Effects of Soil and Water Conservation Practices and Slope Gradient on Selected Soil Physicochemical Properties in Ejersa Watershed, Toke Kutaye District, Ethiopia

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Introduction
In developing countries such as Ethiopia, many people have settled in the highlands due to favorable agricultural and ecological conditions, resulting in high population densities and resource degradation [1]. In fact, erratic and erosive rainfall, steep terrain, deforestation, inappropriate land use, land fragmentation, overgrazing, and poor management are among the factors contributing to land degradation in Ethiopia [2]. Soil erosion on steep slopes was identifed as a major challenge in the highlands of the north and eastern part of Ethiopia [3] and has been escalating related to expanding arable land, high human population, and livestock density [4]. Tis is an observable environmental issue that has led to diminishing agricultural productivity, food insecurity, and rural poverty [5,6]. Soil erosion has also been recognized as one of the major factors challenging sustainability of the Ethiopian agriculture [7] and afected two-thirds of the nation's population and mostly linked to the conversion of forest to agricultural land [8].
Recognizing soil erosion as a major environmental and socioeconomic problem especially on the highlands, the Ethiopian government has implemented several interventions including restoration of degraded lands, reforestation, and integrated physical and biological soil and water conservation (SWC) practices [9]. Consequently, large areas are covered with terraces, stone-faced soil bunds, soil bunds, area closures with diferent grasses, shrubs, and trees planted on protection measures [10]. Nevertheless, despite the extensive concerted eforts made every year through mass mobilization of farmers, little efort has been made to investigate the efectiveness of the soil and water conservation practices in Ethiopia. Tus, this study was conducted to assess the efects of soil and water conservation practices on selected soil physicochemical properties at Ejersa Watershed along the slope gradient.

Description of the Study Area.
Te study was conducted at Ejersa Watershed (Figure 1), which is located at 5 km from Guder Town and 137 km west of Addis Ababa, the capital. Geographically, the watershed is located between 8°58″ and 30°5″N latitude and 37°45″ and 6°29″E longitude at an altitude ranging between 1,880 and 3,194 m.a.s.l., with an average annual rainfall (RF) of 1,300 mm and a mean monthly temperature of 20°C. Te total land area of the watershed is about 366 hectares out of which about 173 hectares were covered by soil and water conservation measures including soil bund, cutof drain, gully stabilization, and tree plantations. About 70% of the total area is moderately steep while the remaining 30% is gentle slope with nitisols (48%), vertisols (27%), and cambisols (25%) constituting the major soil types.

Data Sources.
Te study involved both primary and secondary data. Te primary data were generated through direct feld observation and laboratory analyses of the soil samples collected while secondary data were obtained from relevant documents collected from concerned ofces.

Soil Sampling.
Te study watershed was divided into three separate slope classes (upper >30%, middle from 15 to 30%, and lower <15%) and a total of 18 composite soil samples (9 from treated and 9 from untreated plots) were taken from January 15 to 25, 2022, during the dry season. Samples were collected from four corners and centers of predefned plots of a land with the conservation practices implemented in 2013 and nonconserved areas of the watershed using a 1.2 m soil auger at a depth of 0-20 cm. For determination of bulk density, undisturbed grab soil samples were collected from the center of each slope class by a core sampler to a height of 6 cm and a diameter of 5 cm. Te soil samples collected using auger were later mixed; 1 kg of the samples were bagged separately with appropriate labels and then transported to the Chemistry Department Laboratory at Ambo University for analyses.

Laboratory Analyses.
Te soil samples collected were air-dried, crushed with mortar and pestle, well mixed, and screened through 2 mm sieve with grinding and sieving repeated till all aggregate particles were fne enough to pass through 2 mm sieve. For the analysis of total nitrogen and organic carbon, extra sieving of the soil samples was conducted by a 0.5 mm sieve. Soil moisture content was determined by the gravimetric method [11] and calculated using Diop et al.'s [12] formula as follows: Moisture content (wt%) � wet soil weight − oven dry soil weight oven dry soil weight x100. (1) Soil bulk density (ρ) was determined by the core method [13] and calculated by dividing mass of the oven-dried soil to volume of the sampling core.
Bulk density g cm3 � mass of oven − dried soil sample volume of the sampling core πr 2 h . (2) Soil pH and electrical conductivity were determined by a 1 : 2.5 (soil : water) ratio using a pH meter [14]. Cation exchange capacity, soil organic carbon, and organic matter were determined by the ammonium acetate method [15], Walkley and Black rapid titration method [15], and multiplying percent of the organic carbon by 1.724, respectively. Total nitrogen was determined following the modifed Kjeldahl method [15], while the Olsen extraction method was used to determine available phosphorus as described by Diop et al. [12].

Statistical Analysis.
Signifcance of the diferences in soil physicochemical parameters between the treated and untreated plots was tested using the independent sample Tukey test, while one-way ANOVA was used to compare the parameters among three slope classes. Data analyses were carried out by using the Statistical Package for Social Sciences (SPSS) version 23.

Efects on Soil Physical Parameters
3.1.1. Bulk Density. Te average value of bulk density (BD) of the soil sample taken from the conserved land (1.16 g/m 3 ) was lower than that taken from nonconserved land (1.45 g/ cm 3 ) ( Table 1). Tis might be attributed to accumulation of organic matter on the treated land due to the conservation practices and the washing away of the organic matter by the process of soil erosion from the nontreated land which clearly indicated the benefts of the conservation measures. Te mean value of the soil bulk density was signifcantly diferent (p < 0.01) between treated and untreated lands which is in line with Tanto and Laekemariam [16]. Te mean values of bulk density recorded on higher, middle, and lower slopes of the treated land were 1.21 g/cm 3 , 1.16 g/cm 3 , and 1.13 g/cm 3 , respectively, and the corresponding values for untreated land were 1.62 g/cm 3 , 1.42 g/cm 3 , and 1.31 g/cm 3 (Table 1). Te higher values on upper slopes and the untreated land might be attributed to more severe erosion, which might have washed the organic matter away clearly indicating the positive impact of the treatment. Tis result was supported with the study by Shaf et al. [17] who reported the highest mean value of BD on the upper slope. Nevertheless, slope gradients did not cause signifcant difference (p > 0.05) in the study watershed.

Soil Moisture Content.
Te mean value of the percentage of the soil moisture content (MC) determined in samples collected from treated land (8.72%) was higher than that determined in samples collected from the untreated land (4.44%) and is generally low which might be attributed to collecting samples during the dry season (Table 1) which might be attributed to the runof trapped by the conservation measures and penetrated into the soil, and the values are signifcantly diferent (p < 0.01). Te result is in agreement with the study by Gadana et al. [18] who reported a higher percentage of soil MC from treated land as compared to that from the untreated land. Te MC determined from lower, middle, and higher slopes of the treated area was 10.07%,  9.03%, and 7.07%, respectively, and the corresponding values for the untreated land were 5.11%, 4.79%, and 3.41%, respectively (Table 1). Te highest MC determined at the lower slope as compared to middle and higher slopes might be attributed to the accumulated crop residue and better soil humus, which is line with the study by Gadana et al. [18]. Te values were signifcantly diferent (p < 0.01) among slope classes both in treated and untreated lands.

Cation Exchange Capacity.
Te average cation exchange capacity (CEC) determined in treated land (33.51 meq/100 g) exceeded the amount determined in the untreated land (21.56 meq/100 g) ( Table 2), which might be due to the higher organic matter content in the former, and the values are signifcantly diferent (p < 0.05). Te result was supported with the study by Gebraselassie et al. [21] and Guadie et al. [3] that attributed the matter to the diference in management practices. Te CEC was also signifcantly diferent (p < 0.05) among the slope classes both in treated and untreated land. Te highest mean CEC (45.67 meq/ 100 g) was recorded at the lower slope in the treated land (Table 2), which might be attributed to the high amount of organic matter and clay content at the particular slope class and in accordance with the study by Yitbarek et al. [14] and Bufebo et al. [22].

Soil Organic Carbon.
Te soil organic carbon (OC) content of the conserved soil determined (2.66%) was higher than that of nonconserved soil (2.24%) (

Soil Organic Matter.
Te average value of soil organic matter (OM) determined in treated soil (4.58%) exceeded that from the untreated land (3.86%) ( Table 2) and might be attributed to depletion of the productive soil layer in the latter as compared to the former. Te result agrees well with the study by Tolesa et al. [19] who reported a relatively higher value of soil OM from the conserved land than that from the nonconserved farm land. Te mean values of the soil OM were signifcantly diferent (p < 0.05) between treated and untreated lands. Te highest percentage (4.71%) was obtained from the lower slope, whereas 4.66% and 4.37% were recorded from middle and upper slopes, respectively. Tis might be attributed to translocation of the organic material from the loss zone to the deposition zone. Te result from the one-way ANOVA indicated that soil OM did not show a signifcant diference (p > 0.05) by slope classes.

Total Nitrogen.
Te level of total nitrogen (TN) determined in treated soil (0.24%) exceeded the amount determined in untreated soil (0.15%) ( Table 2) and might be explained by the accumulation of organic matter in the conserved feld. Te results indicated positive impacts of the soil and water conservation intervention on TN and are signifcantly diferent (p < 0.01). Tis fnding is in accordance with previous studies such as studies by Shaf et al. [17], Belayneh et al. [23], Alemayehu and Fisseha [25], and Hailu [26]. Te average values of TN determined in soil samples collected from treated land were 0.26%, 0.24%, and 0.21% at the lower, middle, and upper slopes, respectively (Table 2), and the corresponding values in the untreated land were 0.19%, 0.15%, and 0.12%. Te values were signifcantly diferent (p < 0.01) among slope classes. Te decline in the values with the increasing slope in both treatments might be due to the high rate of erosion on the upper slope and accumulation of organic materials at the bottom portion of the study area, which is in agreement with the study by Tolesa et al. [19] who reported high TN at the lower slope than the upper slope.

Correlations of the Soil Physicochemical Parameters
Determined. Soil moisture content (MC) showed strong positive correlation with pH, EC, CEC, OC, OM, TN, and AvP (Table 3). Soil pH also had a strong positive correlation with EC, CEC, OC, OM, TN, and AvP. Soil EC was positively correlated with CEC, OC, OM, TN, and AvP. Soil OC had a positive correlation with clay, pH, EC, OM, TN, CEC, and AvP, and TN and AvP also showed strong positive correlation. Te positive correlations indicated simultaneous changes (increase or decrease) of the soil physicochemical parameter with the particular parameter in focus. Similar studies conducted in other parts of the country by Demelash and Stahr [29] and Tellen et al. [28] also indicated a strong positive correlation between soil OM and TN which is in agreement with the present fnding.

Conclusion and Recommendation
Higher values of many of the soil physicochemical parameters were recorded in soil samples collected from the treated land and lower slopes, which might be explained by the deposition efects of the soil and water conservation practices and decline in the rate of soil erosion. Results further revealed that most soil physicochemical properties determined such as bulk density, moisture content, pH, electrical conductivity, cation exchange capacity, organic carbon content, organic matter, total nitrogen and available phosphorus were signifcantly positively afected by soil and water conservation practices comprising soil bund, cutof drain, gully stabilization, and tree plantations. Te slope gradients also signifcantly afected all the parameters determined except bulk density, and signifcant values of fertility indicators were recorded in the treated area of the study watershed. Tus, sustainable integrated watershed management should be widely implemented particularly in erosion prone areas in a more coordinated manner to harness the associated multiple benefts.

Data Availability
All necessary data have been included in the manuscript.

Conflicts of Interest
Te authors declare that they have no conficts of interest.