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

Abstract

BackgroundThe 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.ResultsAmong 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 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).ConclusionLand 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.