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Ziziphus spina-christi is a multipurpose tree which grows naturally on the farm lands in Habru district. Since, the effects of this tree on soil and crop have not scientifically quantified; this study has been carried out with the aim of assessing the effect of Ziziphus spina-christi on soil physicochemical properties, grain and biomass yield of sorghum.Five isolated and nearly identical Ziziphus spina-christi tree growing on farm lands with similar site condition were selected and canopy coverage of each tree was divided into four radial transects. Soil samples from three horizontal distances: 1.2m, 2.9m and 15m with two soil depths (0–15cm and 15-30cm) were taken for analysis of soil physico- chemical properties. Three quadrates 1m x1m at each transect and distances were laid for sorghum grain yield and biomass estimation. The result shows that soil pH, EC, CEC and soil texture were not significantly (p>0.05) influenced by Ziziphus spina-christi tree, whereas soil bulk density, soil moisture content, total nitrogen, organic carbon, available phosphorus and exchangeable cation (Mg, Ca and K) were significantly (P0.05).Hence, retaining of this tree on crop land improves the soil fertility status.

The effect of Acacia seyal tree on soil bulk density, soil moisture content, grain and biomass yield of tef (Eragrostis tef) were examined on six selected comparable A. seyal trees on croplands. Composite soil samples (6 trees × 3 distances × 2 soil depths) were collected at three distances from the tree base (1, 2.5 and 9 m) and two soil depths (0-15 and 15-30 cm) in four radial directions. Grain and biomass yield of tef were also sampled from 1 m2 plot at the three distances from where the soils were sampled. The result indicated that soil bulk density (BD) and soil moisture content (SMC) were significantly affected both by distance from the tree base and soil depths. Grain yield of tef and SMC was higher whereas BD was lower under the canopy of A. seyal than that of beyond the canopy. The reduction of grain and biomass yield at the nearest distance may be attributed by competition for light and water between tree and crop. However, biomass yield of associated tef were the same under the canopy and away from the canopy of A. seyal. Therefore, the result indicates that retaining of A. seyal tree on croplands with the integration of food crops, could be an additional benefit for the farmers without competing their crop production. Key words: Acacia seyal, soil physical property, biomass, grain yield of tef.

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.

Background: Soil fertility decline in agricultural land is due to intensive cropping system, shortening of the fallow period, reduced manure application, extensive use of crop residues as fuelor fodder and removal of ground cover. This study investigated the effect of biochar (BC), lime and farmyard manure (FYM) on growth, yield and grain nutrient of wheat on two cropland sites in southern Ethiopia. Result: The effect of site was significant (P < 0.05) for most parameters investigated, indicating the influence of soil type. The effects of BC, lime, and FYM were significant (p ≤ 0.001) on straw yield, grain yield, and total biomass. However, BC (5 tha -1 ) produced the highest straw yield, grain yield, and total biomass. Similarly, BC and lime significantly (P < 0.001) affected plant height, and tiller numbers and productivity. BC (5 tha -1 ) gave the highest plant height, total tillers, and productive tillers, maximum number of seeds per spike, while FYM (10 tha -1 ) produced the highest spike length. Biochar significantly (P < 0.05) increased the concentrations of P and K in wheat grain. The highest effect on grain Ca concentration was from the application of lime while the highest on grain N concentration was from FYM (10 tha -1 ). Conclusion: The effect from BC was attributed to improvements in soil pH, CEC, nutrient availability and water retention. Thus BC has a good potential as a soil amendment for smallholder farm crop production.

The effect of Balanites aegyptiaca, Acacia tortilis and Acacia seyal on soil fertility and microclimate was studied in the central rift valley of Ethiopia. Sampling was done in randomized complete blocks at 1/3 radius, 2/3 radius and at crown edge radius from the tree base and control was maintained at an open area. All soil and microclimatic parameters observed under the canopy distances were significantly different from the open control (P

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.

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.

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

In Central Rift Valley (CRV) of Ethiopia, renewable land resources are under pressure. Acacia woodland clearance, frequent cultivation, crop residue removal and mono-cropping are among the major land mis-management practices in the area. This has largely been affecting the soil quality. The current study is aimed at assessing the impact of Acacia woodland conversion, and the subsequent mis-managements on selected soil physical properties, and their relation to the prevailing wind erosion in the area. For this study, four land use/land cover types, namely protected woodland (PWL), managed pastureland (MPL), parkland agroforestry (PAF) and treeless cropland (TLCL) were considered. Higher (P 0.25 mm) fraction of soil was found under PWL and MPL while higher fraction of micro-aggregates (0.053–0.25 mm) of soil was found under PAF and TLCL. Soil under PAF and TLCL had higher (P < 0.001) proportion of aggregates of < 1 mm, implying potential susceptibility of the soil to wind erosion. Higher soil bulk density (BD) was found in the most top layer (0–15 cm) of TLCL and PAF, and this could be attributed to the trampling effect by animals freely released to these land use types. In the study area, conversion of native woodland to PAF and TLCL and the subsequent mismanagements negatively affected some soil physical properties, thereby enhancing severity of soil erosion by wind. In contrast, although not commonly practiced by smallholder farmers, soil under MPL is more stable and less susceptible to wind erosion.