Background

Environmental degradation caused by inappropriate land use is a worldwide problem that has attracted attention in sustainable agricultural production systems. Ethiopia is considered to be one of the least developed countries where agriculture had always played a central role in the country’s economy. Although agriculture has always been the mainstay of the economy, it is characterized by very low growth rate. The rapidly increasing population has led to a declining availability of cultivable land and a very high rate of soil erosion (Abera 2003). It is apparent that soil is one of the most important and determinant factors that strongly affects crop production. Soil is the foundation resource for nearly all land uses, and the most important component of sustainable agriculture (Mulugeta and Karl 2010). Therefore, assessment of soil quality indicators with respect to land use types, management practices and slope classes is useful and primary indicator for sustainable agricultural land management. Understanding the effect of these factors on soil properties is useful for devising land management strategies. The information can also be used to forecast the likely effects of any potential changes in land use types and management practices on soil properties. It is apparent that the destruction of vegetative cover can promote soil erosion, which eventually increases the magnitude of soil related constraints to crop production. Generally, a sound understanding of land use and management effects on soil properties provides an opportunity to evaluate sustainability of land use systems (Woldeamlak 2003).

There is increasing awareness that soil nutrient depletion from the agro ecosystem is a very widespread problem and an immediate crop production constraint in Ethiopia (Stoorvogel and Smaling 1990; Stoorvogel et al. 1993). A change in land use, poor soil management, topography of the area and socioeconomic activities can negatively affect the potential use of an area and may ultimately lead to land degradation and loss of productivity. Loss of arable land due to soil degradation is a wide spread phenomenon in the highlands of Ethiopia, which accounts for 45% of Ethiopian total land area and 66% of the total land area of Amhara Region (Lakew et al. 2006). Low soil fertility was reported as one of the major factors affecting crop production in west part of Amhara region (Yihenew 2002, 2007). Therefore, the objective of this research was to assess the effects of different land use types, management practices and slope classes on selected soil physico-chemical properties of Zikere watershed, North-western Ethiopia.

Results and discussion

Land use and slope versus soil physical properties

There was significant difference in percentage of sand, silt and clay contents (p < 0.05) among soils of different land use types. The mean value of sand fraction was lowest (18%) under the natural forest and highest (34.76%) in grazing lands (Table 1). Soils in natural forest and plantation forest had clay texture while crop and grazing lands had clay loam texture. This could probably be attributed to the selective removal of clay particles by erosion leaving the sand particles in the crop and freely grazed land. Under sparser vegetation covers, the clay fractions are likely to be lost to processes of erosion and migration down the soil profile (Woldeamlak 2003). In unprotected lands, the finer soil particles will be selectively removed by erosion, thereby increasing the proportion of the coarser particles in the soil which leaves more sand particles. In line with this, Mulugeta (2004) and Belayneh (2009) reported that deforestation, farming practices and intensive grazing change soil texture by aggravating soil erosion.

Table 1 Selected soil physical properties of different land use type and slope classes
Full size table

Bulk density was significantly different (p < 0.05) among land use types and slope classes. The mean value of bulk density was lowest (1.05 t m−3) under the natural forest and highest (1.33 t m−3) in grazing land. High bulk density values in grazing land and cultivated land might be due to the result of excessive wet season livestock trampling and continuous shallow depth cultivation and low organic matter input. This result was in line with the works of Islam and Weil (2000), Woldeamlak (2003), Mulugeta (2004) and Yihenew and Getachew (2013). The reason for the lowest soil bulk density on the forest land as well as in the lower slope position could be due to the higher clay content and accumulation of organic matter. Similarly, less disturbance of the land unlike other land uses could be the other reason.

Land use and slope versus soil chemical properties

Land use changes from forest to crop land resulted in reduction of soil pH. The highest (6.05) and the lowest (5.44) soil pH were recorded under the forest and the cultivated lands, respectively (Table 2). According to Mohammed (2003), the soil in high altitude and higher slopes had low pH values, probably suggesting the washing away of solutes and basic cations from these parts. Continuous cultivation practices, excessive precipitation, steepness of the topography and application of inorganic fertilizer could have attributed as some of the factors which are responsible for the reduction of pH in the soil. Mohammed (2003) also concluded that the lowest value of pH under the cultivated land could be due to the depletion of basic cations in crop harvest and leached to streams in runoff generated from accelerated erosions. The other reason could be higher microbial oxidation that produces organic acids, which provide H+ to the soil solution and, thereby, lowers soil pH. Generally, the pH values observed in the study area are within the ranges of strongly acidic to slightly acidic reactions according the rating of Foth and Ellis (1997).

Table 2 The effect of land use systems and slope classes on selected soil chemical properties
Full size table

The mean OM was significantly different (p < 0.05) among the land use types and slope classes (Table 2). Soil OM content was highest (5.01%) under the natural forest land and lowest (2.57%) on the cultivated land. The result was in agreement with that of Yihenew and Getachew (2013) who reported that lowest organic carbon was registered in cultivated land and highest in the natural forestland. The mean TN was significantly different among land use types and slope classes (p < 0.05). The average values of TN were highest (0.23%) on the natural forest and lowest (0.12%) under the cultivated land. The results were similar with the reports of Yihenew and Getachew (2013). The N contents in the lower slope soils of the forest land were medium; while on cultivated land and grazing lands were low based on classification of Landon (1991).

There was a significant difference of available P (AP) contents among various land uses and slope classes (p < 0.05) (Table 2). The AP content in the cultivated land appeared to be significantly higher than the rest land use types. Hence, the highest (6.18 mg kg−1) and the lowest (1.33 mg kg−1) AP contents were observed under the cultivated and the grazing lands, respectively (Table 2). Similar findings were reported by Gebeyaw (2007) and Woldeamlak (2003). Although the OM content of the cultivated land was lowest, AP content was highest under the cultivated land than the other land use types. This could be due to the application of Diammonium phosphate (DAP) fertilizer on the cultivated land in line with the explanation made by Woldeamlak (2003) and Gebeyaw (2007).

Cation exchange capacity (CEC) values were significantly different among the land use types and slope classes (p < 0.05) (Table 2). The highest (35.44 cmol(+) kg−1) and the lowest (26.08 cmol(+) kg−1) values were observed under natural forest and the cultivated lands, respectively. However, cultivated land and plantation forest showed statistically non-significant difference between them. It is a general truth that both clay and colloidal OM have the ability to absorb and hold positively charged ions. Thus, soils containing high clay and OM contents have high CEC. According to Belayneh (2009), land use highly significantly influenced the change in CEC.

Management practices versus soil properties

The analysis of variance showed the presence of significant difference (p < 0.05) in mean value of bulk density (BD) among management practices. The non-managed plots were found to exhibit significantly higher mean value of BD than the managed plots (Table 3). This could be attributed to the presence of significantly higher OM as a result of conservation measures and manure application. According to Mulugeta and Karl (2010), the non-conserved micro-watershed was found to exhibit significantly higher mean value of BD than the micro-watershed treated with soil and water conservation measures which had been attributed to the presence of significantly higher organic matter as a result of conservation measures. Similar result was reported by Tadele et al. (2013).

Table 3 The effect of management practices on selected soil physico-chemical properties
Full size table

There was significant difference in mean value of pH (p < 0.05) among the treatments (Table 3). The non-managed plot has the lowest pH value than other treatments. Soil reactions for all the soil samples were moderately acidic to slightly acidic (5.63 - 6.36). This result was in agreement with results of Alemayhu (2007) for Anjeni catchment who explained that the mean pH value of both the terraced and the non-terraced farm plots were rated moderately acidic. The same work discussed that the overall acidity situation might have resulted from the moist climatic conditions prevailing in the study areas.

Soil organic matter (OM) contents between the managed and non-managed plots were also significantly different (p < 0.05) (Table 3). The non-conserved plots had significantly lower mean value of OM than all other treatments considered in the study. The result agrees with the finding of Tadele et al. (2011). The variations in mean value of OM could have attributed to the effect of management practices implemented and biomass accumulated. However, physical soil and water conservation (SWC) measures complemented with organic manure application raised soil OM content better than soils with only soil bund construction. Mohammed (2003) showed that manure addition had significantly increased the amount of OM level at a nearby watershed called Anjeni. Total nitrogen content was significantly different among land management practices (p < 0.05) (Table 3). Physical SWC measures complemented with manure gave higher TN compared to other measures and the non-conserved lands. Mulugeta and Karl (2010) stated that physical SWC measures stabilized with nitrogen fixing plants have given much higher TN than other biological measures. They also discussed that the non-conserved land had the smallest mean value of TN compared to the conserved catchment.

The mean values of CEC were highly significantly different (p < 0.05) among land management practices (Table 3). High organic matter and clay contents increase CEC in soils (Yihenew and Getachew 2013). Similarly, Mulugeta and Karl (2010)) supported the idea that high clay soils can hold more exchangeable cations than a low clay containing soils. There was highly significant difference in mean value of AP among the land management practices (p < 0.05). The mean value of the same parameter from managed plots was higher than that of the non-managed plot. According to Mulugeta and Karl (2010), AP was significantly different between the conserved and non-conserved fields. The variation was reported to be due to the soil OM content difference.

Conclusions

From the study, it was possible to conclude that soil physico-chemical properties significantly vary among land use types, land management practices and slope classes. It was apparent that shift in land use types from natural forest to the other land use types had negative effect on soil physical and chemical properties. The overall qualities of the soils under the cultivated land were inferior to soils attributes of the adjacent natural forest, plantation forest and grazing lands. Therefore, integrated land management practices are the most effective ways in reducing soil erosion and increasing soil fertility of cultivated lands.

Methods

Description of the study area

The research was carried out in Zikre watershed, located in the highlands of north-western Ethiopia (Figure 1). The area is located at an approximate geographical coordinate of 10°41'0''- 10°42'15'' N latitude and 37°31'15" - 37°32'45'' E longitude. The watershed lies at Anjeni kebele (lowest administrative unit in Ethiopia), Dembecha district of West Gojjam Administrative Zone, Amhara National Regional State. It is found 15 km north of Dembecha town on the rural road leading to Feres Bet town. A perennial river called Zikre starts in the watershed and flows to the Blue Nile. The watershed covers a total land area of 267 ha and it is inhabited by about 2100 people distributed in the watershed, and consists of 345 households. Zikre watershed lies at altitude ranging from 2368 to 2585 m.a.s.l. The land use of the study area includes approximately 3.37% natural forest, 67.42% of cultivated land, 4.87% of plantation forest, 24.34% of pasture and grazing land. The mean annual rainfall of the areas is about 1834 mm and the mean annual temperature is 16°C. The long-term mean annual minimum and maximum temperatures of the area are 9.0°C and 23.3°C, respectively (Alemayhu 2007).

Figure 1
figure 1

Location map of the study area.

Full size image

Soil sampling

A reconnaissance survey was carried out for identifying representative soil sampling plots after which parallel transects were laid along contour lines. Separate composite samples were collected to evaluate the effect of land use types (natural forest, plantation forest, crop land and grazing land), management practices (cultivated land with 9-year manure plus soil bund, cultivated land with 9-year manure, cultivated land with soil bund only and cultivated land without management practices) and slope classes (upper, middle and lower) on soil physico-chemical properties (Figure 2). For treatments with soil bunds, soil samples were taken at the center of the two successive structures. Non-conserved plots refer to the area under cultivation that is found adjacent to the conserved plots that did not receive any soil conservation measure or manure application. Total samples collected for analysis were twelve with four treatments and three replications. All soil samples were bagged, labelled and transported to the laboratory for preparation and analysis.

Figure 2
figure 2

Land use types and sampling points.

Full size image

Soil analysis

The soil samples were air dried under the shade, ground using pestle & mortar and sieved to pass through 2 mm sieve. Soil bulk density was determined using undisturbed core sampling method following the procedures described in Black (1965). Particle size distribution (soil texture) was determined by the hydrometer method (Bouyoucos 1962) Soil pH was determined in a 1:2.5 soil to water suspension following the procedure outlined in Sahelemedhin and Taye (2000). The organic carbon content was analyzed by wet digestion method using the Walkley and Black oxidation method (Walkley and Black 1934). Soil organic matter (OM) content was calculated by multiplying soil organic carbon content by a factor of 1.724. The total nitrogen (TN) content was determined using the Kjeldahl method (Bremner and Mulvaney 1982); while the available phosphorus (AP) was determined following the Olsen procedure (Olsen et al. 1954). The cation exchange capacity (CEC) was determined after extraction of the samples with 1 N ammonium acetate (Chapman 1965).

Statistical data analysis

Analysis of variance (ANOVA) was done to determine the presence of significant difference among the treatments. In conditions where there was significant difference, mean comparison was performed with Duncan’s multiple range test (DMRT) using SAS software (SAS institute 2002).