Characterization of nutrient release from volcanic rocks in soils from the southern Main Ethiopian Rift

The Ethiopian Rift, with its rugged topography and varied lithology, substantially influences soil physicochemical properties. Nutrient distribution within these landscapes is primarily influenced by rock geochemistry and weathering intensity, though their interactions remain debatable. Consequently, we investigated soil chemical and mineral composition across various volcanic substrates at different altitudes in the Ethiopian Highlands. At elevations up to 1800 m above sea level, kaolinite mineral dominates saprolite, reflecting advanced weathering. A-horizons in this region exhibit low pH levels (KCl/CaCl 2 : 3.7 to 6.3) and medium to high cation exchange capacity (CEC: 11 to 51 cmolc.kg −1 ). A positive correlation between Organic Carbon (Cox: 1.7 to 4.6 wt%) and CEC highlights the role of organic matter in nutrient retention in the A-horizon. By contrast, rift floor soils—modified by Holocene to Pleistocene pyroclastic deposits—show mixed mineralogy of kaolinite, smectites, chlorites, volcanic glass (pumice), feldspars, quartz, and iron hydroxides. Their A-horizons have pH values of 4.0–7.1 (KCl/CaCl 2 ), with medium to low levels of Cox (0.3–2.8 wt%), and CEC ranging from 14 to 58 cmolc.kg −1 . The CEC is positively correlated with clay mineral content, particularly smectite. Here, nutrient availability is closely related to clay mineral content, especially smectite, and the progressive weathering of volcanic glass. These findings indicate contrasting controls on soil fertility: in Highlands, organic matter strongly regulates nutrient availability, whereas in rift floor soils, mineralogy and volcanic glass weathering are the dominant drivers. Together, they highlight the dual importance of organic processes and parent material alteration in shaping soil development across the Main Ethiopian Rift. • We analyzed the chemical properties of soils from the southern Ethiopian Rift and the Ethiopian Highlands. • The intensity and duration of weathering are fundamental determinants of pedogenetic processes. • During weathering, primary minerals and volcanic glass are gradually replaced by secondary minerals like chlorite, clays, and iron hydroxides. • The nutrient status of the soils is strongly influenced by the abundance of smectite and the accumulation of organic matter.