than four decades earlier, in the semi-arid sagebrush steppe of Wyoming. The discrepancies among these studies likely resulted from differences in climate among study sites and in specific soil characteristics. The degree of degradation  Zhong et al., 2003). However, (Nosetto et al., 2006) found that grazing exclosures did not result in significant changes in the total carbon storage in comparison with the adjacent grazed stands, suggesting a slow ecosystem recovery in the  Moreover, across the grazing land management practices, a higher total carbon stock was stored in soil than in the aboveground vegetation. According to Girmay, Singh, Mitiku, Borresen, and Lal (2008), more than 90% of the total carbon stocks were contributed from SOC in wooded grassland of northern Ethiopia.

| Soil physicochemical properties between land management practices and soil depths
The results of the soil texture showed significantly highest silt content in 5 years exclosure as compared to the open grazing land and 10 years exclosure. Despite the differences, the results did not show any relationship between grazing management and soil texture. Soil texture is one of the inherent soil characteristics that changes rarely (Khademolhosseini & Jahromi, 2014). Therefore, the difference in silt content among the grazing management might be due to other factors instead of grazing management. However, the highest sand percentage observed in the open grazing lands might be due to the decrease in ground cover as a result of continuous heavy grazing, which accelerates erosion of fine soil particles (Pei, Fu, & Wan, 2008;Yong-Zhong, Yu-Lin, Jian-Yuan, & Wen-Zhi, 2005).
Areas excluded from grazing had a lower soil bulk density than open grazing lands while there is no significant variation between young and old exclosures in our study, indicating that excluding of livestock from degraded grazing areas significantly decreased soil bulk density. In line with this study, Pei et al. (2008) and Liu, Wu, Su, Gao, and Wu (2017) also found that soil bulk density decreased after 6 years of exclosure and 10 years of exclosure in de- in grazing intensity in a semi-arid steppe of Inner Mongolia. In contrary, Aynekulu et al. (2017) stated that excluding of grazing land had no effect on bulk density in Southern Ethiopian rangelands.
The decrease in bulk density in grazing exclosures may increase soil aeration, water absorption, and water holding capacity and reduces runoff (Kozlowski, 1999;Lal & Kimble, 2001).
Significantly highest OC, TN%, and AP were recorded in the 10 years of grazing exclosure. In line with this study, Pei et al. (2008) found OC, TN, and AP increased significantly with exclosure period. Li et al. (2012) and  also stated grazing exclosure had a potential to restore soil nutrients. A meta-analysis by Xiong et al. (2016) found that soil available nitrogen and soil available phosphorus increased by 52.0% and 21.7%, respectively, in grasslands of China. The highest OC, TN%, and AP recorded from the 10 years of grazing exclusion could be due to the higher accumulation and decomposition of litters into the soil. The results for the OC, TN% and AP was in agreement with the reports of Yimer, Alemu, and Abdelkadir (2015) who stated that the relative increase in the soil parameters in exclosures is due to the management establishment and subsequent increased organic matter accumulation derived from litterfall from the trees/shrubs and herbaceous species biomass and from reduced soil erosion through effective ground cover. Besides, the increases in canopy cover with the increase in exclosure duration could decrease soil nutrient losses by reducing the erosive impact of raindrops and soil erosion (Girmay, Singh, Nyssen, & Borrosen, 2009;Mekuria et al., 2009). In contrary to our result, Aynekulu et al. (2017) found In the open grazing land significantly highest CEC was recorded in the 10-20 and 20-30 cm as compared to 0-10 cm. Overall, soil CEC increased with increasing soil depths. In line with this study, Mekuria and Aynekulu (2013) and  found that exclosures showed significantly higher CEC than the adjacent grazing lands in northern Ethiopia.

| CON CLUS IONS
The establishment of area exclosures on degraded communal grazing lands had positive effect in restoring vegetation biomass, carbon sequestration potentials, and soil nutrients of eroded communal grazing lands. The aboveground biomass and carbon stocks increased with duration of grazing exclusion; however, the open grazing lands and 5 years of grazing exclosure did not differ significantly in our study. A similar pattern was observed for belowground carbon stocks and soil organic carbon stocks, that is, the grazing lands excluded for 10 years from grazing differed significantly with both the 5 years closed area and the open grazing lands. The grass, herbs, and litter carbon stocks were the highest in the 10 years of grazing exclosure, amounting almost more than five times the value recorded in the open grazing lands. Similarly, the overall total carbon stock was highest for the 10 years of grazing exclosure followed by the 5 years of grazing exclosure and open grazing areas. In the present study, higher total carbon stock was stored in soil than in the aboveground vegetation across all grazing land management practices. Therefore, establishment of area exclosures needs to be widely practised in the semi-arid areas of the region to enhance vegetation biomass, carbon sequestration potentials, and soil nutrient contents. Moreover, further studies on temporal and spatial vegetation biomass and carbon stocks need to be thoroughly investigated to capture the whole dynamics of the grazing land ecosystems under various regimes of grazing exclusions in arid and semi-arid environments.

ACK N OWLED G M ENT
The authors are grateful to the Tigray Agricultural Research Institute (TARI) for financial support. The write-up of the paper was supported by the Steps Toward Sustainable Forest Management with the Local Communities in Tigray, Northern Ethiopia (ETH 13/0018) funded by NORAD/NORHED. We also thank Mekelle University.

CO N FLI C T S O F I NTE R E S T
The authors declare no conflict of interest.

AUTH O R CO NTR I B UTI O N S
T.G. conceived and designed the study; T.G. collected the data. T.G, E.B, Z.K.T, and N.S analyzed the data and wrote the paper; E.B., Z.K.T, and N.S. critically reviewed the paper and provided comments on the contents and structure of the paper.

DATA AVA I L A B I L I T Y
Excel files containing all carbon stock and soil properties data have been submitted to Dryad https ://doi.org/10.5061/dryad.v7t77ts.