Assessment of Selected Physicochemical Properties of Soils under Different Land Uses and Topographic Positions at Gola Wachu Subwatershed, Eastern Ethiopia

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Introduction
Rapid population increase and an elongated history of sedentary farming have altered land use types in most countries of the world, especially tropical African countries, and a major cause of environmental deterioration [1,2].Agricultural activities alter the physical, chemical, and biological aspects of soil and are key contributors to soil degradation, due to nutrient mining without commensurate fertilizer inputs [3].In Ethiopia, agriculture is the backbone of the economy, contributing 50% of the gross domestic product (GDP) and 90% of foreign exchange earnings.However, agriculture is under threat because of the unwise use of land resources [4].Due to continuous agriculture and overgrazing, half of Ethiopia's farmland is moderately to strictly degraded and nutritionally depleted [5].
Land degradation in Ethiopia is caused by the cultivation of fragile soils along steep slopes amidst the prevailing erratic and erosive rainfall patterns, and this situation is aggravated by deforestation and inadequate investments in soil conservation practices such as maintenance of vegetation cover, application of organic matter to the soil, leaving the farmland to undergo fallowing, application of external plant nutrients, and controlled grazing [6,7].Consequently, reducing resource depletion, enhancing agricultural output, and attaining food security are the main challenges of the country.Terefore, concerted eforts should be made to conserve the biophysical and socioeconomic environment toward enhancing agricultural productivity while enthroning sustainable agroecosystem.
Various land use and management systems may contribute to diferences in management-responsive soil properties.For instance, soil physicochemical parameters in continuous cultivation may difer from those in land that has been left fallow for a long period of time [8,9].Soil erosion, leaching, nutrient cycling, carbon sequestration, and other biogeochemical processes are infuenced by land-use systems [10,11].Wubie and Assen [12] noted that continuous cultivation and application of acid-forming fertilizers afect the transformation and availability of macro-and micronutrients, which in turn afect crop yields and aggravate leaching losses of nutrients in weathered tropical soils.Soil organic carbon can be infuenced by land use types, and both biotic and abiotic factors have been recognized as important in this regard [13].
Sufcient knowledge of soil properties at the level of subwatershed is vital for tackling local issues in agricultural production with diferent management systems.Such knowledge is necessary for developing suitable methods, which can help Ethiopians overcome many of the issues they face in the agricultural sector and in their eforts to conserve and manage natural resources for long-term development.
Among Ethiopian highlands, East Hararghe is the most variable in terms of landscape and is strictly degraded and hence records low farming productivity compared with other regions [14].In Hararghe region, soil resources are exceptionally diversifed in nature owing to the complicated interaction of soil forming factors and processes [15].
High population density in the Gola Wachu subwatershed in the Kersa District of Eastern Ethiopia causes farms to be fragmented and pushes farmers to cultivate on steep slopes.Te conversions of forest land and pasture lands into agricultural farms, as well as ongoing overgrazing and cultivation, have severely deteriorated the steep slopes.Te cause and extent of the problem in this subwatershed have not been recognized or qualifed, neither is information available on how soil physicochemical attributes in various land uses and topographic positions change in reply to land use changes in the subwatershed.As a result, farmers remain without any information about the nutrient status of their soils to sustain their livelihood, and this problem is ongoing.Currently, agricultural productivity in Gola Wachu subwatershed has started to decline, and fertilizer requirements of the soil have become very high.Tis might be related with periodic changes in land use in this subwatershed.To curb this problem and ensure sustainability of land use in subwatershed, it is imperative to comprehend the status of the various soil properties.
Information on the physicochemical properties of soils across types of land uses and topographic positions at Gola Wachu subwatershed is important for transferring knowledge regarding land use and topographic positions.However, there has been no study in this regard.Terefore, the objective of this research was to assess the selected physicochemical properties of soils from diferent land uses and topographic positions of this subwatershed.Te results of this study are predictable to aid knowledge transmission to stakeholders including farmers, soil scientists, agronomists, and decision-makers.N and 41 °50′0″ to 41 °57′30″ E at an altitude of 1968 to 2127 m above sea level (Figure 1).Te subwatershed covers a total area of 634 ha [16].

Climate.
Gola Wachu subwatershed receives a mean annual rainfall of 665 mm according to the data gathered from National Meteorology Agency (NMA) over a twenty-one-year period .Te pattern of rainfall in the subwatershed was bimodal.Te primary raining season is from July to September, and the short raining season is from March to June.August had the highest average yearly rainfall.Te mean minimum and maximum annual air temperatures in the study area were 12 and 24 °C, respectively, based on 18 years (1997-2014) of climate data acquisition from the National Metrology Agency, with a mean annual air temperature of 18 °C (Figure 2).

Geology and Soils.
Te Kersa district geology is characterized by the Adigrat Formation, which is made up of sandstones and shells, based on the Ethiopian geological map, which was frst available in 1973 at a scale of 1 : 2000000.Te lower half of the landscape is enclosed by the Hamanlei series development, which contains Oxfordian limestone, whereas the top section is covered with less complicated undiferentiated Precambrian rock.Furthermore, according to Mulat et al. [15], the Hararghe highlands are characterized by crystalline bedrocks made primarily of granitic rock and gneiss.Leptosols, Regosols, Cambisols, Luvisols, and Vertisols are the principal soil types found in subwatersheds [15].

Land Use Types, Vegetation, and Farming Systems.
In Gola Wachu subwatershed, various types of land uses are found; the main ones were cultivated, fallow, and grazing land use types (Figure 3).Te proportion of several types of land use found in Gola Wachu subwatershed was also identifed from the area of land use types using ArcGIS 10.4.Te agricultural pattern of the subwatershed is primarily survival agriculture, with crop mixed and livestock production.Livestock is an important aspect of the agricultural system because it provides food, draught power, and revenue to households.Sorghum and maize are the most popular rain-fed feld crops and are commonly intercropped with common beans and khat.In addition, Eucalyptus camaldulensis and Eucalyptus globule trees dominated the vegetation around homesteads (Table 1).Applied and Environmental Soil Science classifed using the DEM of ArcGIS 10.4 versions by considering the altitude of the subwatershed.Te subwatershed was divided into three slope positions based on the altitude levels (Figure 4).Soil samples were collected in three replications from nine sampling plots comprising the selected three types of land use in each of the three topographic positions.Te sampling plots were square plots of 10 m × 10 m area recognized at the midpoint of each plot of the studied types of land use.At each sampling plot, eight subsamples were collected from a depth of 0 to 20 cm using a soil auger.Te sampling was carried out by the two-way diagonal method from the angles and middles of the square plots after which the soil samples were mixed to form a composite sample.Undisturbed soil samples were also collected using steel core samplers of a 100 cm 3 volume.In this way, 27 each of  Tis includes the area utilized for crop production under rain-fed conditions.Mixed cropping, in which khat is intercropped with sorghum, is the most common cropping system.Only a few quantities of OM are reimbursed to the soil, and residue of crop is practically never changed to the soil since it is used for other reasons such as fuel wood, feed of animal, and construction material (e.g., sorghum).Te farmers use mostly chemical fertilizers (DAP, urea) for crop production rather than organic fertilizers 2 Grazing land Land utilized for community cattle grazing and achieved through a controlled system in which livestock is restricted in stalls and fed with a carry and cut system 3 Fallow land Tis comprises land that was formerly heavily cultivated but is now uncultivated and in need of rehabilitation disturbed composite soil samples and undisturbed soil samples were collected for the analyses of relevant soil properties.

Soil Sample Preparation and Laboratory Analyses.
Te disturbed composite soil samples were minimized to 1 kg and packaged in closed plastic bags and tagged, which included proper labeling of land use type, topographic position, collection date, and the sample feld code.Prior to analysis, these disturbed soil samples were air-dried at room temperature, pulverized with a pestle and mortar, and passed through a 2 mm sieve in the laboratory for all soil properties except for total nitrogen and organic carbon.For the analysis of total nitrogen and organic carbon, the soil samples were further passed through 0.5 mm sieve to remove coarser materials.Te undisturbed samples were used to determine bulk density (BD).All soil analyses were carried out following the standard analytical procedures, described as follows.

Analysis of Soil Physical Properties.
Te selected soil physical properties, including soil texture and BD, were determined.Soil texture analysis was performed using the Bouyoucos hydrometer method [17].Te organic matter (OM) was destroyed with hydrogen peroxide H 2 O 2 , and the dispersing agent was sodium hexa-metaphosphate (NaPO 3 ) 6 .Finally, the soil textural classes were determined using the USDA system textural triangle.Te BD of the soil was determined on undisturbed (core) soil samples using Blake's [18] techniques which involved dividing the masses of the oven-dried soils (g) by their corresponding volumes (cm 3 ).

Analysis of Soil Chemical Properties.
Selected soil chemical properties such as pH, electrical conductivity (EC), cation exchange capacity (CEC), exchangeable bases (Ca, Mg, Na, and K), total nitrogen content, organic matter (OM), available P, and micronutrients (Fe, Cu, Zn, and Mn) were analyzed.In a supernatant suspension of 1 : 2.5 soil-to-water ratio, the pH (pH-H 2 O) of the soil was potentiometrically determined with a glass electrode and pH meter [19].Te EC (dS/m) was obtained from a suspension prepared for pH analysis [19].Te wet oxidation method was used to analyze soil organic carbon content [20].Soil OM was approximated from soil organic carbon with multiplying the latter by 1.724.
Total nitrogen (TN) was measured titrimetrically according to Jackson's description of the Kjeldahl method (1973).Te ratio of soil organic carbon-to-total nitrogen (C : N ratio) was obtained.After leaching soil samples with 0.5 M sodium bicarbonate (NaHCO 3 ) at pH 8.5 by using the Olsen extraction method, available phosphorus was determined calorimetrically using a spectrophotometer [21].Te CEC was evaluated using the ammonium acetate (C 2 H 7 NO 2 ) saturated sample and then replenished with sodium from a penetrated NaCl solution after extra ammonium was removed by repetitive alcohol washing [22].
Using 1N ammonium acetate, exchangeable basic cations (Ca, Mg, K, and Na) were extracted at pH 7 [22].Using an atomic absorption spectrophotometer (AAS), exchangeable Ca and Mg were determined from the ammonium acetate extract, whereas exchangeable K and Na were determined using a fame photometer from the same extract [22].Te ratio of the sum of exchangeable bases to CEC multiplied by 100 was used to calculate the percent base saturation.According to Serstu and Taye Bekele [23], the accessible micronutrients in the soil (Fe, Mn, Zn, and Cu) were extracted using the diethylene triamine penta acitic acid (DTPA) and subsequently measured using an AAS.

Statistical Analysis.
To determine statistical diferences in soil properties in types of land use and topographic positions, the laboratory analysis data were subjected to twoway analysis of variance (ANOVA) using the general linear model (GLM) procedure of the statistical analysis system (SAS) software version 9.1.3[24].Fisher's test of least signifcant diference (LSD) was used to compare and separate signifcant means at P < 0.05.

Selected Physical Properties of Soils under Diferent Land
Uses and Slope Positions 3.1.1.Soil Texture.Soil texture characteristics including sand, clay, and silt contents did not show signifcant differences (P < 0.05) between diferent types of land use and topographic positions in Gola Wachu subwatershed (Table 2).Tus, the textural classes of all soils of the three types of land use in each of the three topographic positions were clay loam.Soil texture is an inherent soil characteristic that is predominantly regulated by soil forming processes or soilforming factors, specifcally the parent material [15], and it can be considered a soil parameter that is not signifcantly altered in the short run by types of land use and management of soil [25].
Although there were no signifcant diferences in soil texture, a nominally high clay content (38%) was verifed in cultivated land of the lower topography, which might be as a consequence of severe cultivation, clearing, and leveling of farming felds.Te comparatively lower percentage of clay particles in grazing and fallow land uses on the upper topography may be due to less erosion and less transportation of materials (especially fner soil particles) from one place to another.In general, there were no signifcant variations among the three topography and land uses.Tis indicates that the infuence of land use at diferent topographic positions on soil texture is minimal.Te results agree with those reported by Tewabe [3], showing relatively only slight variations in soil texture along diferent topographic positions and in various land use types.According to the critical levels suggested by Hazelton and Murphy [26], the clay fraction of the soil of the Gola Wachu subwatershed was moderate, the sand fraction was high, and the silt fraction ranged from low to moderate.

Bulk Density.
Bulk density (BD) was signifcantly diferent among types of land use at each topographic position.Te information presented in (Table 2) shows that BD was higher (1.33 g/cm 3 ) signifcantly in cultivated land of upper topography and lower (1.19 g/cm 3 ) in grazing land of lower topography.Te highest BD in cultivated land on the upper topography could be associated with exhaustive tillage activities, which might provisionally lose the plowed layer of soil and in the long term lead to increases in bulk density and decreases in the amount of OM content, which result in poor soil structure.Poor soil structure infuences the waterholding capacity, which results in the degradation of soil quality.However, lower BD in grazing land on the lower topography might be owing to the outcome of high OM, which results in high porosity owing to better structure or aggregation.In addition, bulk density was signifcantly higher in upper topography of cultivated land as compared to lower topography which might have been a diference due to high soil erosion, low organic matter content, and low porosity in the upper slope than the middle and lower topographies.Similar fndings have been reported by Ayoubi et al. [27] and Mulat et al. [15].
According to the critical level specifed by Hazelton and Murphy [26], the bulk density of Gola Wachu subwatershed was ranged between low to moderate (1.19 to 1.33 g/cm 3 ).Tus, the bulk density values noted for the soils in the subwatershed were in the normal range.Tis may not limit root penetration and/or plant growth, air circulation, and the accessibility of highly movable vital nutrients such as P and K.

Selected Chemical Properties of Soils under Diferent Land
Uses and Slope Positions

Soil pH and Electrical Conductivity (EC).
Soil pH (H 2 O) and electrical conductivity mean values showed signifcant variation among the types of land use in each topographic position.Te soil pH showed as signifcantly diferent at (P ≤ 0.05).Te higher soil pH value (6.92) was found in the lower topography of grazing land, while the lower soil pH value (5.89) was recorded on the upper topography of cultivated land (Table 3).Tis might be related to less soil erosion in grazing land than in cultivated and fallow land and higher deposition of basic cations in the lower topography.In addition, the higher soil pH value in grazing land may be related to inadequate removal of basic cations through leaching and erosion.In other words, the case for lower soil pH value under cultivated as well as fallow land on upper topography might be due to the reduction of basic cations through continuous usage of acid forming inorganic fertilizers and intensive cultivation.Tese fndings are similar to those of Gebreselassie et al. [28], who found low pH values in the soils of cultivated land, which was attributed to extensive farming and the usage of acid forming inorganic fertilizers.In addition, Gonfa et al. [29] revealed that lower pH values in cultivated land were owed to lessening of basic cations in harvesting crop and high microorganism oxidation that produces organic acids, which deliver H ions to the solution of the soil and thus drop the pH of the soil.
Based on the rating of soil pH suggested by Tadesse [30], the values of soil pH noted in the three land uses at three topographic positions of the subwatershed were ranged from moderately acidic to neutral.
Electrical conductivity (EC) was also diferent in land use types and topographic positions (Table 3).Considering the impact of land use types at each topographic position on EC, the lowest EC (0.02 dS/m) was recorded on cultivated land of upper topography and the highest EC (0.068 dS/m) was recorded on grazing land of lower topography.Te electrical conductivity shows an increasing trend down the topography.Te case for the highest EC recorded in grazing land on the lower topography may be t as it contained the highest quantity of basic cations.In contrary, the cultivated upper topography contained the lowest quantity of basic cations, which may have been depleted by concentrated cultivation and washed away of basic cations by leaching and erosion.According to the rating recognized by the US Salinity Laboratory Staf [31], the soils of Gola Wachu subwatershed fall under nonsaline (low EC, <2 dS/m) conditions.
Generally, the EC values in soils of various types of land use in varied topographic positions are not in range that could cause salinity problems and damage the growth of plants.Besides, the commonly low EC values could be accredited to the powerful process of leaching, which eradicates base forming cations from the soil.

Soil Organic Matter. Soil organic matter (SOM)
showed signifcant variation in land use types at each topographic position (P ≤ 0.05).Higher organic matter content was found in the lower topography of grazing land (1.98%) and lower OM content in upper topography of farming land (1.15%) (Table 3).Tis might be associated with lessened soil degradation and highest OM in grazing land than in fallow and cultivated lands.Similarly, a study in the Bollen watershed, Northwestern Ethiopia, by Kefale [3] indicated that the comparatively low organic matter content noted in cultivated lands soils may be associated with Applied and Environmental Soil Science generally poor practice of applying OM to soil, complete removal of the biomass from cropped felds, steep relief that aggravates removal of the organic material by erosion, and intensive cultivation practices.Uzoh et al. [32] also suggest that organic matter is deposited down through the soil profle.
Te OM content showed an increasing trend down topographies for each type of land use in the study area.Te case for the lower OM content in the upper slope could be correlated to the steep topography that aggravated the removal of organic material by erosion, and the higher organic matter contented in the lower slope position could be due to less erosion of soil and high organic matter deposition.
According to the critical level specifed by Tadesse [30], the soil OM content of various land use types in each topographic position of the subwatershed was categorized as low.Tis result indicates that the status of soil fertility of the subwatershed is depleted and requires some management intervention to ensure sustainable use of the soil resources in the subwatershed.

Total
Nitrogen and C/N Ratio.Te total nitrogen (TN) content of the soils was signifcantly (P ≤ 0.05) infuenced by the type of land use at each topographic position.Te total nitrogen mean values were higher (0.18%) in grazing land on the lower topography and lower (0.12%) in cultivated land on the upper topography (Table 3).Te highest total nitrogen in grazing land of the lower slope might be associated with the involvement of OM added through litter from the dispersed trees, including acacia species that contribute to fxation of nitrogen, urine, and feces of animals during grazing.In contrast, the lower TN in cultivated and fallow land of upper topography might be due to the low OM content caused by the absence of litter fall and crop residue removal.Tis outcome agrees with the fndings of Kefale [3].Crop residues have been shown to have the advantage of immobilizing nitrogen, which lowers nitrogen mineralization [33].
Following the rating recommended by Tadesse [30], the total N content of the diferent types of land use with slope positions was in range of low to moderate.Te total nitrogen content of the grazing land and fallow land of soils of each topography was moderate, whereas it was low in the cultivated land of upper topography soils.
Te carbon-to-nitrogen (C/N) ratio in soil is a better indicator of newly added residues.It had a direct impact on decomposition of residue and the cycling of nitrogen in our soils.Te soil C/N ratios of the Gola Wachu subwatershed were not signifcantly infuenced by land use type across the topographic positions (p > 0.05).However, a slight numerical variation was observed.Higher carbon-to-nitrogen ratios were found on grazing land on the lower topography, and a lower carbon-to-nitrogen ratio was detected on cultivated land on upper topography (Table 4).Te highest C/N ratio in grazing land could be related to the higher OM content through the adding of manure and fall of litter from woody species.Te lowest C/N ratio in the upper topography of the cultivated land could be related with the depletion of OM content.Higher and lower carbon-tonitrogen ratios were verifed in the lower and upper topographies of each land use, respectively.Tis could imply higher OM and total nitrogen in the lower topography than in the middle and upper topographies.Generally, plants can take up nitrogen because of this carbon-to-nitrogen ratio.

Available Phosphorus.
Available phosphorus (P) was afected signifcantly (P ≤ 0.05) by types of land uses across topographic position.Te higher (7.08 mg/kg) and lower (2.04 mg/kg) available phosphorus contents were found at lower topography of grazing and upper topography of cultivated lands, respectively (Table 3).Tis may be due to high OM concentration in lower topography soils of grazing land that releases P at the time of mineralization.In soils, organic compounds raise the availability of P by forming inorganic phosphates that can further simply assimilated by plants.Te cause of low available phosphorus content for fallow and cultivated lands as well as upper topography zone of the subwatershed might be in line with the outcomes stated by Mbibueh et al. [5] that accessibility of P in greatest soils of Ethiopia was depleted by the infuences of fxation and erosion.Tis study is also reliable with the studies by Mulat et al. [15] as well as Gonfa et al. [29], who found that alteration in plant cover, nutrient cycling, and biomass CEC � cation exchange capacity; PBS � percent base saturation; LSD � least signifcant diferent; NS � nonsignifcant; means within a row and column followed by a diferent letter in superscripts are signifcantly diferent at P ≤ 0.05 according to Fisher's LSD.Applied and Environmental Soil Science production in the ecosystem can impact soil P dynamics.Nonetheless, the fndings of this study difered from those of Kifu and Beyene [34], who stated that accessible P was the highest in cultivated land soils than in grassland soils.Longstanding manure and house refuse treatments, as well as the resulting rise in microbial activity, were blamed by the authors for the greater concentration of accessible P discovered in cultivated land.Besides, considering the topographic position of the highest and lowest available phosphorus was verifed on lower and upper topographies of each land use, respectively (Table 3).Tis might be owing to the fact that normally available phosphorus is powerfully attached to particles of soil and is thus simply transported during erosion down the topography.Te result is also in harmony with Gadana et al. [35] who stated that the most widespread phosphorus loss in Ethiopia is owing to erosion of soil particularly in the highland areas.
Based on Landon [36], the soil available phosphorus level of <4 mg/kg is valued as low, 5-7 mg/kg as intermediate, and >8 mg/kg is valued as high.Terefore, the available phosphorus level of the soils in the subwatershed was in the range of low to medium.

Exchangeable Bases.
Exchangeable Ca and Mg were signifcantly diferent (P ≤ 0.05) between types of land uses in each topographic position.Te highest exchangeable Ca (21.98 cmol (+)/kg) was noted under grazing land use type at lower topography, and lower exchangeable Ca (18.23 cmol (+)/kg) was detected under cultivated land of upper topography (Table 4).Higher exchangeable Ca on grazing land of lower topography might be due to high CEC and OM.Te lower exchangeable Ca soils of cultivated land in upper topography may be related to low pH and SOM (Table 3).Also, lower exchangeable Ca might be related to extensive removal by harvesting crop with no/little OM input into the soil.Tese results are consistent with Gonfa et al. [29], who found that agriculture increases Ca 2+ leaching, particularly in acidic tropical soils.Donis and Assefa [37] found decreased Ca in the cultivated felds of surface horizon, which they attributed to Ca removal during crop harvest, excessive leaching from continuous cultivation, and OM decomposition.Generally, higher accumulation of exchangeable Ca was observed at the lower topographic positions and may be owed to particle movement from the upper position to the lower position and the prevalence of closely fat to gently undulating topography at the lowest soil sampling site.Te ratings given by FAO [38] on the exchangeable Ca soils of the subwatershed fall in the range of medium.
Exchangeable Mg varied in response to variation types of land use among topographic positions.Te mean values of exchangeable Mg value in the grazing land use type were 9.85, 9.93, and 10.14 cmol (+)/kg on upper, middle, and lower positions, respectively, and those under the cultivated land use type were 7.85, 8.22, and 9.08 cmol (+)/kg on upper, middle, and lower positions, respectively, and those under the fallow land use type were 8.92, 9.04, and 9.72 cmol (+)/kg on upper, middle, and lower positions (Table 5).Te highest (9.97 cmol (+)/kg) mean values of exchangeable Mg were found in grazing land soils followed by fallow land, whereas the lowest one (8.38 cmol (+)/kg) was found in cultivated land (Table 4).Tis might be related to higher leaching of exchangeable Mg based on the upper topographic position, and intensive cultivation and land clearing in cultivated land may lower the amount of this exchangeable Mg.Te exchangeable Mg reduced in the grazing-to-cultivated land that may be related to higher soil OM observed in the grazing land.Besides, comparatively less exchangeable Mg detected in the cultivated land soils might be for the lower SOM and intensive farming which is the reason for leaching and deletion in crop harvest.Tis agrees with the study of Gonfa et al. [29], who found which intensive agriculture increases Ca 2+ and Mg 2+ depletion, particularly in acidic tropical soils.Based on FAO [38], exchangeable Mg of the soils in the research area is in the high to very high range.
Exchangeable K diverse in response to diferent types of land was used at each topographic position.Te higher exchangeable K (1.17 cmol (+)/kg) was noted in the grazing type of land use of lower topography and the lower K + (0.56 cmol (+)/kg) was detected under the cultivated land of upper topography (Table 4).Te higher exchangeable K might be due to existence of various tree species types in the grazing land that drives the cation over their deep roots, whereas the lower exchangeable K in the agricultural/cultivated land was possibly due to great K + removal by soil erosion and intensive cultivation.According to Gonfa et al. [29], high levels of weathering and intense farming, as well as the usage of acid forming inorganic fertilizers (urea and diammonium phosphate), infuence the distribution of potassium in soils and contribute to its reduction.Tis could be probable case for the comparatively lower exchangeable K in the soils of CL, especially at upper topography.In contrary, exchangeable K shows an increasing trend from upper slope to lower topographic position.Tis variation might be related to higher soil erosion and leaching on upper topography in contrast with lower and middle topography.
According to the FAO [38] critical value, the exchangeable potassium of soils of the subwatershed was in the range of medium to high.Te detected exchangeable K mean values of soil in the subwatershed fall in the range of medium in the cultivated land and fallow land while high in the grazing land use type.
Exchangeable Na also shows variation amongst types of land use in each topographic location.It was higher (0.92 cmol (+)/kg) in the grazing land of lower topography and lower (0.65 cmol (+)/kg) in the cultivated land of upper topography (Table 4).Te lower exchangeable sodium in upper topography of the cultivated land may be related to soil erosion, leaching, and deposition of soil particles on the upper topographic position to lower topographic position.As ratings by FAO [38], exchangeable Na mean values soils were from medium to high in the subwatershed.Te recorded exchangeable Na mean values of soils of the subwatershed dropped in the range of medium in the cultivated land use type of upper topography, while they were high in fallow and grazing land use types of all topographic positions.
Generally, investigation by Gebrekidan and Negassa [39] indicated that diferences in spreading of exchangeable bases depend on particle size distribution, mineral existence, the weathering of degree, climatic conditions, management practices of soil, intensity of cultivation, and development degree of soil and parent material as when the soil is formed.Moreover, restricted recycling of crop residue and dung in the soil, deforestation, leaching, much less usage of chemical fertilizers, decreasing fallow periods or extensive cropping, and erosion of soil have contributed to the reduction of basic cations and CEC in the cultivated land as compared to the grazing land.

Cation Exchange Capacity and Percent Base
Saturation.Cation exchange capacity (CEC) mean values of soils in subwatershed were signifcantly (P ≤ 0.05) infuenced by types of land use and topographic positions.Te highest (52.06 cmol (+)/kg) and lowest (46.04 cmol (+)/kg) CEC mean values were observed in grazing of lower topography and cultivated land use of upper topography, respectively (Table 5).Te CEC mean values in the cultivated land of upper topography reduced chiefy due to a decrease in the OM content.Te higher cation exchange capacity in the grazing land of lower topography might be due to joint impact and involvement of organic matter and amount and types of clay content in the soil.Te fndings are in agreement with those of Lemma et al. [40,41], who found that cation exchange capacity of soil varies as land uses change.Moreover, Gebreselassie et al. [28] and Kifu and Beyene [34] concluded that higher CEC in the grazing land was caused by high organic matter and clay content.Te CEC values reveal an increasing trend from the upper to lower topographic position (Table 4).Tis may be linked to the increment of clay and OM content of the upper topographic and lower topographic positions.As per ratings of the cation exchange capacity of soil by Hazelton and Murphy [26], the CEC soil in the subwatershed was classifed as very high in all types of land use in each topographic position.
Percentage base saturation (PBS) does not signifcantly (P > 0.05) vary by types of land use in each topographic position.However, relatively higher mean values (70.72%) and the lower mean values (57.69%) of PBS were recorded under the grazing land of lower topography and cultivated land of upper topography, respectively (Table 4).Te case for high PBS content in the grazing land may be associated with high SOM of the soil, and that of the low PBS content observed in the cultivated land of upper topography might be associated with the lower pH and lower SOM content.Seemingly, Abate [42] recommended that diference in PBS might be due to diference in pH, soil organic carbon content, particle size distribution, intensive of cultivation, parent materials, soil management practices, slope, and leaching.Processes that modify the amount of basic cations have an efect on percent base saturation in general.According to the saturation rating given by Hazelton and Murphy [26], percent base saturation soils in the subwatershed was were the range of moderate to high indicating the presence of weakly to moderately leaching conditions.

Extractable Micronutrients (Fe, Mn, Cu, and Zn).
Te same to other soil properties analyzed in this study and the four extracted micronutrients (Cu, Mn, Fe, and Zn) were statistically diferent in soils of various types of land use in each of the three topographic positions.Te highest contents of available Fe (15.48 mg/kg soil), Mn (22.17 mg/kg soil), Cu (1.37 mg/kg soil), and Zn (0.89 mg/kg soil) were observed in the grazing land of lower topographies, while the lowest (13.23, 19.07, 1.18, and 0.47 mg/kg soil) contents of Fe, Mn, Cu, and Zn were recorded in the cultivated land of upper topographies, respectively (Table 5).Te lower extracted Fe, Mn, Cu, and Zn contents of the cultivated land of upper especially upper topography may be owing to the minor SOC content and leaching of extracted micronutrients by erosion.Tis outcome is in harmony with the fnding of Abate [42] who stated that the diference in the intensity of leaching, possibly high erosion and rainfall in that specifc microclimate, might also be accountable for the lower level of micronutrients.Furthermore, these diferences of extracted micronutrients of the subwatershed agreed with the study of Gebrekidan and Negassa [39] who stated that micronutrients are afected by diferent land uses variously.
Micronutrient availability is specifcally sensitive to alter in the soil environment, according to Wajahat et al. [43].OM, soil pH, sand, and clay content are aspects that infuence the level of micronutrients.In addition to these, intensity of farming, properties of soil drainage, type of soil, erosion, and leaching could also be accountable for the diference in soil micronutrient content.Te recent study was also likewise examined that variances in the content of extracted micronutrients in each land use type may be related to the efect of several aspects, such as anthropogenic and environmental factors, soil pH, parent material, soil texture, SOC, CEC, and available P in soils, which infuence

Figure 3 :
Figure 3: Land use map of the study area using the satellite image of Landsat 8 OLI.
2.1.1.Location.Tis study was carried out at Gola Wachu subwatershed which is situated in the Kersa district, East Hararghe zone, Oromia Region, Ethiopia.It is located 482 km east of Addis Ababa and 44 km west of Harar city.Geographically, the subwatershed lies between 9 °20′0″ to 9

Table 2 :
Soil texture and bulk density under diferent land uses in each topographic position.
BD � bulk density; LSD � least signifcant diference; NS � nonsignifcant; means within rows and columns followed by diferent letters in superscripts are signifcantly diferent at P ≤ 0.05 according to Fisher's LSD.

Table 3 :
Soil pH-H 2 O, EC, OM, TN, C/N, and Av.P under different land uses in each topographic position.

Table 4 :
Soil exchangeable bases, cation exchange capacity, and percent base saturation under diferent land uses in each topographic position in the subwatershed.

Table 5 :
Extractable micronutrients of soils in diferent land uses in each topographic position of the study area.