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Concentrations of exchangeable bases and cation exchange capacity in soils of cropland, grazing and forest in the Bale Mountains, Ethiopia
Forest Ecology and Management 256 (2008) 1298–1302
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Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco
Concentrations of exchangeable bases and cation exchange capacity in soils of cropland, grazing and forest in the Bale Mountains, Ethiopia Fantaw Yimer a,b,*, Stig Ledin b,1, Abdu Abdelkadir a,2 a b
Wondo Genet College of Forestry & Natural Resources, P.O. Box, 128, Shashemane, Ethiopia Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 August 2007 Received in revised form 17 June 2008 Accepted 23 June 2008
Conversion of native forest ecosystem to cropland has considerably degraded the soil nutrient levels in the Bale Mountains, south-eastern highlands of Ethiopia. This study investigated the effects of land use change through conversion of native forest to cropland and/or grazing land on soil pH (H2O), base cations (Ca2+, Mg2+, K+, Na+), CEC and percentage base saturation (PBS) in three adjacent land-use types: cropland, grazing land and native forest. A total of 108 soil samples (3 replications 3 land-use types 4 profiles 3 soil depth layers, 0–0.2, 0.2–0.4 and 0.4–1.0 m) were collected for laboratory analyses. Results showed that soil pH, Na+ and K+, CEC, and PBS varied significantly with respect to land use and soil depth while Ca2+and Mg2+ varied with soil depth (r < 0.05). Conversion of native forest ecosystem to cropland during a 15-year period significantly increased soil pH and PBS while reducing Na+ and K+. The CEC in the cropland was reduced by 37.7% (2.6% per annum) compared to the native forest, which could be attributed to the decline in organic matter concentrations. If such rapid declines in soil nutrient concentrations continue unabated, the soils will reach at the point of no return within a few decades. Although the effect of grazing on most of the properties was found to be minimal, adapting the number of stock to the carrying capacity of the land and thereby enhancing the natural regeneration, combined with proper cropland management practices could help restoring soil nutrients for sustainable agricultural production and ecosystem functions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nutrient dynamics Deforestation Land conversion Nutrient loss Soil property changes Land degradation
1. Introduction A change in land use, mainly through conversion of natural vegetation to cropland and/or grazing, influences many natural phenomena and ecological processes (Turner, 1989), leading to considerable changes in soil properties. Clearing of forests and their subsequent conversion into cropland deprives soil of its nutrient supply and storage, as well as its biological components (Sombroek et al., 1993; Wairiu and Lal, 2003; Rasiah et al., 2004). Studies (e.g. Lugo et al., 1986; Lepsch et al., 1994) indicate that conversion of native vegetation into cultivation in tropical regions causes important changes in soil properties, including loss of organic matter, increase in bulk density and decreases in exchangeable cations and base saturation. These changes have
* Corresponding author at: Wondo Genet College of Forestry & Natural Resources, P.O. Box, 128, Shashemane, Ethiopia. Tel.: +251 461 109900; fax: +251 461 109983. E-mail addresses: [email protected] (F. Yimer), [email protected] (S. Ledin), [email protected] (A. Abdelkadir). 1 Fax: +46 18 672795. 2 Fax: +251 461 109983. 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.06.047
been shown to affect soil fertility (Piccolo et al., 1994; Tiessen et al., 1994; Fernandes et al., 1997; Neill et al., 1997; Dominy et al., 2002). In most tropical soils, cation exchange capacity (CEC) and exchangeable base concentrations become limiting factors in soil productivity (Saikh et al., 1998). Consequently, many agricultural soils in the tropics are now below their potential production levels (Sombroek et al., 1993). Grazing is by far the most common land use practice in virtually all parts of the world (Raun and Peterson, 1986). The potential of these grazing lands is undermined by exploitative land use practices due to rapidly increasing human and livestock populations (Bunderson, 1986). Grazing influences plant species composition, net primary productivity, above- and below-ground nutrient allocation in plants, nutrient cycling and water infiltration (Burke et al., 1998). In areas where over-grazing has seriously degraded vegetation cover and primary production, soil carbon content is lower due to low levels of plant residues, increased erosion losses and reduced organic matter inputs. Such practices have caused widespread soil degradation, thereby causing a decrease in soil carbon and related nutrient concentrations (Bruce et al., 1999).
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Fig. 1. Location map of the study area.
This report complements a previous study (Yimer et al., 2007) in which the same samples were analysed for changes in soil organic carbon, total nitrogen concentrations and carbon–nitrogen ratios. Here, further studies were carried out to assess the effects of converting native forest into cropland and grazing on soil exchangeable bases, CEC and percentage base saturation in the Afromontane vegetation zone of the Bale Mountains, southeastern Ethiopia. 2. Materials and methods 2.1. Study site The study was carried out on the southern slope of the Bale Mountains, situated between 68450 N and 398450 E and approximately 60 km south of Goba town, Ethiopia (Fig. 1). The study site lies between 3000 and 3200 m above sea level and receives a mean annual precipitation of 1064 mm (Yimer et al., 2006a,b). The estimated mean annual temperature ranges from 13.3 to 14.1 8C. Geologically, the area consists of rocks of volcanic origin welded with volcanic ash materials (Mohr, 1971; Berhe et al., 1987), weathered to mainly black (10YR 2/1, moist) to very dark brown (7.5YR 2.5/2, moist) sandy loam to loam in the A-horizon and sandy loam to loam–clay loam in the B-horizon soils under the natural forest (Yimer et al., 2006b). According to an earlier reconnaissance study (Weinert and Mazurek, 1984), andosols were considered as the most prevalent soils in the Bale massif. Andosols have a unique combination of physical and chemical properties (e.g. low bulk density, large variable charge, large water storage, high phosphate retention and high accumulation of organic matter) (Shoji et al.,
1993; Yimer, 1996; Delvaux et al., 2004; Yimer et al., 2006a). Selected soil properties of the study area are presented in Table 1. The natural vegetation (hereafter referred to as ‘native forest’) is dominated by Schefflera abyssinica and Hagenia abyssinica and the understorey consists of small trees and shrubs such as Brucea antidysenterica, Cassipoourea malosana, Rubus apetalus, Dombeya torrida, Allophyllus abyssinicus, Rapanea simensis, Euphorbia dumalis, Vernonia urticifolica and Echinops macrochaltus. The ground cover of this forest site is rich in herbaceous plants. Farming in the study area comprises a mix of crop and livestock production systems, but nutrient flows between the two are predominantly one-sided, with feeding of crop residues to livestock around the homesteads. Instead of dispersing dung in fields, farmers used to burn in households, suggesting a negative nutrient balance. Crop production through forest clearance started in 1991 (oral communication with elder farmers). Barley, which is a major food crop, is cultivated almost continuously below 3300 m and may extend far above depending on the soil and slope. In the barley cultivation, the soil is pulverized twice through oxen ploughing 2 months earlier from the summer ‘‘kremt’’ rainfall. Grazing is carried out mainly on the communal grazing fields and on cropland after harvest. 2.2. Soil sampling and laboratory analyses Three blocks of land each containing the three land-use types (cropland that had been under barley cultivation for 15 years; communal grazing that had never been cultivated; and native forest) were selected within three altitudinal zones ranging from 3000 to 3150 m at an interval of 50 m from each other. Four soil
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Table 1 Soil organic carbon (%), textural fractions (%) and bulk density (B.d., g cm3) in relation to land use type and soil depth (mean S.E.) Treatment
Property SOC
B.d.
Sand
Silt
Clay
Land use type Crop land Grazing Native forest
3.7 (0.3)b 5.3 (0.3)a 5.3 (0.3)a
0.95 (0.02)a 0.89 (0.03)a 0.79 (0.02)b
45.4 (1.5)b 50.0 (1.5)ab 52.1 (2. 0)a
41.4 (0.8)a 41.8 (1. 5)a 38.7 (1. 4)a
13.1 (1.4)a 8.2 (1.0)b 9.2 (1.2)b
Soil depth (m) 0–0.2 0.2–0.4 0.4–1.0
6.0 (0.3)a 4.6 (0.3)b 3.7 (0.3)c
0.78 (0.02)b 0.90 (0.02)a 0.95 (0.03)a
53.2 (1.5)a 49.4 (1.5)ab 44.8 (1.9)b
39.3 (1.2)a 41.0 (1.1)a 41.6 (1.1)a
7.4 (0.7)b 9.6 (0.9)b 13.6 (1.6)a
Means within columns followed by different letters are significantly different (p < 0.05) with respect to land use and soil depth.
Table 2 Summary of two-way ANOVA results for soil pH (H2O), base cations (mmol kg1), CEC (mmol kg1) and percentage base saturation (PBS) in relation to land use and soil depth Source of variation
d.f.
Land use (LU) Depth (D) LU D Error
2 2 4 98
pH
Na
K
Ca
Mg
CEC
PBS
MS
p
MS
p
MS
p
MS
p
MS
p
MS
p
MS
p
6.450 0.575 0.493 0.237
<0.001 0.093 0.089
0.360 0.013 0.010 0.038
<0.001 0.708 0.905
3.541 2.517 0.676 0.695
0.008 0.030 0.427
13.986 196.56 6.947 16.795
0.438 <0.001 0.798
2.051 21.841 3.844 1.824
0.329 <0.001 0.086
2397.42 881.000 32.478 37.379
<0.001 <0.001 0.486
3110.14 771.037 86.260 31.553
<0.001 0.015 0.744
profiles in each of the land-use types were opened within similar topographical conditions such as landscape position and percentage slope. A total of 108 soil samples (3 replications 3 land-use types 4 profiles 3 soil depth layers, 0–0.2, 0.2–0.4 and 0.4– 1.0 m) were collected during the cropping period for laboratory analyses. Samples for chemical analyses were passed through a 2-mm soil sieve. Exchangeable base cations were extracted with 1N ammonium acetate at pH 7. Calcium and magnesium were determined by atomic absorption spectrophotometry, while sodium and potassium were analysed by flame emission spectrophotometry (Black et al., 1965). Soil pH was measured with combined electrodes in a 1:2.5 soil:water suspension. Cation exchange capacity was quantified titrimetrically by distillation of ammonium displaced by sodium (Chapman, 1965) at the National Soil Laboratory and Research Centre in Addis Ababa, Ethiopia. Percentage base saturation (PBS) was calculated by dividing the sum of the charge equivalents of the base cations (Ca2+, Mg2+, K+ and Na+) by the CEC of the soil and multiplying by 100. Soil organic carbon data from our earlier report (Yimer et al., 2007) were also used to evaluate the associations with the soil CEC. Statistical differences in soil properties analysed among landuse types and soil depths were tested using two-way analysis of variance (ANOVA) following the general linear model (GLM) procedure of SPSS Version 12.0.1 for Windows (Julie, 2001). Means that exhibited significant differences were compared using Tukey’s Honest Significance Difference (HSD) at 5% probability level. Linear regression analysis was performed to examine the relationship between some soil properties.
in its consistence; and has clear and smooth to wavy boundaries. Across all land-use types and altitudinal ranges, there was no significant difference in the clay content in the top 0.2-m depth of soil. However, differences were observed in soil below 0.2-m depth with respect to altitudinal range; lower in the upper than in the middle altitudinal ranges. The study results showed that land use type significantly affected soil pH, concentrations of base cations (Na+ and K+), CEC and percentage base saturation (Table 2). There were also significant differences in the concentrations of bases (K+, Ca2+ and Mg2+), CEC and PBS between the soil depths. Soil pH and concentrations of base cations from different land-use types and soil depths are presented in Table 3. Soil pH values in the native forest and grazing lands were lower than in cropland soils. The concentrations of Na+ and K+ were lower in cropland than in grazing and native forest. For all land-use types, the overall mean concentrations of K+, Ca2+ and Mg2+ were higher in the topsoil (0– 0.20 m depth) than in the subsoil below 0.2-m depth. The CEC and PBS of soils from different land-use types and soil depths are presented in Table 3. The CEC of the soils was lowest in cropland, followed by grazing land, and highest in the native forest. CEC values were significantly correlated with soil organic carbon concentration (r2 = 0.66, p < 0.001), and increased with increasing soil organic carbon (Fig. 2). Percentage base saturation was higher in cropland than in either native forest or grazing land. It was also higher in the topsoil than in the subsoil. Percentage base saturation was correlated positively with some selected soil properties. Soil pH accounted for 67% (r2 = 0.67, p < 0.001) of the variation in PBS, followed by Ca2+ (r2 = 0.59, p < 0.001) and Mg2+ (r2 = 0.39, p < 0.001) in the 1.0 m soil profile.
3. Results 4. Discussion The morphology of the surface horizon soil under forest and grazing was marked by black (10YR 2/1, moist), and by very dark grayish brown (7.5YR 3/2, moist) colour under cultivation. Irrespective of the land-use types, the soil is generally characterized by slightly gravelly sandy loam to loam; moderately fine to medium granular structure; non-sticky/non-plastic to slightly sticky/slightly plastic (wet), friable (moist), soft (dry), and smeary
The soil textural fractions showed variation with land-use types and altitudinal ranges. In this soil, the overall clay content was slightly larger and sand content tended to be smaller in cultivated as compared to grazing and forest. Cultivation promotes further weathering processes as it shears and pulverizes the soil and changes the moisture and temperature regimes (Reicosky and
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Table 3 Soil pH (H2O) and concentrations of base cations (mmol kg1), CEC (mmol kg1), and percentage base saturation in relation to land use type and soil depth (mean S.E.) Variable
pH Na K Ca Mg CEC PBS
Land use type
Soil depth (m)
Cropland
Grazing
Native forest
0–0.2
0.2–0.4
0.4–1.0
6.0 0.1 0.5 7.8 2.6 26.1 42.7
5.6 0.2 1.0 6.6 2.6 37.3 27.7
5.2 0.2 1.0 7.3 3.0 42.0 25.7
5.8 0.2 1.1 9.8 3.6 40.2 37.4
5.6 0.2 0.9 6.6 2.4 34.9 29.5
5.5 0.2 0.6 5.3 2.2 30.1 29.3
(0.1)a (0.1)b (0.1)b (0.6)a (0.2)a (1.1)c (2.7)a
(0.1)b (0.1)a (0.2)a (0.5)a (0.2)a (1.0)b (1.4)b
(0.1)c (0.1)a (0.1)a (1.0)a (0.4)a (1.5)a (2.5)b
(0.1)a (0.1)a (0.2)a (0.9)a (0.3)a (1.6)a (2.8)a
(0.1)a (0.03)a (0.2)ab (0.7)b (0.2)b (1.5)b (2.3)b
(0.1)a (0.1)a (0.1)b (0.4)b (0.2)b (1.4)c (2.5)b
Means within rows followed by different letters are significantly different (p < 0.05) with respect to land use/soil depth.
Fig. 2. Relationship between soil organic carbon and CEC.
Forcella, 1998). The differences in textural fractions were also due to some variations in the pedogenic processes, especially in relation to rate of weathering; slower in the upper than in other altitudinal ranges. Our findings revealed that once the native forest is converted to cropland and grazing land-use types, significant changes occurred in soil properties. A change in land use from native vegetation to cropland tends to increase soil pH in both the surface and subsurface layers (Lumbanraja et al., 1998). In our study, the increase in soil pH in cropland soils might be related to the addition of potash through the traditional slash and burn practices. According to Moraes et al. (1996), burning releases nutrients in the ash, which increases soil pH. As a response of conversion of native forest to cropland, concentrations of base cations, mainly Na+ (75%) and K+ (53.8%) were significantly lower than in the other land-use types investigated due to removal by the harvest. The lower concentrations of base cations in the subsoil layers compared with the topsoil suggest that vegetation pumps bases from the subsoil to the topsoil. The higher topsoil base concentration (in spite of the lack of fertiliser amendment) could also be related to the slash and burn process. The range of Mg2+ concentration in the soils under the different land-use types was 2.6–3.0 mmol kg1, which is considerably higher than the critical level of 0.5 mmol kg1 reported for both tropical and temperate soils (Landon, 1991; McAlister et al., 1998). Changes in the CEC of soils due to land use changes can be quite considerable. In the present study, the CEC values across all landuse types varied significantly due to differences in the amounts of soil organic matter (carbon) concentrations. According to Yimer et al. (2007), soil organic carbon concentrations in the 1.0 m soil layer varied significantly with respect to land use type and soil depth. The regression analysis also revealed the strong association between soil organic carbon concentrations and CEC. Taking the fairly low clay content into consideration, it is obvious that the contribution to CEC by organic substances is critical. Topsoil after
15 years of cultivation showed a 36.7% reduction in CEC compared with topsoil in the adjacent native forest. Studies elsewhere (e.g. Saikh et al., 1998) also reported that soils cultivated for 5 and 16 years showed a decline in CEC of 43 and 27%, respectively, compared with forest soils. The high CEC value in the native forest soils is consistent with other findings showing a strong relationship between CEC and concentrations of soil organic carbon (e.g. Tegene, 2000; Eshetu et al., 2004). This finding has some important implications because it indicates that soil CEC is not likely to be increased to any significant level except by improving the organic carbon concentration in cultivated soils. The percentage base saturation of the soils studied here was significantly higher in the cropland than in native forest and in grazing land. Base cations stored in wood and shrubs are released at burning and replace the Al3+ released through H+ buffering. Naturally, the decrease in exchange sites and in organic matter concentrations and the increase in base cations also leads to higher base saturation. 5. Conclusions This study found that as a response of conversion of native forest to cropland significant changes followed in the chemical properties of soils. Soils collected from three land-use types in the Bale Mountains shown marked differences in their pH, concentration of base cations, CEC and percentage base saturation. Although not pronounced, free grazing has had effect on the CEC and other properties of the soil. Therefore, improving the existing land use practices and implementing better cropland management as well as introducing controlled grazing system could help to restore the soil nutrient levels for sustainable agricultural production and ecosystem functioning. Acknowledgements This study was financed through SIDA support to Wondo Genet College of Forestry, Ethiopia. Special thanks to Dr. Mulualem Tigabu, Faculty of Forest Sciences/SLU, Umea˚, and to an anonymous reviewer for their valuable comments on the manuscript. We also thank staff members of the Bale Mountains National Park headquarters; late Sileshi Kifle and the Rira village communities for their assistance during the fieldwork. References Berhe, S.M., Desta, B., Nicoletti, M., Teferra, M., 1987. Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia. Journal of Geological Society 144, 213–226. Black, C.A., Evans, D.D., White, J.L., Ensminger, L.E., Clark, F.E., 1965. Methods of Soil Analysis. Part 1. Physical and Mineralogical Properties including Statistics of Measurement and Sampling. Am. Soc. Agro. Inc.,, Madison, WI. Bruce, J.P., Frome, M., Haites, E., Janzen, H., Lal, R., Paustin, K., 1999. Carbon sequestration in soils. Journal of Soil and Water Conservation 54 (1), 382–389.
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Concentrations of exchangeable bases and cation exchange capacity in soils of cropland, grazing and forest in the Bale Mountains, Ethiopia Fantaw Yimer a,b,*, Stig Ledin b,1, Abdu Abdelkadir a,2 a b
Wondo Genet College of Forestry & Natural Resources, P.O. Box, 128, Shashemane, Ethiopia Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 August 2007 Received in revised form 17 June 2008 Accepted 23 June 2008
Conversion of native forest ecosystem to cropland has considerably degraded the soil nutrient levels in the Bale Mountains, south-eastern highlands of Ethiopia. This study investigated the effects of land use change through conversion of native forest to cropland and/or grazing land on soil pH (H2O), base cations (Ca2+, Mg2+, K+, Na+), CEC and percentage base saturation (PBS) in three adjacent land-use types: cropland, grazing land and native forest. A total of 108 soil samples (3 replications 3 land-use types 4 profiles 3 soil depth layers, 0–0.2, 0.2–0.4 and 0.4–1.0 m) were collected for laboratory analyses. Results showed that soil pH, Na+ and K+, CEC, and PBS varied significantly with respect to land use and soil depth while Ca2+and Mg2+ varied with soil depth (r < 0.05). Conversion of native forest ecosystem to cropland during a 15-year period significantly increased soil pH and PBS while reducing Na+ and K+. The CEC in the cropland was reduced by 37.7% (2.6% per annum) compared to the native forest, which could be attributed to the decline in organic matter concentrations. If such rapid declines in soil nutrient concentrations continue unabated, the soils will reach at the point of no return within a few decades. Although the effect of grazing on most of the properties was found to be minimal, adapting the number of stock to the carrying capacity of the land and thereby enhancing the natural regeneration, combined with proper cropland management practices could help restoring soil nutrients for sustainable agricultural production and ecosystem functions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nutrient dynamics Deforestation Land conversion Nutrient loss Soil property changes Land degradation
1. Introduction A change in land use, mainly through conversion of natural vegetation to cropland and/or grazing, influences many natural phenomena and ecological processes (Turner, 1989), leading to considerable changes in soil properties. Clearing of forests and their subsequent conversion into cropland deprives soil of its nutrient supply and storage, as well as its biological components (Sombroek et al., 1993; Wairiu and Lal, 2003; Rasiah et al., 2004). Studies (e.g. Lugo et al., 1986; Lepsch et al., 1994) indicate that conversion of native vegetation into cultivation in tropical regions causes important changes in soil properties, including loss of organic matter, increase in bulk density and decreases in exchangeable cations and base saturation. These changes have
* Corresponding author at: Wondo Genet College of Forestry & Natural Resources, P.O. Box, 128, Shashemane, Ethiopia. Tel.: +251 461 109900; fax: +251 461 109983. E-mail addresses: [email protected] (F. Yimer), [email protected] (S. Ledin), [email protected] (A. Abdelkadir). 1 Fax: +46 18 672795. 2 Fax: +251 461 109983. 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.06.047
been shown to affect soil fertility (Piccolo et al., 1994; Tiessen et al., 1994; Fernandes et al., 1997; Neill et al., 1997; Dominy et al., 2002). In most tropical soils, cation exchange capacity (CEC) and exchangeable base concentrations become limiting factors in soil productivity (Saikh et al., 1998). Consequently, many agricultural soils in the tropics are now below their potential production levels (Sombroek et al., 1993). Grazing is by far the most common land use practice in virtually all parts of the world (Raun and Peterson, 1986). The potential of these grazing lands is undermined by exploitative land use practices due to rapidly increasing human and livestock populations (Bunderson, 1986). Grazing influences plant species composition, net primary productivity, above- and below-ground nutrient allocation in plants, nutrient cycling and water infiltration (Burke et al., 1998). In areas where over-grazing has seriously degraded vegetation cover and primary production, soil carbon content is lower due to low levels of plant residues, increased erosion losses and reduced organic matter inputs. Such practices have caused widespread soil degradation, thereby causing a decrease in soil carbon and related nutrient concentrations (Bruce et al., 1999).
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Fig. 1. Location map of the study area.
This report complements a previous study (Yimer et al., 2007) in which the same samples were analysed for changes in soil organic carbon, total nitrogen concentrations and carbon–nitrogen ratios. Here, further studies were carried out to assess the effects of converting native forest into cropland and grazing on soil exchangeable bases, CEC and percentage base saturation in the Afromontane vegetation zone of the Bale Mountains, southeastern Ethiopia. 2. Materials and methods 2.1. Study site The study was carried out on the southern slope of the Bale Mountains, situated between 68450 N and 398450 E and approximately 60 km south of Goba town, Ethiopia (Fig. 1). The study site lies between 3000 and 3200 m above sea level and receives a mean annual precipitation of 1064 mm (Yimer et al., 2006a,b). The estimated mean annual temperature ranges from 13.3 to 14.1 8C. Geologically, the area consists of rocks of volcanic origin welded with volcanic ash materials (Mohr, 1971; Berhe et al., 1987), weathered to mainly black (10YR 2/1, moist) to very dark brown (7.5YR 2.5/2, moist) sandy loam to loam in the A-horizon and sandy loam to loam–clay loam in the B-horizon soils under the natural forest (Yimer et al., 2006b). According to an earlier reconnaissance study (Weinert and Mazurek, 1984), andosols were considered as the most prevalent soils in the Bale massif. Andosols have a unique combination of physical and chemical properties (e.g. low bulk density, large variable charge, large water storage, high phosphate retention and high accumulation of organic matter) (Shoji et al.,
1993; Yimer, 1996; Delvaux et al., 2004; Yimer et al., 2006a). Selected soil properties of the study area are presented in Table 1. The natural vegetation (hereafter referred to as ‘native forest’) is dominated by Schefflera abyssinica and Hagenia abyssinica and the understorey consists of small trees and shrubs such as Brucea antidysenterica, Cassipoourea malosana, Rubus apetalus, Dombeya torrida, Allophyllus abyssinicus, Rapanea simensis, Euphorbia dumalis, Vernonia urticifolica and Echinops macrochaltus. The ground cover of this forest site is rich in herbaceous plants. Farming in the study area comprises a mix of crop and livestock production systems, but nutrient flows between the two are predominantly one-sided, with feeding of crop residues to livestock around the homesteads. Instead of dispersing dung in fields, farmers used to burn in households, suggesting a negative nutrient balance. Crop production through forest clearance started in 1991 (oral communication with elder farmers). Barley, which is a major food crop, is cultivated almost continuously below 3300 m and may extend far above depending on the soil and slope. In the barley cultivation, the soil is pulverized twice through oxen ploughing 2 months earlier from the summer ‘‘kremt’’ rainfall. Grazing is carried out mainly on the communal grazing fields and on cropland after harvest. 2.2. Soil sampling and laboratory analyses Three blocks of land each containing the three land-use types (cropland that had been under barley cultivation for 15 years; communal grazing that had never been cultivated; and native forest) were selected within three altitudinal zones ranging from 3000 to 3150 m at an interval of 50 m from each other. Four soil
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Table 1 Soil organic carbon (%), textural fractions (%) and bulk density (B.d., g cm3) in relation to land use type and soil depth (mean S.E.) Treatment
Property SOC
B.d.
Sand
Silt
Clay
Land use type Crop land Grazing Native forest
3.7 (0.3)b 5.3 (0.3)a 5.3 (0.3)a
0.95 (0.02)a 0.89 (0.03)a 0.79 (0.02)b
45.4 (1.5)b 50.0 (1.5)ab 52.1 (2. 0)a
41.4 (0.8)a 41.8 (1. 5)a 38.7 (1. 4)a
13.1 (1.4)a 8.2 (1.0)b 9.2 (1.2)b
Soil depth (m) 0–0.2 0.2–0.4 0.4–1.0
6.0 (0.3)a 4.6 (0.3)b 3.7 (0.3)c
0.78 (0.02)b 0.90 (0.02)a 0.95 (0.03)a
53.2 (1.5)a 49.4 (1.5)ab 44.8 (1.9)b
39.3 (1.2)a 41.0 (1.1)a 41.6 (1.1)a
7.4 (0.7)b 9.6 (0.9)b 13.6 (1.6)a
Means within columns followed by different letters are significantly different (p < 0.05) with respect to land use and soil depth.
Table 2 Summary of two-way ANOVA results for soil pH (H2O), base cations (mmol kg1), CEC (mmol kg1) and percentage base saturation (PBS) in relation to land use and soil depth Source of variation
d.f.
Land use (LU) Depth (D) LU D Error
2 2 4 98
pH
Na
K
Ca
Mg
CEC
PBS
MS
p
MS
p
MS
p
MS
p
MS
p
MS
p
MS
p
6.450 0.575 0.493 0.237
<0.001 0.093 0.089
0.360 0.013 0.010 0.038
<0.001 0.708 0.905
3.541 2.517 0.676 0.695
0.008 0.030 0.427
13.986 196.56 6.947 16.795
0.438 <0.001 0.798
2.051 21.841 3.844 1.824
0.329 <0.001 0.086
2397.42 881.000 32.478 37.379
<0.001 <0.001 0.486
3110.14 771.037 86.260 31.553
<0.001 0.015 0.744
profiles in each of the land-use types were opened within similar topographical conditions such as landscape position and percentage slope. A total of 108 soil samples (3 replications 3 land-use types 4 profiles 3 soil depth layers, 0–0.2, 0.2–0.4 and 0.4– 1.0 m) were collected during the cropping period for laboratory analyses. Samples for chemical analyses were passed through a 2-mm soil sieve. Exchangeable base cations were extracted with 1N ammonium acetate at pH 7. Calcium and magnesium were determined by atomic absorption spectrophotometry, while sodium and potassium were analysed by flame emission spectrophotometry (Black et al., 1965). Soil pH was measured with combined electrodes in a 1:2.5 soil:water suspension. Cation exchange capacity was quantified titrimetrically by distillation of ammonium displaced by sodium (Chapman, 1965) at the National Soil Laboratory and Research Centre in Addis Ababa, Ethiopia. Percentage base saturation (PBS) was calculated by dividing the sum of the charge equivalents of the base cations (Ca2+, Mg2+, K+ and Na+) by the CEC of the soil and multiplying by 100. Soil organic carbon data from our earlier report (Yimer et al., 2007) were also used to evaluate the associations with the soil CEC. Statistical differences in soil properties analysed among landuse types and soil depths were tested using two-way analysis of variance (ANOVA) following the general linear model (GLM) procedure of SPSS Version 12.0.1 for Windows (Julie, 2001). Means that exhibited significant differences were compared using Tukey’s Honest Significance Difference (HSD) at 5% probability level. Linear regression analysis was performed to examine the relationship between some soil properties.
in its consistence; and has clear and smooth to wavy boundaries. Across all land-use types and altitudinal ranges, there was no significant difference in the clay content in the top 0.2-m depth of soil. However, differences were observed in soil below 0.2-m depth with respect to altitudinal range; lower in the upper than in the middle altitudinal ranges. The study results showed that land use type significantly affected soil pH, concentrations of base cations (Na+ and K+), CEC and percentage base saturation (Table 2). There were also significant differences in the concentrations of bases (K+, Ca2+ and Mg2+), CEC and PBS between the soil depths. Soil pH and concentrations of base cations from different land-use types and soil depths are presented in Table 3. Soil pH values in the native forest and grazing lands were lower than in cropland soils. The concentrations of Na+ and K+ were lower in cropland than in grazing and native forest. For all land-use types, the overall mean concentrations of K+, Ca2+ and Mg2+ were higher in the topsoil (0– 0.20 m depth) than in the subsoil below 0.2-m depth. The CEC and PBS of soils from different land-use types and soil depths are presented in Table 3. The CEC of the soils was lowest in cropland, followed by grazing land, and highest in the native forest. CEC values were significantly correlated with soil organic carbon concentration (r2 = 0.66, p < 0.001), and increased with increasing soil organic carbon (Fig. 2). Percentage base saturation was higher in cropland than in either native forest or grazing land. It was also higher in the topsoil than in the subsoil. Percentage base saturation was correlated positively with some selected soil properties. Soil pH accounted for 67% (r2 = 0.67, p < 0.001) of the variation in PBS, followed by Ca2+ (r2 = 0.59, p < 0.001) and Mg2+ (r2 = 0.39, p < 0.001) in the 1.0 m soil profile.
3. Results 4. Discussion The morphology of the surface horizon soil under forest and grazing was marked by black (10YR 2/1, moist), and by very dark grayish brown (7.5YR 3/2, moist) colour under cultivation. Irrespective of the land-use types, the soil is generally characterized by slightly gravelly sandy loam to loam; moderately fine to medium granular structure; non-sticky/non-plastic to slightly sticky/slightly plastic (wet), friable (moist), soft (dry), and smeary
The soil textural fractions showed variation with land-use types and altitudinal ranges. In this soil, the overall clay content was slightly larger and sand content tended to be smaller in cultivated as compared to grazing and forest. Cultivation promotes further weathering processes as it shears and pulverizes the soil and changes the moisture and temperature regimes (Reicosky and
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Table 3 Soil pH (H2O) and concentrations of base cations (mmol kg1), CEC (mmol kg1), and percentage base saturation in relation to land use type and soil depth (mean S.E.) Variable
pH Na K Ca Mg CEC PBS
Land use type
Soil depth (m)
Cropland
Grazing
Native forest
0–0.2
0.2–0.4
0.4–1.0
6.0 0.1 0.5 7.8 2.6 26.1 42.7
5.6 0.2 1.0 6.6 2.6 37.3 27.7
5.2 0.2 1.0 7.3 3.0 42.0 25.7
5.8 0.2 1.1 9.8 3.6 40.2 37.4
5.6 0.2 0.9 6.6 2.4 34.9 29.5
5.5 0.2 0.6 5.3 2.2 30.1 29.3
(0.1)a (0.1)b (0.1)b (0.6)a (0.2)a (1.1)c (2.7)a
(0.1)b (0.1)a (0.2)a (0.5)a (0.2)a (1.0)b (1.4)b
(0.1)c (0.1)a (0.1)a (1.0)a (0.4)a (1.5)a (2.5)b
(0.1)a (0.1)a (0.2)a (0.9)a (0.3)a (1.6)a (2.8)a
(0.1)a (0.03)a (0.2)ab (0.7)b (0.2)b (1.5)b (2.3)b
(0.1)a (0.1)a (0.1)b (0.4)b (0.2)b (1.4)c (2.5)b
Means within rows followed by different letters are significantly different (p < 0.05) with respect to land use/soil depth.
Fig. 2. Relationship between soil organic carbon and CEC.
Forcella, 1998). The differences in textural fractions were also due to some variations in the pedogenic processes, especially in relation to rate of weathering; slower in the upper than in other altitudinal ranges. Our findings revealed that once the native forest is converted to cropland and grazing land-use types, significant changes occurred in soil properties. A change in land use from native vegetation to cropland tends to increase soil pH in both the surface and subsurface layers (Lumbanraja et al., 1998). In our study, the increase in soil pH in cropland soils might be related to the addition of potash through the traditional slash and burn practices. According to Moraes et al. (1996), burning releases nutrients in the ash, which increases soil pH. As a response of conversion of native forest to cropland, concentrations of base cations, mainly Na+ (75%) and K+ (53.8%) were significantly lower than in the other land-use types investigated due to removal by the harvest. The lower concentrations of base cations in the subsoil layers compared with the topsoil suggest that vegetation pumps bases from the subsoil to the topsoil. The higher topsoil base concentration (in spite of the lack of fertiliser amendment) could also be related to the slash and burn process. The range of Mg2+ concentration in the soils under the different land-use types was 2.6–3.0 mmol kg1, which is considerably higher than the critical level of 0.5 mmol kg1 reported for both tropical and temperate soils (Landon, 1991; McAlister et al., 1998). Changes in the CEC of soils due to land use changes can be quite considerable. In the present study, the CEC values across all landuse types varied significantly due to differences in the amounts of soil organic matter (carbon) concentrations. According to Yimer et al. (2007), soil organic carbon concentrations in the 1.0 m soil layer varied significantly with respect to land use type and soil depth. The regression analysis also revealed the strong association between soil organic carbon concentrations and CEC. Taking the fairly low clay content into consideration, it is obvious that the contribution to CEC by organic substances is critical. Topsoil after
15 years of cultivation showed a 36.7% reduction in CEC compared with topsoil in the adjacent native forest. Studies elsewhere (e.g. Saikh et al., 1998) also reported that soils cultivated for 5 and 16 years showed a decline in CEC of 43 and 27%, respectively, compared with forest soils. The high CEC value in the native forest soils is consistent with other findings showing a strong relationship between CEC and concentrations of soil organic carbon (e.g. Tegene, 2000; Eshetu et al., 2004). This finding has some important implications because it indicates that soil CEC is not likely to be increased to any significant level except by improving the organic carbon concentration in cultivated soils. The percentage base saturation of the soils studied here was significantly higher in the cropland than in native forest and in grazing land. Base cations stored in wood and shrubs are released at burning and replace the Al3+ released through H+ buffering. Naturally, the decrease in exchange sites and in organic matter concentrations and the increase in base cations also leads to higher base saturation. 5. Conclusions This study found that as a response of conversion of native forest to cropland significant changes followed in the chemical properties of soils. Soils collected from three land-use types in the Bale Mountains shown marked differences in their pH, concentration of base cations, CEC and percentage base saturation. Although not pronounced, free grazing has had effect on the CEC and other properties of the soil. Therefore, improving the existing land use practices and implementing better cropland management as well as introducing controlled grazing system could help to restore the soil nutrient levels for sustainable agricultural production and ecosystem functioning. Acknowledgements This study was financed through SIDA support to Wondo Genet College of Forestry, Ethiopia. Special thanks to Dr. Mulualem Tigabu, Faculty of Forest Sciences/SLU, Umea˚, and to an anonymous reviewer for their valuable comments on the manuscript. We also thank staff members of the Bale Mountains National Park headquarters; late Sileshi Kifle and the Rira village communities for their assistance during the fieldwork. References Berhe, S.M., Desta, B., Nicoletti, M., Teferra, M., 1987. Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia. Journal of Geological Society 144, 213–226. Black, C.A., Evans, D.D., White, J.L., Ensminger, L.E., Clark, F.E., 1965. Methods of Soil Analysis. Part 1. Physical and Mineralogical Properties including Statistics of Measurement and Sampling. Am. Soc. Agro. Inc.,, Madison, WI. Bruce, J.P., Frome, M., Haites, E., Janzen, H., Lal, R., Paustin, K., 1999. Carbon sequestration in soils. Journal of Soil and Water Conservation 54 (1), 382–389.
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