3.1. Effects of Soil Bund and Stone-Faced Soil Bund on Soil Physical Properties
Soil texture: Clay, silt, and sand fractions were significantly affected (
p ≤ 0.05) by soil bund (SB), stone-faced soil bund (SFSB), and slope gradients (
Table 1). The overall mean sand fraction was found to be high in the upper (>15%) and low in the lower (<9%) slope positions. However, the silt and clay fractions were higher in the lower (<9%) slope positions. In general, sand content increases as slope gradient increases, and clay and silt content decreases as slope gradient increases. This could be due to the selective removal and transport of fine soil particles such as clay and silt by water erosion to the lower slope, leaving the coarser materials onsite in the upper slope positions. The result agreed with the reports of [
11] that showed an increase in sand and decline in silt and clay contents with an increase in slope gradient in the Weday watershed, eastern Ethiopia. According to [
48], sands are easily detachable but difficult to transport; in contrast, silt and clay are easily transportable although they are difficult to detach by runoff water.
On the other hand, a statistically significant difference (
p ≤ 0.05) was found in clay, silt, and sand proportion between treated and untreated fields. The overall mean percentage of clay and silt content was significantly higher in the treated than the untreated fields, whereas the sand fraction was significantly lower in the treated than the untreated fields (
Table 1). This might be due to the accumulation of fine-textured clay and silt fractions behind the constructed structures. The result concurs with the findings of [
26] in Rwanda, [
27] in southern Ethiopia, and [
23] in northwest Ethiopia, in which higher clay and silt proportions were found in fields treated with SWCPs than the untreated fields.
Soil bulk density (BD): BD showed a statistically significant difference (
p ≤ 0.05) between the treated and untreated fields and among slope positions (
Table 1). BD was found to be lower in fields treated with SB and SFSB than the control. Higher BD in the untreated fields could be associated with the absence of SWCPs that exposed the soil to erosion and consequently to the removal of organic carbon from the topsoil layer. This finding was in line with those of [
24] and [
5], which showed significantly lower BD values in the treated micro-watersheds than the untreated in Adaa Berga district, western Ethiopia, and Ambachia watershed, northern Ethiopia, respectively.
Similarly, BD showed a statistically significant variation (
p ≤ 0.05) at different slope positions. It was found to be lower in lower (<9% slope) than in the upper (>15% slope) positions. As slope gradient increases, BD increases, which could be associated with low soil organic matter content. [
24] reported lower BD in cultivated fields of lower slope positions than in the upper slope in Adaa Berga district, Western Shewa, Ethiopia. Other studies [
12,
49] in the Goromti watershed and in the Guto Gida District, Western Ethiopia, respectively, also reported the direct relationship of BD and slope gradient. This study also showed significant and negative correlation of BD with clay fraction (r = −0.76 **); significant and positive correlation with sand fraction (r = 0.73 **), and significant and negative correlation with organic matter (r = −0.70 **). The reason could be associated with variations in soil organic matter content, which has an inverse relationship with soil BD.
Soil porosity: Soil porosity showed a statistically significant difference (
p ≤ 0.05) at different slope positions (
Table 1). In general, the values of soil porosity decrease as the slope gradient increases. The lowest soil porosity in the upper slope fields (>15%) might be due to the intensive cultivation and soil erosion, which reduces the soil organic matter content and total pore volume of the soil. The result agrees with those of [
39], which reported lower total porosity in steep slope than in gentle slope fields as a result of high BD, low clay content, and low organic matter content in the Dawja watershed, northwest Ethiopia.
A statistically significant difference (
p ≤ 0.05) was found in soil porosity between the treated and untreated fields. Lower soil porosity (59.83 ± 2.43) was found in the untreated fields than the treated fields. The low soil porosity in the untreated fields might be due to the low organic matter content of the soil as a result of soil erosion that caused higher BD. This result agreed with the findings of [
23] in northwest Ethiopia that low soil porosity in the untreated/control field was due to the removal of soil organic matter and exposure of the subsoil as a result of soil erosion. The total soil porosity showed a significant and positive correlation with organic matter OM (r = 0.70 **) and a significant and negative correlation with bulk density (r = −1.00 **; Table 6). The highest soil porosity was recorded in fields located in the lower slope position (<9%) having the highest clay content showing the positive effect of clay content on soil porosity. This result was in line with [
40] in which the lowest total soil porosity (46.42%) was recorded in fields having steep slope, while the highest total soil porosity (50.10%) was recorded on fields having a gentle slope in the Dawja watershed, northwest Ethiopia. Soil texture, bulk density, and porosity didn’t show a statistically significant difference (
p ≤ 0.05) between SB and SFSB, which might be due to the similarity in the age of the practices.
3.2. Effects of Soil Bund and Stone-Faced Soil Bund on Soil Chemical Properties
Soil Reaction (pH): Soil pH showed a statistically significant difference (
p ≤ 0.05) between the treated and untreated fields (
Table 2). It was 6.51 ± 0.32 behind the SB, and 6.48 ± 0.26 behind the SFSB and 5.90 ± 0.48 in the control treatment. This might be due to the effect of soluble bases and organic matter removal through sheet erosion from the control fields due to the absence of SWCPs, as it was reported by [
11] in the Weday watershed, eastern Ethiopia. Similarly, [
28] indicated low pH values in the untreated fields due to the low base saturation percentage and low sediment organic matter (SOM) content and high pH value in the sediment accumulation zone behind the SWCPs of the treated fields in the Anjeni watershed, central highlands of Ethiopia. In general, as per the ratings of [
50], the soil pH in the Lole watershed was slightly acidic (5.9–6.65), which is suitable for crop production, as most nutrients for field crops are available at pH values between 5.5–7.0 [
33].
The variations in soil pH were also statistically significant (
p ≤ 0.05) in different slope positions. The overall mean value of soil pH was found to be low in the upper slope (>15%), and high in the lower (<9%) slope positions. As the slope gradient increases, soil pH decreases. This might be due to the influence of the slope gradient through its effect of facilitating soil erosion and the leaching of soluble base cations, which in turn increased the concentration of H
+ ion in the soil solution and reduced soil pH. This result agreed with the findings of [
23] in Simada district, northwest Ethiopia. The difference in pH across the slope could also be associated with the distribution of SOM and CEC, as pH is positively and significantly correlated with SOM, CEC, and clay fraction (r = 0.66 **, r = 0.62 and r = 0.72, respectively).
Cation Exchange Capacity (CEC): CEC showed a statistically significant difference (
p ≤ 0.05) between the treated and untreated fields. Soils in the treated fields showed significantly higher CEC than the untreated fields. This finding implies that CEC was significantly influenced by the implementation of SWCPs, which might be due to the accumulation of SOM behind SWCPs. This was confirmed by the significant and positive correlation of SOM (r = 0.80 **) and clay content (r = 0.74 **) with CEC (Table 6). The result is in line with the reports of [
23] and [
11], in which higher mean CEC values were found in the treated than in the untreated fields in Adaa Berga district, central Ethiopia and the Weday watershed, eastern Ethiopia, respectively. Therefore, as per the ratings established by [
51], the CEC value in the Lole watershed was found to be high, which might be linked to the higher content of the clay particles.
On the other hand, the CEC values showed a statistically significant difference (
p ≤ 0.05) at different slope gradients. It was found to be low in the upper slope positions (>15%), and high in the lower (<9%) slope positions. As the slope gradient increased, the CEC value decreased. This might be due to the removal of basic cations from the upper slope and accumulation in the lower slope positions. This result is in line with the findings of [
23] and [
11], in which higher CEC values in the lower slope were found than those in the upper slope positions in Adaa Berga district, central Ethiopia and the Weday watershed, eastern Ethiopia, respectively.
Organic Carbon (OC): Soil organic carbon (SOC) showed a statistically significant difference (
p < 0.05) between the treated and untreated fields (
Table 2), which might be associated with sediment accumulation due to SWCPs and crop residues in the treated fields. According to [
42], due to SWCPs, high SOC (3.69%) was found in the treated Tsegur Kidanemihret micro-watershed compared with the untreated Tsegur Eyesus micro-watershed (2.24%), northwest Ethiopia. In general, as per the ratings of [
51], SOC content was found to be low in the Lole watershed, which might be due to intensive tillage, continuous cropping, and the removal of crop residues.
SOC also showed a statistically significant variation (
p < 0.05) between the different slope positions (
Table 2). Higher SOC was recorded at lower than higher slope gradients. SOC showed an inverse relationship with slope gradient; i.e., as slope gradient increases, SOC declines. This might be associated with the removal of organic matter from the higher slope areas and its subsequent deposition in the lower slope areas via water erosion. The result agrees with [
30] and [
13], who found that fertile soil deposition at a lower slope favored high crop biomass and residue, as well as SOC, in Mesobit-Gedba northern Ethiopia, and the Zikre watershed, northwest Ethiopia, respectively.
Total Nitrogen (TN): TN showed a statistically significant difference (
p ≤ 0.05) at different slope positions (
Table 3). High TN was recorded in the lower slope than in the higher slope gradients. This might be due to the removal of organic matter from the steep slopes via soil erosion. Similar results were reported by [
13,
24] in the Zikre watershed, Adaa Berga district, and by [
39] in the Dawja watershed, northwest Ethiopia.
Similarly, TN showed a statistically significant difference (
p ≤ 0.05) between the treated and untreated fields (
Table 3). The treated fields showed higher TN values than the untreated fields, which could be associated with the implementation of SWCPs that maintain soil fertility by decreasing the removal of SOC and TN through soil erosion. This finding is in line with [
31] and [
29], who found that higher TN content was recorded in treated fields compared with untreated fields in southern Ethiopia and northwest Ethiopia, respectively. The Pearson correlation coefficient also revealed that TN significantly and positively correlated with SOM (r = 0.80 **) (Table 6). This is because SOM is the main source of TN. TN also correlated positively with SOC because of increased biomass production, litter quantity, and organic matter decomposition. In general, TN was low in the untreated fields and medium in the treated fields, as per the ratings suggested by [
51], indicating that nitrogen is a limiting plant nutrient in the study area. This might be due to the limited use of nitrogen-containing inputs such as commercial fertilizer, plant residues, and animal manure.
Available Phosphorus (Av-P): Av-P showed a statistically significant difference between the treated and untreated fields (
Table 3). Low Av-P from untreated fields was due to continuous cultivation without SWCPs, extractive crops biomass harvest, and soil erosion, as indicated by the findings of [
11] and [
29,
31] in eastern and southern Ethiopia, respectively. Av-P also showed a positive and significant relationship with SOM and TN (r = 0.85). The Av-P of the study watershed was medium based on the rating of [
51].
Av-P significantly varied at different slope gradients (
p ≤ 0.05). Higher mean Av-P was recorded in the lower slope gradients than in the upper ones, which might be due to the washing out of topsoil and organic matter from the higher slope gradients and their subsequent accumulation at the lower gradient/deposition zone, which agrees with the findings of [
11] in the Weday watershed, eastern Ethiopia, reference [
40] in the Dawja watershed, northwest Ethiopia, and [
31] in Wenago district, southern Ethiopia. In contrast to this result, [
12] and [
28] reported that the mean values of Av-P were not significant at different slope gradients in the Goromti watershed, western Ethiopia, and the Anjeni watershed, central highlands of Ethiopia, respectively.
Available Potassium (Av-K): Av-K showed a statistically significant difference between treated and untreated fields (
p < 0.05;
Table 3). Higher Av-K was recorded in the lower slope (<9%) than in the upper slope (>15%) positions due to the transportation of potassium by erosion from steep slope areas to gentle/low slope, as reported in the findings of [
36] in Rwanda. In contrast to this, [
12] in the Goromti watershed, western Ethiopia revealed that Av-K didn’t show a significant difference (
p < 0.05) at different slope gradients. In general, TN showed significant difference, but Av-p and AV-k didn’t show a significant difference between fields treated with SB and SFSB.