3.2. Effect of land-use change on selected soil properties
3.2.1. Soil Physical properties
Soil texture
As the result in Table 2 revealed, the mean value of the percentage of sand in cultivated land was 61% and that of forest land was found to be 52% at depth of 0-20cm. The highest percent of sand was found in cultivated while the lowest was found in grazing land (50%). While, at the depth of 20-40cm the percent of sand under cultivated was 58%, forestland (45%), and that of grazing lands was 54%.
The mean values of silt at depth of 0-20cm under cultivated, grazing, and forest lands were 33.5%, 38%, and 43.5% respectively. While, at the depth of 20-40cm it was 30.5%, 37%, and 44% respectively. The percentage value of clay soil at depth of 0-20cm under cultivated, grazing, and forest lands were 5.5%, 10%, and 6.5% respectively. While, at depth of 20-40cm, the clay content under cultivated, grazing, and forest lands was 11.5%, 9%, and 11% respectively. The textural class for cultivated land at depth of 0-20cm and 20-40cm were sandy clay loam and sandy loam respectively. For grazing land, the textural class at both depths (0–20 and 20-40cm) was loam soil. The textural class at depth of 0–20 and 20-40cm was sandy loam. Unlike sandy soil, clay soil was higher in forest land than both cultivated and grazing lands. Despite, slightly the higher silt content was observed in grazing land and the lower was in cultivated land which agrees with research Lechisa et al. (2014) who reported that the highest silt content was observed forest land and the lower was in cultivated land.
In this finding, the highest sand contents (61%) in the cultivated land was in contrast to the result of Mesgistu et al. (2017) who said that the highest sand and silt were found in forest land in Warandhab area, Horo Guduru Wollega Zone, Oromia, Ethiopia. However, the highest silt contents (44%) at the depth of 20-40cm in forest soil land agrees with this finding. The highest sand content in cultivated land is perhaps due to a high amount of rainfall in the area that washed away the finer soil particles (clay) leaving behind the sand fractions (Getahun et al. 2016).
Table 2
Textural Class of the land-uses
Land Use | Depth (cm) | % Sand | % Silt | % Clay | Textural class |
Cultivated land | 0–20 | 61 | 33.5 | 5.5 | Sandy clay loam |
| 20–40 | 58 | 30.5 | 11.5 | Sandy loam |
Grazing land | 0–20 | 52 | 38 | 10 | Sandy loam |
| 20–40 | 54 | 37 | 9 | Sandy loam |
Forest land | 0–20 | 50 | 43.5 | 6.5 | Loam soil |
| 20–40 | 45 | 44 | 11 | Loam soil |
Bulk Density |
As indicated in Table 2, the bulk density (BD) values were slightly different for the land use types. The mean value of BD under forest, cultivated and grazing land at the depth of 0-20cm were 1.08 g cm− 3, 1.62 g cm− 3, and 1.63 g cm− 3 respectively. Whereas, at depth of 20-40cm it was 1.26 g cm− 3, 1.60 g cm− 3, and 1.31 g cm− 3 respectively. Comparatively cultivated land had significantly (p < 0.05) higher (1.62 g cm− 3) BD at depth of 0–20 cm. This is might be due to the compaction of the topsoil by cultivation or due to low organic matter. While forest land had the lowest (1.08 g cm− 3) bulk density which could be due to the presence of high organic matter. An increase in root penetration and biological activity might have decreased bulk density in forest soils. The result indicated that surface soil had significantly lower bulk density than subsurface soil. This could be due to the presence of higher organic matter in surface soil (Table 3). This agreed with the findings of Terefe et al. (2020) where the topsoil had less BD than the subsurface soil. This could also be due to overgrazing of the grazing land and the use of machinery or intensive agricultural practices in the cultivated land.
Soil Moisture
The moisture content (%) of the surface soils (0-20cm) of the study area is shown in Table 3. Accordingly, forest land, cultivated land, and grazing land have the mean value 20.6%,10.84%, and 14.76% respectively and at the depth of 20-40cm, the mean value of moisture in forest land, cultivated land, and grazing land were 25.85%, 10.86% and 15.12% respectively. The surface (0-20cm) soil had lower moisture content than that of the lower depth (20-40cm). This is might be due to solar heating at topsoil than the subsurface soil. Forest land had significantly higher moisture content, whereas cultivated land had the lowest moisture content; this implies that the soil under forest was covered by vegetation which results in low evaporation; water stays in the soil. This result agreed with (Duiker and Lal 2000; Post and Kwon 2000; Knowles and Singh, 2003; Baker, 2007) who reported soil protected by the superficial layer of organic matter improves the capture and the use of rainfall through increased water absorption and infiltration and decreased evaporation from the soil surface. This leads to reduced runoff and soil erosion with higher soil moisture throughout the season compared to the disturbed soils left unprotected (Bationo et al. 2007).
Table 3
Effects of land-use change on soil bulk density and soil moisture
Land uses | Depth (cm) | BD (g/cm3) | Moisture (%) |
Cultivated land | 0–20 | 1.62ab | 10.84e |
| 20–40 | 1.60b | 10.86e |
Grazing land | 0–20 | 1.63a | 14.76d |
| 20–40 | 1.31c | 15.12c |
Forest land | 0–20 | 1.08e | 20.6b |
| 20–40 | 1.26d | 25.85a |
p(0.05) | | 0.02 | 0.01 |
3.3. Soil Chemical properties
Soil pH
Soil pH was found to be significantly (p < 0.05) affected by land-use change and soil depth. Forest soil has the highest mean value of pH (6.20) at the depth of 20-40cm, whereas it was significantly lower (4.63) in cultivated lands at the depth of 0-20cm. The lower pH at the cultivated land when compared with forest land and grazing lands implies that the depletion of basic cations in crop harvest and the continuous use of ammonium-based fertilizers such as diammonium phosphate (NH4)2HPO4 in cultivated lands make the soil acidic. The oxidation of these fertilizers by soil micro-organisms produce inorganic acids. This acid releases H+ to the soil solution that in turn lowers the pH of the soil. In general, as the depth increase, the pH value decrease (acidity increase). This is because of the larger organic matter content observed in the surface soils across the land uses and the humified organic matter can bind tightly with Al3+ and reduces their activity in the soil solution which raises the soil pH. The present study agreed with (Ristow et al. 2010) who reported as the soil pH decreases, most desirable crop nutrients become less available while others, often undesirable, become more available and can reach toxic levels. The basic cations such as Ca2+ and Mg2+ also have shown a decreasing trend with increasing depth. The adsorption of these cations on the colloidal complex gives replacement of H+ and Al+ 3 which lowers the percentage of acid saturation which increases the soil pH.
Soil Organic Carbon
As presented in (Table 4), soil organic carbon (SOC) was significantly (p < 0.05) affected by land-use change and soil depth. In both soil depths (0-20cm and 20-40cm), SOC was lower in cultivated fields as compared to other land uses. The analysis of the effect of soil depth showed the highest SOC (3.79%) under forest soils at 0-20cm, While the lowest SOC (1.19%) was recorded in cultivated soils at the depth of 20-40cm. This agreed with Lal (2002) and Mengistu and Fassil (2010) who reported that soil organic carbon storage and soil nutrient availability are greater in forest land than in the cropland and pastures that replace them. Extensive deforestation and the conversion of natural forests into agricultural lands in the Ethiopia ecosystem led to a significant decline in organic matter levels. Woldeamlak and Stroonsnigder (2003) reported the conversion of forest land into grazing land and cultivated land led to a large decrease of organic matter at Chemo Watershed Ethiopia. Most cultivated soils of Ethiopia are poor in organic matter contents due to the low amount of organic materials applied to the soil and complete removal of the biomass from the field (Yihenew 2002), and due to steep relief condition, intensive cultivation, and excessive erosion hazards (Nyssen et al. 2004). In agreement with this, all agricultural fields in the study area had low organic carbon content according to the classification presented in Landon (1984). Tekalign (1991) also stated that an organic matter content of less than two (2%) as an indication of soil degradation for tropical soils involving a high risk of soil erosion.
Total nitrogen
The result presented in (Table 4) showed there were significant (p < 0.05) variations in total nitrogen among the land uses and soil depth. The mean value of total nitrogen at the depth of 0-20cm for cultivated land, forest land, and grazing land was 0.11%, 0.33%, and 0.20% respectively. While, at the depth of 20-40cm it is 0.10%, 0.27% and 0.15% respectively. Total nitrogen increased in the order of cultivated land < grazing land < forest land in the study area. The present finding agreed with (Nega 2006), who reported that the average total Nitrogen (N) increased from cultivated to grazing and forest land soils, which again declined with increasing depth from surface to subsurface soils. Since forest land had higher OM, higher TN was recorded in forest land than cultivated and grazing lands at both depths (Table 4). This agreed with the findings of Hazelton et al. (2007) where the total nitrogen was measured higher in soils with high organic matter.
Generally, cultivated soils had significantly lower TN at all depth as compared to grasslands and forest lands, which indicated continuous cultivation, ultimately reduced the total nitrogen contents in the soil. Due to land-use shifts from forest land to cultivated land, the TN content was declined, and also it was declined with increasing soil depth. This finding agreed with the findings of Nega (2006) in Senbat sub-watershed, western Ethiopia, where average TN increased from cultivated to grazing and forest soils. The minimum change of TN under cultivated land compared to forestland and grazing land shows that fertilizer applications may not have replaced the total N lost due to harvest removal, leaching, and humus losses associated with cultivation (Eyayu et al. 2009).
According to Malo et al. (2005), the lower level of nitrogen in cultivated fields compared to other land use implied that fertilizer additions have not replaced the TN lost due to harvest removal, leaching, and humus losses associated with cultivation. The continuous cultivation could have also aggravated OC oxidation and loss of N in cultivated fields resulting in the lowest contents.
Available Phosphorus
As shown in Table 4, the available P content (mg kg− 1) of the surface soils (0-20cm) in forest soil, cultivated soil, and grazing soil had the mean value 27.83, 12.63, and 4.17 respectively and at the depth of 20-40cm the mean value of available P in forest soil, cultivated soil and grazing land soil is 24.53, 10.43 and 2.43 respectively. The surface (0-20cm) soil has significantly (p < 0.05) higher available P content than that of the lower depth. The available P content of soils significantly declined due to the conversion of natural forests into grazing lands and farmland. The high content of available P in the forest land could be due to the high content of soil OM resulting in the release of organic phosphorous thereby enhance available P under forest land. Similarly, this result is in agreement with the findings of Abad et al. (2014) who reported that the available P was high in forest land as compared to pasture land and cultivated land at 0-30cm soil depth.
The higher available P content at both depths under forest land is likely the consequences of the long term litter accumulation and the associated increase in microbial activity. The results were in agreement with those of Materechera and Mkhabela (2001) who reported that organic matter influence P in soil solution by complexing P from the adsorption site in ligand exchange and increase the mobility of inorganic P, particularly in acid soils, by decreasing the chemical activity of iron and aluminum. The lack of available P in the soils limits the growth of plants. Cultivated land should be supplied with inorganic fertilizer to increase the concentration of P in the soil solution that is required by crops. Phosphorus is involved in several key plant functions, including energy transfer (Uchida 2000).
High availability of P existed in forest land and the lowest was in cultivated land. This is might be the pool of available P could be trees in the forest land with abundant microorganisms that return via litterfall to the soil (Asmamaw and Mohammed 2013). In agricultural land, available phosphorus (P) was reduced most probably due to a decline in both % SOC, CEC, and soil acidification (Achalu et al. 2012; Nisar et al. 2013). This finding also agrees with the result of Birhanu et al. (2016) who reported alteration of forest land to agricultural land decreased the availability of phosphorus in western Ethiopia.
In contrast to this finding, Mengistu et al. (2017) found higher available P in the soil of cultivated land as the result of crop residue left on cultivated land and later plowed in properly. In forests land, fallen vegetation cover and natural pruning of tree take time to return into the soil to decompose and increase SOM content, which lead to increases available Phosphorous content. Consequently, available P content was significantly (P < 0.05) different among different land-use systems (Alemayehu and Beyene 2013).
Exchangeable Cations (Ex.K, Ca and Mg)
The exchangeable K of soil under study area was significantly (p < 0.05) affected by land use type and depth of the soil (Table 4). Exchangeable K content at the depth of 0-20cm in forest land, cultivated land, and grazing land was 1.51 cmol(+) Kg− 1, 1.04 cmol(+) Kg− 1, and 1.13 cmol(+) Kg− 1respectively. At the depth of 20-40cm, Exchangeable K of forest land, cultivated land, and grazing land were 0.79, 0.88, and 0.44 cmol (+) Kg− 1 respectively. Soils of forest land had the highest exchangeable K (1.51 cmol(+) Kg− 1) content than soils of cultivated and grazing lands at the surface soil ( depth of 0-20cm) and exchangeable K content of the soil declined with increase in depth. This result is supported by previous findings that indicate the intensity of cultivation, and the use of acid-forming inorganic fertilizers affect the distribution of K in the soil system and enhance its depletion (Malo et al. 2005). Similarly, Yitbarek et al. (2013) reported that the exchangeable K of soil is higher in forest land than cultivated and grazing lands.
The analysis of variance results indicated that the exchangeable Ca was significantly (p < 0.05) affected by land-use types and soil depth. The presence of such significant variation on exchangeable Ca could be due to different management practices. At the depth of 0-20cm, the mean values of exchangeable Ca under forest, cultivated and grazing lands were 23.30 cmol (+) Kg− 1, 9.27 cmol (+) Kg− 1, and 14.43 cmol (+) Kg− 1respectively. However, at the depth of 20-40cm its mean value under forest, cultivated and grazing lands was 12.27Cmol (+) Kg− 1, 6.87 cmol (+) Kg− 1, and 8.17 cmol (+) Kg− 1 respectively. This implies that the exchangeable Ca was higher at the surface soil depth than that of the subsurface soil depth. This could be due to the abundance of animal and plant residues at the surface layer of soil than the subsurface soil layer. This is agreed with the findings of Alemayehu and Beyene (2013) who in their results revealed that the exchangeable Ca content of the soil was higher on the surface soil layer than the subsurface soil layer due to the association of biological activity and accumulation from plant residues. In contrast, Bore and Bedadi (2015) reported that the exchangeable Ca was increasing with increasing soil depth since it is susceptible and the possibility of easily leach downward by runoff and water percolation.
The analysis of variance results indicated the exchangeable Mg was significantly (p < 0.05) affected by land-use types and by soil depth. The exchangeable Mg concentrations followed a similar trend as that of Ca under the land uses. The mean value of exchangeable Mg at depth of 0-20cm under forest, cultivated, and grazing land uses were 12.70, 5.83, and 6.77 cmol(+) Kg− 1respectively. While, at the depth of 20-40cm, the mean values of exchangeable Mg in forest land, cultivated land, and grazing land were 5.47, 4.20, and 5.83 cmol(+) Kg− 1respectively. The highest and lower values of exchangeable Mg were found under grazing land and cultivated land respectively. The low exchangeable Ca and Mg under cultivated land might be due to leaching, soil erosion, and crop harvest as it was reported by Negassa (2001).
Cation Exchanging Capacity (CEC)
The analysis of variance results (Table 4) revealed that the CEC of the soils was significantly (p < 0.05) affected by the land-use change and the depth of the soil. The mean value of CEC under forest land, cultivated land, and grazing land at the depth of 0-20cm were 37.55, 25.20, and 27.31 cmol(+) Kg− 1 respectively. For the depth of 20-40cm, the mean values of forest land, cultivated land, and grazing land were 24.10, 24.04, and 23.86 cmol(+) Kg− 1 respectively (Table 4). This implies that the highest CEC value was found in the surface soil layer than subsurface. The significantly higher CEC in forest land might be due to the presence of higher soil organic matter. This result is in line with the findings of Yitbarek et al. (2013) who reported that the CEC of soil was higher in forest land compared to that of the adjacent cultivated and grazing lands in Abobo area, Western Ethiopia. This depletion of OC in farmland could be due to intensive cultivation. These results were also in agreement with the findings of Boke (2004) in Kembata-Tembaro southern Ethiopia and Negassa (2001) in Bako area, western Ethiopia
The CEC values of the soil decreased with depth consistently from forest land to cultivated land. Similarly, Nigussie et al. (2012) reported that the CEC of soil was higher in the subsurface of the soil layer under the adjacent forest, cultivated and grazing lands. But the result of the present study is unparalleled of the findings of Alemayehu and Beyene (2013) who reported that the CEC of the soil was not significantly affected by soil depth at the depth of 0-15cm and 15-30cm under adjacent maize, Enset and grasslands in Southern Ethiopia.
Table 4
Effects of land-use change on soil chemical properties
Soil depth (0-20cm) |
Land use | pH | % SOC | %TN | AvP | Ex.Ca | Ex.Mg | Ex.K | CEC |
Farm land | 4.63c | 1.33e | 0.11e | 12.63c | 9.27d | 5.83c | 1.13b | 25.20b |
Forest land | 6.02a | 3.79a | 0.35a | 27.83a | 23.30a | 12.70a | 1.51a | 37.55a |
Grazing land | 5.14b | 2.29c | 0.20c | 4.17e | 14.43b | 6.77b | 1.04b | 27.31c |
Soil depth (20-40cm) |
Farm land | 5.20b | 1.19f | 0.10f | 10.43d | 6.87e | 4.20a | 0.88cd | 24.04b |
Forest land | 6.02a | 3.10a | 0.28b | 24.52b | 12.27c | 6.17b | 0.79c | 24.10c |
Grazing land | 5.13b | 1.70d | 0.15d | 2.43f | 8.17a | 5.47a | 0.44e | 23.86b |
p(0.05) | 0.03 | 0.05 | 0.01 | 0.05 | 0.02 | 0.04 | 0.016 | 0.045 |