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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.

Grazing exclosures have been promoted as an effective and low-cost land management strategy to recover vegetation and associated functions in degraded landscapes in the tropics. While grazing exclosures can be important reservoirs of biodiversity and carbon, their potential in playing a dual role of conservation of biodiversity and mitigation of climate change effects is not yet established. To address this gap, we assessed the effect of diversity on aboveground carbon (AGC) and the relative importance of the driving biotic (functional diversity, functional composition and structural diversity) and abiotic (climate, topography and soil) mechanisms. We used a dataset from 133 inventory plots across three altitudinal zones, i.e., highland, midland and lowland, in northern Ethiopia, which allowed local- (within altitudinal zone) and broad- (across altitudinal zones) environmental scale analysis of diversity-AGC relationships. We found that species richness-AGC relationship shifted from neutral in highlands to positive in mid- and lowlands as well as across the altitudinal zones. Structural diversity was consistently the strongest mediator of the positive effects of species richness on AGC within and across altitudinal zones, whereas functional composition linked species richness to AGC at the broad environmental scale only. Abiotic factors had direct and indirect effects via biotic factors on AGC, but their relative importance varied with altitudinal zones. Our results indicate that the effect of species diversity on AGC was altitude-dependent and operated more strongly through structural diversity (representing niche complementarity effect) than functional composition (representing selection effect). Our study suggests that maintaining high structural diversity and managing functionally important species while promoting favourable climatic and soil conditions can enhance carbon storage in grazing exclosures.

In developing countries, fuelwood extraction is one of the main drivers of deforestation, forest degradation, and greenhouse gas (GHG) emissions. This study aimed to investigate the use, management, and attitudes towards biogas technology in rural communities in Ethiopia and assess its’ potential in reducing the consumption of biomass fuels and emissions of GHG. A questionnaire survey sampled from 40 biogas user and 40 non-user households was used to collect data. The biogas flow rate was measured using a gas flow meter from three random biogas digesters. Biogas and non-biogas user households reported similar socioeconomic perspectives. Our results showed that 92.5% of biogas user households and 77.5% of non-users had a positive attitude towards biogas technology, however, 52.5% of the non-users lacked information about the technology while 25% of the non-users were deterred by biogas installation costs. The average biogas consumption of each household was from 496 to 566 m3 yr−1 for cooking and from 106 to 124 m3 yr−1 for lighting. Biogas user households consumed fuelwood and crop residue to the same extent as non-user households for baking foods. However, biogas user households differed significantly from non-user households in biomass fuel use for non-baked foods owing to the suitability of biogas stoves. Each biogas user household reduced their fuelwood and charcoal use by about 2410 kg yr−1 and 379 kg yr−1, respectively, and decreased their kerosene use (for lighting) by about 9.5 L yr−1. The GHGs emitted by each household was estimated at 10,242 kg CO2e yr−1 and 16,266 kg CO2e yr−1 for biogas user and non-user households, respectively. Thus, each household with a biogas digester has the potential to reduce 6024 kg CO2e yr−1 of GHG emission. Biogas energy can offer an opportunity to mitigate GHGs emissions through reduced pressure on forests. Its’ contribution can more than double by improving the biogas stove design to fit the traditional baked foods.

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.

This study was undertaken to evaluate the effect of tree species on soil properties. Soil δ15N, total N, C:N ratio, base cations and available P were analyzed under farmland, native forest and tree plantations established on abandoned farmland at Belete forest. Changes associated with land use were evaluated using a comparative approach. Forest clearing followed by continuous cultivation of annual crops resulted in a significant decline in total N, base cations and available P within 0–10 or 0–20 cm depth. Assuming a linear rate of loss, total N declined by 90.6 kg ha−1 year−1 or by 38% (6.8 Mg ha−1) of the level in native forest during the 75 years of cultivation. The 15N enrichment of soil and litter N in the farmland may indicate losses of N through leaching and harvest. The degree of change in soil properties under exotic trees after 20 years of afforestation of former farmland depended on tree species. Total N within 0–50 cm depth increased by 5.7 Mg ha−1 under Cupressus lusitanica and 2.0 Mg ha−1 under Pinus patula. The decrease in δ15N value with afforestation was greater for C. lusitanica than for P. patula, indicating that N cycling under C. lusitanica progressed more towards ‘native forest like’ conditions. Under C. lusitanica, exchangeable Ca2+ increased significantly at 0–5 cm. Exchangeable K+ increased significantly within 0–30 cm depth under both C. lusitanica and P. patula. The increased C:N ratio under these tree species was attributed to recent organic matter (OM) addition. The soil under Eucalyptus grandis, established on former mixed land use (pasture plus farmland), had nearly similar δ15N, TN, exchangeable Ca2+ and K+ to native forest. The residual effect of 15N-depleted C4 pasture grasses might explain the lack of difference in soil δ15N profile below 10 cm depth between E. grandis and native forest. Greater OM inputs by E. grandis probably account for the greater C:N ratio relative to native forest soil.