Characteristics of Nitisol profiles as affected by land use type and slope class in some Ethiopian highlands

Background: The success of soil management depends on understanding of how soils respond to agricultural land use practices over time. Nitisols are among the most extensive agricultural soils in the Ethiopian highlands but soil degradation threatens their productive capacity. In this study, the effects of two land use systems, intensive cereal and agroforestry systems, and slope class on physical and chemical characteristics of some Nitisol profiles were investigated. In total 12 sample profiles were described and soil samples were collected from each of the identified master horizon. Soil physical characteristics evaluated were particle size distribution, structural aggregate stability, water holding capacity and bulk density. Chemical characteristics determined were exchangeable bases and cation exchange capacity, soil pH and the contents of organic carbon (OC), total nitrogen (TN), available phosphorus (AP) and some micronutrients. Results: Among the physical characteristics, land use and slope significantly (p < 0.05) affected particle size distribution and plant available water content. The mean sand (28%) and silt (26%) particles in the intensive cereal system were significantly (p < 0.05) higher compared to 15% sand and 18% silt in the agroforestry system. Conversely, the mean values of fine grained texture materials including 39% fine sand, 42% fine silt and 67% clay in the agroforestry system were significantly higher than 30% fine sand, 21% fine silt and 46% clay in the cereal system. Similarly, the lower slope had significantly (p < 0.05) higher fin texture materials (39% fine sand, 30% fine silt, and 63%) clay) compared to 17% fine sand, 14% fine silt and 51% clay fractions in the upper slope. The proportion of water stable aggregate (WSA) were highlight (63–94%) and there was no significant difference between land types and slope classes. Following from high structural aggregate stability, the soils have high water holding capacity that ranged from 22 to 32% at PWP to 34–49% at FC while plant available water content (AWC) was in the 120–230 mm m−1 range. Considering the chemical characteristics, land use significantly affected soil pH, total nitrogen (TN), exchangeable magnesium (Mg2+), potassium (K+), percent base saturation (PBS), and available micro nutrients—iron (Fe2+), manganese (Mn2+) and zinc (Zn2+). The mean pH value (5.29) in the intensive cereal system strongly acidic while the pH value for the agroforestry system (6.12) was taken moderately acidic. The mean OC content was 2.0 and 2.1% for the intensive cereal and agroforestry systems that were rated very low. The mean TN values were 0.15 and 0.22% for intensive cereal and agroforestry systems that were taken as low to very low. Similarly the mean values for AP were 8 and 10 mg kg−1 for cereal and agroforestry systems that were rated low. On the other hand, the CEC, exchangeable bases (Ca2+, Mg2+, K+) and PBS of the soil were rated high while Na+ appeared only in trace amount, and there was no significant difference between land use type and slope classes except for Mg2+, K+ and PBS. Mean values of Mg2+ and K+ (15 and 3 cmol(+) kg−1) and PBS (75%) in the agroforestry system were significantly higher than those in the cereal system (6 and 1.6 cmol(+) kg−1 of Mg2+ and K+ and 51% PBS). Among micronutrients, land use significantly (p < 0.05) affected © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Open Access *Correspondence: [email protected]; [email protected] Centre for Environmental Science, College of Natural and Computational Sciences, Addis Ababa University, Addis Ababa, Ethiopia Page 2 of 15 Elias Environ Syst Res (2017) 6:20 Background In Ethiopia, diversities in state factors such as topography, parent materials, climate and vegetation have resulted in the development of 18 soil types of which Nitisols are among the most extensive soils (FAO 1984; Elias 2016). Indeed, more than half of all the Nitisols of tropical Africa are found in the Ethiopian highlands followed by Kenya, Congo and Cameroon (Stocking 1988; FAO 2001). Different reports provide different area estimates of Nitisols in the Ethiopian highlands (FAO 1984; Zewdie 2013). The most recent survey puts the extent of Nitisols to cover about one million hectares that account for 31% of the agricultural lands in the Ethiopian highlands (Elias 2016). The soils are particularly extensive in the south-western and north-central highlands representing 64 and 25% of the agricultural landmass, respectively (Fig. 1). Nitisols are among the most productive agricultural soils along with Vertisols, Luvisols, and Planosols (Stocking 1988). They support the bulk of the cereal and livestock production in the Ethiopian highlands. More importantly, the production of coffee (Cofea arabica), the most important export commodity in Ethiopia, relies almost exclusively on Nitisols. In addition, the large proportion of tea production comes from strongly acidic Nitisols in the western part of the country (Elias 2002). However, soil nutrient and organic matter depletion, acidification and soil erosion losses as result of inappropriate land use practices have become major cause of concern for agricultural soils in the Ethiopian highlands (Elias 2002; IFPRI 2010). In particular, due to the land form of occurrence (high to mountains relief hills with moderately steep slopes) and intensive cereal cultivation and cattle grazing, Nitisols have become prone to degradation in spite of their high structural aggregate stability to resist erosion (FAO 2001; Elias 2016). Land use changes and cultivation of fields without adequate conservation practices, low levels of fertilizer application and failure to recycle crop residues are among the causal factors. Traditionally, soil fertility is replenished through fallow cycles of up to 20 years during which time the land gains fertility through atmospheric deposition, biological fixation and the supply of fresh organic matter and nutrients to the soils (Smaling and Braun 1996; Elias et al. 1998). As population increases, fallow periods are either shortened or abandoned altogether resulting in continuous cultivation of the land. In some parts of the Ethiopian highlands, steep slopes with gradients as steep as 50% are cultivated without installing adequate conservation measures (Assen and Tegene 2008). Often, resource-poor farmers have a short time horizon, i.e., they are primarily concerned with the crop and animal production of the forthcoming season than the long-term productivity of the soil. Longer-term processes that adversely affect agricultural sustainability such as depletion of soil organic matter and nutrient stocks are less visible and perhaps less noteworthy by farmers (Hailu et al. 2015; Elias 2016). As result, land degradation has become a major policy concern in Ethiopia that is experiencing one of the highest rates of soil erosion and nutrient depletion in Africa (Elias et al. 1998; Hailu et al. 2015; Laekemariam et al. 2016). The rate of soil erosion losses, 130 tons ha−1 for cultivated fields, was estimated to be one of the highest in Africa (FAO 1986; Elias 2016). The depletion rate of macronutrients, −122 kg N ha−1, −13 kg P ha−1 and −82 kg K ha−1, was estimated to be high (Haileslassie et al. 2005). The field level nutrient balances on Nitisols reported from southern Ethiopia (−102, −45 and −67 kg N, P and K ha−1) are even more threatening (Elias 2002). Among the unsustainable land use practices farmers that fuel soil degradation include low and unbalanced available Fe2+, Mn2+ and Zn+. The mean values of Fe2+ (97 mg kg−1) and Mn2+ (68 mg kg−1) in the agroforestry system were taken as excessively high while they were moderately sufficient (37, 39 mg kg−1, respectively) in the cereal system. Slope effects were significant for OC, TN and AP having higher mean values (2.5% OC, 0.22% TN and 17 mg kg−1 AP) in the lower slope than in the upper slope (1.5% OC, 0.13% TN and 8 mg kg−1 AP). Conclusion: Land use and slope had significant effect on some soil physical and chemical characteristics. The land use practices in the intensive cereal system are adversely affecting important soil characteristics as compared to the soil under the agroforestry system. These include alteration of particle size distribution, strongly acidic soil reaction, organic matter and nutrient depletion (N, P, K and Zn) and low plant available water content. Among the inappropriate land use practices include repeated cultivation to create fine seedbed that predisposes the soil to erosion, unbalanced fertilizer application, rotation of maize with potato that are depleting soil nutrient stocks (e., K and Zn), and removal of crop residues from fields. Therefore, a more balanced fertilizer blend application that contain N, P, K and Zn combined with liming to raise soil pH, organic matter management and integrated soil water conservation are recommended.