Introduction

Soil and water conservation (SWC) practices have been given a high priority and attention, which is the main effect of improving soil fertility and reducing soil erosion. Soil and water conservation practices play a great role in promoting sustainable soil quality and productivity (Deng and Shangguan 2021). It is related to not only the improvement of soil fertility but also the sustainable development of the agricultural sector and the economy of the country at large (Abiye 2022). Moreover, it is vital to reduce soil erosion and runoff velocity, trap sediment and nutrients, improve water quality, enhance downstream water, reduce sedimentation, and improve land productivity (Belayneh et al. 2019). However, SWC practices are not effective in the develo** countries because of different socioeconomic factors such as lack of finance, lack of training, and farmers’ perception (Asfaw and Neka 2017). Furthermore, it results in a major variation of soil physicochemical properties (Selassie et al. 2015 and Assaye 2020).

Land degradation and soil erosion remains one of the biggest environmental problems in the world, particularly in the develo** countries (Lal 2014). It is the main problem forcing agricultural development in sub-Saharan Africa (Wolka et al. 2018). Besides, land degradation and soil erosion are the major challenges that adversely affect agricultural production in Ethiopia (Belay et al. 2015). Furthermore, soil erosion and land degradation have adverse effects on the soil physicochemical properties (Belayneh et al. 2019). Soil erosion accelerates the degradation of land that occurs through water and wind erosion (Hurni et al. 2016; Seifu and Elias 2018). The majority of the farmers in rural areas are subsistence oriented in cultivation of crops, but the cultivated lands are impoverished soils that are highly susceptible to soil erosion (Shiene 2012). The average annual rate of soil loss in cultivated land accounts for 42 t ha−1 year−1 in Ethiopia (Hurni 1993). The main factors that initiate the degradation of land and soil erosion in the Agemi watershed include chemical fertilizer, lack of SWC practices, and removal of crop residues from cultivated lands. Although several research efforts on evaluating the effect of physical SWC practices on the soil physicochemical properties (Erkossa et al. 2018; Guadie et al. 2020; Sinore and Doboch 2021) have been made in different parts of Ethiopia, site-specific empirical evidence is limited in the Agemi watershed. The findings of this study play an essential role in providing relevant evidence for policy-makers on the impacts of different SWC practices on soil properties and insuring food security. Hence, this study explored the effect of physical SWC practices and slope gradient on the soil physicochemical properties in the Agemi watershed of northwestern Ethiopia.

Materials and methods

Study area

The study was conducted in the Agemi watershed of northwestern Ethiopia (Fig. 1). This is situated at 12° 29′ 00″–13° 1′ 00″ north and 34° 50′ 00″–35° 10′ 00″ east. The altitude ranges from 2372 to 2880 m.a.s.l. It is far from 400 km northwest of Addis Ababa. Moreover, the area of the watershed covers about 798 ha (Fig. 1).

Fig. 1
figure 1

The location map of the study area

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Topographically, the study area is classified based on the FAO 2006 guidelines as gentle slope (2–8%), slo** (8–15%), moderately steep slope (15–30%), and steep slope (30–50%). Based on FAO (2006), the dominant soil of the Agemi watershed is Nitosols, which is a deep, well-drained and red tropical soil. According to the national meteorology agency data (2000–2019), the annual mean average rainfall is 1200–1800 mm. Likewise, the minimum and maximum temperatures are 17.15 °C and 24.48 °C, respectively (Fig. 2).

Fig. 2
figure 2

The mean monthly rainfall and temperature of the study area

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Preparation of soil samples

Farmland treated with SWC practices (stone-faced soil bund (SFSB) and soil bund (SB), slope gradients, and untreated farmland (control)) were identified in the study area. Farmers reported that the SWC structures had 10 years old. According to FAO (2006), three slope classes, including lower (5–10%), middle (10–15%), and upper slope class (> 15%), were selected. Hence, the soil samples were collected from both treated and untreated farmland with three slope classes. Soil samples were collected to scale down by 0.5 m from the upper bund and scale up by 0.5 m from the lower bund between two successive bunds (Belayneh et al. 2019). The soil samples were carefully isolated from unwanted materials like gravel soil, plant, and animal residues. Both soils of disturbed and undisturbed samples were collected at the top of 0–20 cm soil depth from four corners and one center of square plots. The undisturbed soil samples were taken using a core sampler of known volume to determine the bulk density of soil, whereas disturbed soil samples were taken using an auger for the analysis of soil physicochemical properties. Thus, 1 kg of composite soil samples were collected for each sample plot. Therefore, 27 composite soil samples were collected from the three slope classes and three farmlands with three replications. Then, the soil samples were oven-dried, crushed, and passed in to a 2-mm sieve for all parameters excluding total nitrogen and organic carbon that passed through a 0.5-mm sieve.

Analysis of soil physicochemical properties

The bulk density of the soil was estimated using a core sampler, and the soil was oven-dried at 105 °C for 24 h (Black 1965). According to Brady and Weil (2008), the total porosity (TP) of the soil was calculated by Eq. (1). Besides, soil texture was analyzed using the hydrometer method (Bouyoucos 1962).

$$TP\left(\%\right)=\left(1-\frac{\mathrm{Bulk\;density}}{\mathrm{Particle\;density}}\right) \times 100\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots$$
(1)

Soil pH (H2O) was measured using a pH meter with a 1:2.5 soil-to-water ratio (Van Reeuwijk 2002). Furthermore, organic carbon was determined by potassium dichromate (K2Cr2O7) oxidation method, while the soil organic matter (SOM) was the percentage of organic carbon by 1.724 (Sertsu and Bekele 2000). Total nitrogen was determined by the Kjeldahl method (Bremner & Mulvaney 1982), whereas available phosphorus was determined by the Olsen method (Olsen et al. 1954). Exchangeable Ca and Mg were measured by an atomic absorption spectrophotometer, while exchangeable Na and K were measured by a flame photometer (Rowell 1994). Likewise, cation exchange capacity (CEC) was determined by leaching of ammonium acetate (Chapman 1965). Moreover, the percent base saturation (PBS) was calculated the sum of basic cations divided by CEC (Bohn et al. 2001).

Statistical analysis

The effects of physical SWC practices and slope gradient on the soil properties were statistically analyzed using the SAS 9.4 software following the two-way ANOVA procedure. Besides, the mean separation was done using the least significant difference (LSD) at a 5% significant level.

Results and discussion

Effects of physical SWC practices and slope gradient on soil physicochemical properties

Soil texture

Soil texture showed a significant variation between SWC practices and slopes (Table 1). The content of sand was lower on treated farmland (29%) than on untreated farmland (35%) (Table 1). This could be the removal of fine particles by soil erosion on untreated land due to the absence of physical barriers. Finer soil particles have been removed by erosion and increasing the proportion of the coarser particles (Terefe et al. 2020). Furthermore, Ademe et al. (2017) and Hishe et al. (2017) confirmed that the lower sand and the higher clay content were obtained from lands treated by SWC practices. The lower value of clay (27%) was obtained from untreated farmland and the higher clay (37%) was observed in treated farmland with SFSB followed by SB (Table 1). This is linked to the effect of SWC practices on farmland to reduce soil erosion (Guadie et al. 2020). Moreover, sand content was higher (35%) and lower (29%) in the upper and lower slope classes, respectively (Table 1). This might be because fine soil particles are transported by soil erosion from steep slopes and deposit onto gentle slopes. The result agrees with Aytenew (2015) and Jembere et al. (2017) who stated that sand particles increase with increasing slope.

Table 1 Effects of physical SWC practices and slope gradient on the soil physical properties
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Soil bulk density

The BD of soils showed a significant variation (p < 0.01) between SWC practices and slopes (Table 1). The higher BD of soil (1.4 g cm−3) was recorded on untreated farmland than treated farmland by SWC practices (Table 1). This might be the presence of SWC practices in the farmland that increase organic matter and decrease BD. This finding is in line with Challa et al. (2016) and Guadie et al. (2020), who showed that lower BD on treated farmland with SFSB and SB than on untreated farmland. Besides, research conducted by Habtamu (2015) shows that untreated land exhibits higher BD than soil under treated farmland. Besides, the lower BD of soil (1.2 g cm−3) was recorded in lower slope class than upper slope class (1.3 g cm−3) (Table 1). This could be linked to organic matter from the upper slope being removed and deposited on the lower slopes. The findings of this study agree with Husen et al. (2017) and Gadisa and Hailu (2020), who stated that the lower BD on the lower slope than on the upper slope.

Total porosity (TP)

The highest (51.5%) and the lowest (48.8%) TP of soil were observed under treated land than on untreated farmland, respectively (Table 1). This could be soils on untreated farmland that has undergone soil structural degradation due to the removal of soil organic matter by soil erosion Guadie et al. (2020). Moreover, the highest (51.2%) TP in soil was recorded on the lower slope than the middle slope (50.4%) and upper slope class (49.3%) (Table 1). This might be due to the movement of fine particles and OM from upper slope to lower slope classes. The report of Aytenew (2015) showed that the effect of slopes creates the variation of TP.

Soil pH (H2O)

The pH of the soil was affected by SWC practices and slopes (Table 2). The soil pH was highest (5.8) on the lower slope of farmland treated by SFSB and lowest (5.6) on the lower slope of land treated with SB, followed by untreated farmland (5.5) (Table 2). Relatively lower soil pH value might be associated with loss of basic cations by water erosion, and the higher pH value could be due to cations preserved by SWC practices. The result agrees with the findings of Ademe et al. (2017) and Guadie et al. (2020) who stated that treated farmland has a higher pH value than untreated farmland. Likewise, Teressa (2017) reported that soil pH was higher on treated farmland and lower on untreated farmland.

Table 2 Effects of physical SWC practices and slopes on the soil chemical properties
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Soil organic matter

Soil organic matter (SOM) was significantly affected by SWC practices and slopes (Table 2). The lowest (2.4%) and highest (3.1%) SOM were recorded on untreated and treated farmland with SFSB, respectively (Table 2). The higher SOM could be related to SWC practices and the lower vulnerability of soil erosion on treated farmland. In contrast, the lowest SOM might be because of the loss of SOM materials by water erosion due to the lack of physical barriers on the untreated farmland. This result agrees with the findings of Challa et al. (2016) and Belayneh et al. (2019), who reported that untreated land had a lower SOM than treated land. Besides, the higher mean value (3.0%) of SOM was recorded on the lower slope than on the upper and middle slope classes (Table 2). It could be linked to the transport of SOM to the lower slope class because of water erosion. The findings of Aytenew (2015) and Chota (2019) stated that the quantity of OM was higher on the lower slope than on the upper slope.

Total nitrogen (TN)

The content of TN under treated farmland with SFSB and SB was higher than untreated farmland (Table 2). The highest TN could be associated with high SOM content on treated farmland because of SWC practices. This result agrees with the findings of Challa et al. (2016) and Belayneh et al. (2019) who reported that TN was higher on treated farmland than on untreated farmland as a result of SWC practices. Moreover, the highest TN (0.3%) was recorded from the lower slope, while the lowest value (0.1%) was recorded from the upper slope. This could be due to the removal of SOM, plant residues, and animal debris from the upper slopes and accumulating on the lower slope. The findings of Lelago et al. (2016) noted that the lower and higher values of TN were recorded under steep and gentle slopes, respectively. The findings of Wubie and Assen (2020) also stated that the highest amount of TN was obtained from a lower slope.

Available phosphorus (AP)

The AP was affected by SWC practices and slopes (Table 2). The highest AP was observed under treated farmland, while the lowest AP was detected in untreated farmland (Table 2). The lower AP results from continuous cultivation without soil management that accelerates soil erosion. Similar findings by Degu et al. (2019) and Guadie et al. (2020) states that AP was higher under treated farmland due to SWC practices. Besides, the highest AP (6.1 ppm) was recorded on the lower slope, while the lowest (5.7 ppm) was recorded on the upper slope. This could be linked to SOM accumulating by soil erosion from the upper slope to the lower slope. The result agrees with the findings of Ademe et al. (2017) and chota (2019), who reported that the higher AP was recorded on the lower slope rather than the upper slopes.

Exchangeable bases (Ca+2, Mg+2, Na+, K+)

Basic cations such as Ca+2, Mg+2, Na+, and K+ were significantly varied by SWC practices and slopes (Table 3). The highest mean values of Ca+2, Mg+2, Na+, and K+ were recorded on treated farmland than untreated farmland. This might be due to SWC practice reducing soil erosion and decreasing leaching of basic cations (Table 3). This result agrees with the findings of Mengistu et al. (2016) and Degu et al. (2019) who showed that the higher basic cations (Mg2+, Ca2+, Na+, and K+) were recorded on treated farmland than on untreated farmland. Also, the lowest and highest basic cations were recorded on the upper and lower slopes, respectively (Table 3). It could be associated with soil erosion, mineral transport from the upper slope to the lower slope. The result was agreed with the findings of Ademe et al. (2017); Kehali et al. (2017) and chota (2019), which showed a higher value of basic cations, were recorded on the lower slopes than on the upper slopes in different parts of Ethiopia.

Table 3 Effects of physical SWC practices and slope gradient on exchangeable bases, CEC and PBS
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Cation exchange capacity (CEC)

The CEC of soil was affected by SWC practices and slopes (Table 3). The lower CEC (31.6 cmol ( +) kg−1) was obtained from untreated farmland, and the higher CEC (38.7 cmol ( +) kg−1) was acquired on treated farmland (Table 3). The lower CEC could be removal of clay particles by soil erosion in the study area. The result agrees with Wolka et al. (2016) and Degu et al. (2019) who stated that CEC was higher under treated land than untreated land. The higher CEC content might be due to SWC practices to reduce soil erosion and increase clay content. Likewise, the lower CEC was obtained from the upper slope and the higher value was obtained from the lower slope class (Table 3). The lower CEC could be clay soil transported by erosion to the lower slope. This study agrees with the findings of Kehali et al. (2017) and Chota (2019) who noted that the accumulation of CEC was higher on the lower slopes than on the upper slopes.

Percent of base saturation (PBS)

The lower (48.7%) and the higher (55.3%) PBS were obtained from untreated and treated farmland, respectively (Table 3). The lower PBS could be the removal of basic cations by the action of surface erosion, and the higher PBS could be retained cation by SWC practices. Besides, PBS was higher on the lower slopes, followed by the middle and upper slope classes. It could be linked to the accumulation of clay minerals that were transported from the upper slopes. Similar findings by Aytenew (2015) and Beshir et al. (2015) stated that the higher PBS was recorded in the lower slope class because of transport minerals by erosion from the upper to the lower slope.

Conclusions

The findings of this study showed that physicochemical properties of soils have a significant variation between soil and water conservation (SWC) practices and slopes. The physicochemical properties of soils such as clay, total porosity, pH, soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), cation exchange capacity (CEC), and basic cations (Ca2+, Mg2+, Na+ and K+) were higher on treated farmland with stone-faced soil bund (SFSB) than soil bund (SB) followed by untreated farmland (control). Moreover, the higher values of clay, pH, CEC, SOM, TN, AP, and basic cations were recorded on the lower slope than on the middle and upper slopes. Farmland treated with SFSB plays a great role in improving soil fertility than SB followed by untreated farmland. Therefore, SWC practices such as SFSB should be implemented on untreated farmland to reduce soil erosion and amend soil fertility in the study watershed.