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The establishment of grazing exclosures is widely practiced to restore degraded agricultural lands and forests. Here, we evaluated the potential of grazing exclosures to contribute to the “4 per 1000” initiative by analyzing the changes in soil organic carbon (SOC) stocks and sequestration (SCS) rates after their establishment on degraded communal grazing lands in Tigray region of Ethiopia. We selected grazing areas that were excluded from grazing for 5 to 24 years across the three agroecological zones of the region and used adjacent open grazing lands (OGLs) as control. Soil samples were collected from two depths (0–15 cm and 15–30 cm) and SOC and aboveground C stocks were quantified in both exclosures and OGLs. The mean SOC stock and SCS rate in exclosures (0–30 cm) were 31 Mg C ha−1 and 3 Mg C ha−1 year−1, which were respectively 166% and 12% higher than that in the OGLs, indicating a positive restoration effect of exclosures on SOC storage. With increasing exclosure age, SOC stock and SCS rate increased in the exclosures but decreased in the OGLs. Higher SOC stock and SCS rate were recorded in 0–15 cm than in 15–30 cm. The relative (i.e., to the SOC stock in OGLs) rates of increase in SOC stocks (70–189‰ year−1) were higher than the 4‰ year−1 and were initially high due to low initial SOC stock but declined over time after a maximum value of SOC stock is reached. Factors such as aboveground biomass, altitude, clay content and precipitation promoted SOC storage in exclosures. Our study highlights the high potential of exclosures for restoring SOC in the 0–30 cm soil depth at a rate greater than the 4‰ value. We argue that practices such as grazing exclosure can be promoted to achieve the climate change mitigation target of the “4‰” initiative.

Considering the importance of soil organic carbon (SOC) and the scarcity of data on how soil management influences its storage in the region, this study assessed the long-term impact of different soil management systems on SOC in southern Ethiopia using the DAYCENT model. The conservation management systems considered were minimum tillage, crop residue (CR) retention, fertilization and their combinations. We parameterized the model with data from studies in the literature. We then modeled conventional cropping system for smallholding farms over a 30-year period (1991–2020) as the business as usual scenario (BAU). Then we assessed the impact of alternative conservation management scenarios compared with the BAU scenario. Our results indicated that the conservation management scenarios increased SOC at 0–20 cm depth in the range 0.34–9.71 Mg C ha−1 over 30 years when compared to BAU practices. The individual effect of fertilization, CR retention or minimum tillage management practices on SOC stock were lower than the response of the combined conservation management practices. The combined 50%–75% CR retention, no-tillage (NT), and 32 kg N ha−1 fertilization provided the highest SOC sequestration. These combinations, increased SOC in the range 8.10–9.71 Mg C ha−1 over 30 years equivalent to rates of 0.27–0.32 Mg C ha−1 yr−1. While long-term empirical data from field experiments are lacking, model results suggest that the combined 50–75% CR retention, NT, and increased N fertilization have a potential to increase SOC sequestration in resource-limited smallholding croplands. The results may be useful for researchers, policy maker and other stakeholders.

In the arid, low biomass producing areas of Ethiopia, Acacia woodlands suffered a severe degradation due to exploitation for various uses, and conversion to grazing and cultivated lands. However, little is known on the impact of agricultural land uses on soil organic carbon (SOC), total nitrogen (TN) stocks, and other soil quality indicators. This study was planned to evaluate SOC and TN stock changes under parkland agroforestry (PAF), managed pastureland (MPL), and treeless cropland (TLCL) regimes by considering the remnant protected woodland (PWL) as a reference. We found that SOC and TN stocks were significantly higher in PWL and MPL areas. Conversion of Acacia woodlands to MPL, PAF, and TLCL resulted in the loss of SOC stock by 23, 50, and 56%, respectively. Higher SOC and TN stocks were found under PWL (144.3 Mg ha−1) and MPL (108.2 Mg ha−1). Significant changes in available phosphorous (P), exchangeable cations, and cation exchangeable capacity were observed following the woodland conversion...

Interest in mixed-species plantations in the tropics has increased because they appear to provide a wider range of options, such as yield, biodiversity, nutrient cycling, and C sequestration than pure stands. Pure stands of Pinus patula Schlecht. and Charm., Juniperus procera Hochst., and Grevillea robusta A. Cunn., and mixed stands of P. patula/G. robusta, P. patula/J. procera, and P. patula/Podocarpus falcatus R. Br. at Wondo Genet in S Ethiopia were studied to examine (1) the impact of mixed-species plantations on soil chemical properties, and (2) the impact of mixed-species plantations on the nutritional status of constituent trees. Soil (0–50 cm depth) and foliage samples were collected from four random plots (100 m2) in each of the pure and mixed-species plantations. Soil samples were analyzed for organic C, N, and base cations. Foliage samples were analyzed for nutrients (N, P, K, Ca, and Mg). There were little significant differences in soil chemical properties and foliar nutrient concentrations of trees between the pure and mixed stands. Among pure stands, J. procera and G. robusta differed in soil exchangeable Ca++ and K+ at 0–5 cm soil depth and in foliar P and Ca concentration. After 18 and 24 y, mixed stands did not influence soil chemical properties and tree nutrition differently than pure stands. This may be due to additive interaction in mixed-species stands and the similarity of the constituent tree species in foliar nutrient concentration and their impact on soil chemical properties.

Models are important research tools for predicting the build-up of soil organic carbon (SOC), because they provide an increased insight into factors that are involved in the build-up process. The CO2Fix (v. 3.1) model was used to examine the influence of litter production, litter quality and microclimate on differences in SOC accretion under exotic tree species established on farmland in southwestern Ethiopia. The SOC storage under Cupressus lusitanica was larger than that under the other two investigated species ( Pinus patula and Eucalyptus grandis ). This was mainly because of the higher total litter input and higher proportion of fine woody litter (branches and coarse roots) in the Cupressus stand. SOC accretion was greater under Pinus than under Eucalyptus . However, the total litter input in the Pinus and Eucalyptus stands was nearly the same. The difference between the Pinus and Eucalyptus stands was best explained by the fact that Pinus produced more fine woody litter than did Eucalyptus . Litter quality and microclimate only accounted for a minor part of the differences in SOC storage in the Cupressus , Pinus and Eucalyptus stands. Therefore, the results suggested that total litter input and the proportion of fine woody litter were the main factors that accounted for the inter-specific differences in SOC accretion.

At Belete forest in southwestern Ethiopia (7°33′N, 36°35′E), tree plantations were established on abandoned farmland, which was previously mainly used for maize cultivation. Total carbon and 13 C analyses were used to evaluate the changes in soil organic carbon (SOC) pools associated with land use changes using a comparative approach. Forest clearing followed by continuous cultivation of crops caused a loss of 43% (75.4 Mg ha − 1 ) total SOC and 73% (128.4 Mg ha − 1 ) forest derived SOC after nearly 75 years. The net loss of SOC was lower because of addition of 53.0 Mg ha − 1 of SOC of C 4 crop origin (mainly maize) to the farmland. On the other hand, afforestation of farmland led to a net accretion of SOC of 69.6 and 29.3 Mg ha − 1 after 20 years under Cupressus lusitanica and Pinus patula stands, respectively . The SOC accretion of plantation origin amounted to 63.3 Mg ha − 1 under C. lusitanica and 24.2 Mg ha − 1 under P. patula. Contrary to the results obtained in some other studies, the SOC of C 4 origin did not decline in these stands. This could be attributed to pasture grasses of C 4 origin that took over after land abandonment and continued to grow under the tree canopies. The grasses could thus have compensated for the SOC loss. SOC might also have been close to a steady state under the pre-plantation period. Based on the SOC amount found in a reference stand of native forest, afforestation with Eucalyptus grandis during 20 years, preceded by 20 years of cultivation and 35 years of pasture, returned the total SOC to nearly pre-deforestation levels. SOC accumulation rates of 1–3.2 Mg ha − 1 y − 1 are apparently possible 20 years after afforestation of an abandoned farmland but the accumulation rate is species dependent.

Changes in soil C and total N stocks were investigated following reforestation of previously cultivated soil in comparison with soil subjected to continuous cultivation and soil under an adjacent natural forests in south central highlands of Ethiopia. Two of the most widely planted tree species in the highlands of Ethiopia, namely Eucalyptus saligna and Cupressus lusitanica , were considered in the plantation treatments. Soil C and total N contents in the upper 0–10 and 10–20 cm soil layers were significantly different in the order: Natural forest > C. lusitanica > E. saligna > Farmland. Differences in soil C and total N contents among the sites for soil depths greater than 20 cm were negligibly small and statistically not significant. Soil C and total N stocks in the upper 0.80 m mineral soil also varied significantly in the same order as above. Estimated average annual soil C accruals were 156 and 37 g C m -2 yr -1 for Cupressus lusitanica and Eucalyptus saligna , respectively. The results demonstrate that reforestation of former arable soils in the dry Afromontane region of Ethiopia could yield significant restoration of soil C and total N that are lost in the process of natural forest conversion into arable lands and subsequent cultivation. However, the two plantation species differed considerably with respect to the rate of soil C and total N accrual. This suggests that proper selection of tree species will considerably affect the magnitude and rate of soil C sequestration. Keywords : continuous farming, Cupressus lusitanica , Eucalyptus saligna , soil carbon sequestration, soil carbon loss SINET: Ethiopian Journal of Science Vol. 28(2) 2005: 99-108