Earth Sciences
Volume 5, Issue 6, December 2016, Pages: 96-103

Soil Organic Carbon and Total Nitrogen Stock Dynamics in the Enset Dominated Farming System of Southwestern Ethiopia

Bahilu Bezabih1, *, Biniam Tesfaye2, Asrat Fikre3

1Department of Natural Resources Management, College of Agriculture and Veterinary Medicine, Jimma University, Jimma,Ethiopia

2Department of General Forestry, College of Agriculture and Natural Resource, Mizan Tepi University, Mizan Tepi, Ethiopia

3Water Resource Research Center, Arbaminch University,  Arbaminch, Ethiopia

Email address:

(B. Bezabih)

*Correspondingauthor

To cite this article:

Bahilu Bezabih, Biniam Tesfaye,  Asrat Fikre. Soil Organic Carbon and Total Nitrogen Stock Dynamics in the Enset Dominated Farming System of Southwestern Ethiopia. Earth Sciences. Vol. 5, No. 6, 2016, pp. 96-103. doi: 10.11648/j.earth.20160506.12

Received: September 3, 2016; Accepted: September 21, 2016; Published: November 9, 2016


Abstract: The study was aimed to investigate soil organic carbon (SOC) and total nitrogen (TN) dynamics among different land use systems in the Essera district of Dawuro zone, southwestern Ethiopia. Landscape of the district was dominantly covered with enset (Ensete ventricosum) farming system. For this study, three representative land use types namely, Enset farm, woody and cultivated lands were considered. For each land uses, a plot of 25x25m size was marked as a sample plot to collect soil samples in an‘X’ design (from the middle and four corners of the plot). Accordingly, both composite and core sampled soils were gathered from the five subsequent soil depths (i.e.0-5, 5-10, 10-15, 15-20 and 20-25cm). Consequently, analysis of variance was conducted by using SAS version 9.2. Moreover, a mean separation for each parameter was made using LSD (Least Significant Difference) test. The result confirmed that soil organic carbon (SOC) and total nitrogen (TN) stocks were significantly influenced by topographic variation, land use types and soil depths. The highest SOC and TN stocks were observed in the lower slope position, enset farm land and upper soil depths. Regarding to land use difference, 18.65 and 13.50 t/ha SOC stock were observed in the enset and cultivated farm land respectively. TN was also highest in the enset farm land as compared with cultivated and woody land. In contrast, the lowest soil organic carbon stocks were recorded in the upper slope position of cultivated land and lower soil depths. Both SOC and TN stocks were significantly decreased from the upper soil layers to lower soil depths. Soil physical properties were also significantly influenced by topographic position, land use difference and soil depths. Soil clay contents were highest in the enset and woody land while lowest in the cultivated land. However, soil bulk density, soil silt and sand fractions were highest in the cultivated land. Moreover, soil clay content was significantly increased from upper to lower slope position and vice versa for soil bulk density, soil silt and sand fraction. In conclusion, based on the confirmed result, it deserves to improve soil fertility management under different land use types and slope categories, so that soil organic carbon, total nitrogen and soil clay contents could be improved accordingly.

Key words: Soil Organic Carbon, Total Nitrogen, Farming System, Land Uses, Soil Depth


1. Introduction

Soil has three important and inter related physical, chemical and biological properties, which makes it functionally complete resources. The inherent characteristics of soil which are mainly the resultant of parent material and climate undergo subtle change due to different land management practices. Some inherent soil properties such as soil pH and texture as well as climatic conditions have substantial influences on soil properties transformation [1]. Types of land use, intensity of cultivation and fertilizer sources are major factors responsible for soil properties transformation. Among this factors, soil management has a marked influence on soil organic matter (SOM) dynamics in agricultural system [22]. Land-use practices affect the distribution and supply of soil nutrients by directly altering soil properties and by influencing biological transformations in the rooting zone. Although its consequences vary, land conversion frequently leads to nutrient losses when it disrupts surface and mineral horizons (e.g.,by mechanical disturbance) and reduces inputs of organic matter [24].

In agricultural ecosystem soil organic matter (SOM) and total nitrogen (TN) are an essential ecosystem component which are major determinants of soil fertility whose dynamics are affected by soil management practices, and are closely related to soil productivity [22],[13]. Primary plant production and soil biological activity are the two main biological processes governing inputs and outputs of SOM in the system, and the equilibrium between both determines the SOM cycling dynamics [20]. Soil organic carbon also associated with other major nutrients especially phosphorus and nitrogen, and hence influences recycling and availability of nutrients for agricultural production system sustainability [2].

Declining soil fertility and land degradation in the forms of soil erosion is threatening agricultural productivity, which is the main stay of the economy, in Ethiopia [16]. However, despite the general recognition of the problem of soil degradation and its impact on agricultural productivity, there are few efforts to quantify the extent, rate and process of soil fertility depletions under various land use systems and management practices [8]. It is estimated that there is a 1-2% reduction of soil productivity annually from the crop lands in the Ethiopian highlands due to land degradation [14]. One aspect of land degradation leading to such decline of soil productivity is loss in organic carbon and total nitrogen due to erosion, intensive decomposition, or leaching. In countries like Ethiopia where there is serious food insecurity due to impoverished soils, the importance of soil organic matter maintenance need not be overemphasized [15]. Therefore, the major objective of this study was to investigate current status of soil organic carbon and total nitrogen dynamics under different land use types and topographic positions in the Enset dominated farming system of Southwestern Ethiopia

2. Methodology

2.1. Description of the Study Area

The study area is situated in the Essera district of Dawuro zone in Southern Nations, Nationalities and Peoples Regional State (SNNPRS) (Figure 1). It is situated in the Omo basin at about 507km South west of Addis Ababa, the capital of Ethiopia.The area is topographically undulating and rugged.The Dawuro Zone covers total area of 4436.7km2and lies between 6.59-7.34°N latitude and 36.68 to 37.52°E longitudes, with an elevation ranging 501-3000m [17]. The zone has 5 districts with a total population of 398, 796. It has also three agro-Ecological zones identified as: kola, Weynadega and Dega occupying 55.6%, 41.4% and 3% respectively. The annual mean temperature ranges between 15.1 and 27.5°C. The rain fall is a bimodal type: the short rainy season is between (February and March) and the long season between (May and September). The average annual rainfall ranges from 1201 to 1800mm. The regional data on land utilization shows that, 38.4% is cultivated land, 13.39% grazing land, 16.81% forest, bushes and shrub land, 17.09% cultivable and 14.31% is covered by others. The live stock resource of the zone was estimated to be 313,094 cattle, 113,554 sheep, 45,703 goats, 7,081 horses, 1,934 mules, 5,064 donkey, 157,996 chicken, and 28, 557 traditional hives [5]. According to [9] soil classification, the dominant soil of the region is Humic Nitisols with a clay and clay loam texture with dark redish brown color.

Figure 1. Map of the study area.

2.2. Farming System

Farming systems of the region (area) is broadly classified in to cereal based farming system and Enset- Root crops complex in combination with different agro- forestry systems. Enset is a plant native to Ethiopia that is often referred to as the false banana because, not surprisingly, of its resemblance to the banana plant. It is grown in the less arid highlands of the south western region of Ethiopia. The major annual food crops grown in the area, includes cereals (maize, sorghum, barley, wheat, and teff), pulses (beans, peas). Maize and wheat followed by beans and peas were grown in the highest proportion in the area. The most common agro-forestry tree species, in the grazing and enset farm in the study area includes, Cordia africana, Millettia ferruginea, Ficus spp., Grevillea robusta, Acacia spp, Coffee (C.arabica) and some fruit trees like Mangifera indica L., and Persea Americana. Musa acuminate and root crop (potatoes and taro) are also grown in a considerably especially in enset farm land.

2.3. Methods of  Data Collection and Analysis

2.3.1. soil Sampling and Soil Laboratory Analysis

Three representative land use types namely, cultivated land, Enset farm and woody lands were selected from three slope positions, lower (3-8%), middle (8-15%), and upper (15-25%). From each slope positions and land use types a plot with 25x25m size was marked as a sample plot following a method applied by [7]. The soil samples were then taken from five points in an ‘X’ design (from the middle and four corners of the plot) and from five subsequent soil depths (i.e.0-5, 5-10, 10-15, 15-20 and 20-25cm). Accordingly, from each land use types both composite and core sampled soils were collected for soil chemical and physical analysis. For soil chemical analysis the samples were air dried and grounded to pass through a 2-mm sieve. Total SOC was quantified by wet digestion with a mixture of potassium dichromate and H2SO4, under external heating [25]. Soil TN was measured after sulfuric digestion followed by Kjeldahl distillation [23]. Soil bulk density was determined by the soil core method which is the ratio of oven dried mass of soil to core volume, and soil texture (sand, silt, and clay contents) was obtained by sieving and decantation procedures according to [21]. Soil texture was estimated using Hydrometer method [10] after destroying organic matter by adding hydrogen peroxide (H2O2) and dispersing the soil through adding sodium hexameta phosphate(NaPo3)6.

Total organic C and TN stocks were calculated by the formula:

Organic C or N stockt/ha (toneperhectare)=organic C or

N content (dagkg-1)*ds*th

Where,ds is the soil bulk density

(tm-3) and th is the thickness of the soil layer (m)[11]

2.3.2. Data Analysis

The comparisons among land use types, topographic positions and soil depths were conducted by using SAS version 9.2 software packages for analysis of variance (ANOVA). When the analysis of variance (ANOVA) showed significant differences (P<0.05) among the various land use types, soil depths and slope position for each parameter, a mean separation for each parameter was made by using LSD (Least Significant Difference) test.

3. Result and Discussion

3.1. Soil Physical Properties

The over all mean of several soil properties significantly influenced by land use types, slope position and soil depths. Soil physical properties includes soil bulk density, sand, silt and clay fraction were significantly affected by slope position (Table1&2). The highest soil bulk density was recorded in the upper slope position which could be due to less addition of crop  residue and trampling effect of livestock. Because of small land holding size grazing intensity was very high in the upper slope position. Similarly, sand, silt and clay proportion were also significantly influenced by land uses. Enset and woody lands have highest clay content and lowest silt and sand fraction. In opposite to this, cultivation land has lowest clay and highest silt and sand fraction (Figure 4). In the study area enset farm land receives more attention because it is considered as a food security crop. As a result, there is frequent addition of organic matter, crop residue, cow dung and house hold residue in the enset farm land. As soil organic matter improved, soil clay content might be increased and silt and sand content decreased (Figure 3). However, soil bulk density was not significantly different due to land use difference.

Table 1. The influence of topographic positions,land use types and soil depth on selected soil physical and chemical properties.

Slope
oc(%) soc(t/ha) TN(%) TNS(t/ha) BD(gm/cm3) Sand(%) silt(%) clay(%)
lower 2.76a 17.05a 1.66a 10.31a 1.25b 24.44a 35.28b 40.28a
middle 2.84a 18.97b 1.89b 7.54b 1.38a 27.50b 33.78b 38.72a
upper 1.93b 13.15c 0.63c 4.23c 1.41a 30.00c 39.61a 30.39b
Land use
cultivation 1.99b 13.5a 13.5a 1.67a 1.39a 34.00a 37.89a 28.61c
enset 2.86a 18.65b 18.65b 12.19c 1.35a 22.56b 34.67b 42.78a
woody 2.68a 17.03c 17.03c 8.22c 1.29a 25.39c 36.61a 38.00b
Depth
0-5cm 3.3a 18.8a 1.69a 9.78a 1.15a 23.78a 35.78a 40.44a
5-10cm 3.01a 18.57a 1.61a 9.45a 1.24a 25.22ab 36.11a 38.67a
10-15cm 2.66b 17.22a 1.24a 7.87a 1.31abcd 26.22bc 36.00a 37.78ab
15-20cm 2.35bc 16.3ab 1.06ab 7.20ac 1.41bcd 28.11c 36.33a 35.56bc
20-25cm 2.02cd 14.88bc 0.78ab 5.69ab 1.49c 29.56d 36.33a 34.11c
25-30cm 1.72d 12.58d 0.56abc 4.15c 1.78d 31.00de 36.78a 32.22cd

Values followed by different letters are statistically different (P≤0.05).

Table 2. The interaction effect of topographic positions and land use types on SOC, TN stock and selected soil physical properties.

LowerSlopePosition
landuse Oc(%) Soc(t/ha) TN(%) TNS(t/ha) BD(gm/cm3) Sand(%) silt(%) clay(%)
cultivation 2.31b 13.31b 0.51a 2.90a 1.16a 30.17a 36.83a 33.00b
Enset 2.93a 19.46a 2.59b 16.99b 1.37a 18.83b 35.50a 45.67a
woody 3.05a 18.38a 1.87c 11.02c 1.24a 24.33a 33.50a 42.17a
MiddleSlopeposition
cultivation 2.14b 15.94b 0.19a 1.39a 1.51a 35.67a 33.50a 30.83b
Enset 3.49a 21.22a 1.99b 11.96b 1.23b 22.67b 32.33a 45.00a
woody 2.89a 19.75a 1.38c 9.27b 1.39ab 24.17b 35.50a 40.33a
UpperSlopeposition
cultivation 1.53a 11.24b 0.10a 0.71a 1.51a 36.17a 41.83a 22.00b
Enset 2.15f 15.25a 1.10b 7.60b 1.46ab 26.17b 36.17b 37.67a
woody 2.1f 12.96b 0.72c 4.37c 1.26b 27.67ab 40.83a 31.50a

Values followed by different letters are statistically different(P≤0.05).

With respect soil depths soil bulk density and sand fraction were significantly increased from upper soil depth of 0-5 cm to lower soil depth of 25-30 cm. In contrast soil clay fraction was significantly decreased from upper soil depth to lower soil depth. However, soil silt fraction was not significantly different due to soil depth variation. The highest sand and silt proportion (36.67 and 38.67%) were observed on the sub layers of cultivated land (25-30cm) which might be due to high leaching of silt and sand particles down the profile in the cultivated land. In generally for all land uses, clay content decreased while sand and silt contents increased with increasing soil depths (Table3).

Table 3. The interaction effect of land use types and soil depths on SOC, TN and selected soil physical properties.

Depth(cm) Oc(%) Soc(t/ha) TN(%) TNS(t/ha) BD(gm/cm3) Sand(%) silt(%) clay(%)
cultivation 0-5 2.58a 15.21a 0.41a 2.19a 1.14a 30.33a 37.67a 32.00a
5-10 2.38a 15.1a 0.37a 2.21a 1.23a 31.33a 38.33a 30.33a
10-15 2.12a 14.68a 0.3a 1.94a 1.35ab 32.33a 38.00a 29.67a
15-20 1.88a 14.26a 0.23b 1.66b 1.49b 34.67a 36.67a 28.67a
20-25 1.62b 12.61b 0.17bc 1.29b 1.55bc 36.67ab 36.33a 27.00ab
25-30 1.37b 9.14b 0.12c 0.71c 1.61c 38.67b 37.33a 24.00b
Enset 0-5 3.93a 22.59a 2.67a 15.1a 1.16a 19.67a 33.33a 47.00a
5-10 3.45a 21.31a 2.64a 16.29a 1.25a 20.67a 35.00a 44.33c
10-15 2.97a 18.86a 1.97bc 12.46b 1.29a 22.33a 34.00a 43.67c
15-20 2.62b 17.81b 1.8c 12.41b 1.41a 23.33a 34.67a 42.00c
20-25 2.27bc 16.77bc 1.31de 9.78c 1.5b 24ab 35.67a 44.33c
25-30 1.92c 14.53c 0.93e 7.08c 1.51b 25.33a 35.33a 39.33d
woody 0-5 3.4a 18.6a 2.03a 11.07a 1.09a 21.33a 36.33a 42.33a
5-10 3.22a 19.29a 1.83a 10.85a 1.20a 23.67a 35.00a 41.33a
10-15 2.88a 18.14a 1.47b 9.20a 1.26ab 24.00a 36.00a 40.00a
15-20 2.54b 16.82b 1.43bc 7.54bc 1.32b 26.33ab 37.67a 36.00b
20-25 2.17bc 15.25bc 0.86cd 5.99cd 1.39cd 28.00b 37.00a 35.00bc
25-30 1.86c 14.06c 0.61d 4.66d 1.5d 29.00b 37.67a 33.33c

Values followed by different letters are statistically different (P≤0.05).

3.2. Soil Organic Carbon and Total Nitrogen Stock

Soil organic carbon and total nitrogen stocks were significantly influenced by slope position, land use types and soil depth (Table 1 & Figure 2). The average mean value of soil organic carbon stocks were 17.05, 18.97 and 13.15t/ha for lower, middle and upper slope positions respectively. Similarly, the average mean value of total nitrogen stocks were 10.31, 7.54 and 4.23t/ha for lower, middle and upper slope position respectively (Table1). The result revealed that soil organic carbon and total nitrogen stocks were significantly decreased from lower to upper slope positions. The decreased soil organic carbon and total nitrogen stock in the upper slope position could be attributed to poor soil fertility management and the steepness of the topography which accelerates soil erosion.

Regarding to land use types, the average SOC stocks were 13.50, 18.65, 17.03t/ha and total nitrogen stock 1.67, 12.19, 8.22t/ha for cultivated, enset woody land respectively (Table 1). The result confirmed that the highest mean value of soil organic carbon and total nitrogen stock were recorded in the enset farm land and the second highest value were in the woody land while the lowest mean value were in the cultivated land. The declining soil organic carbon and total nitrogen stocks in the cultivated land might have been aggravated by the insufficient inputs of organic substrate from the farming system due to residue removal and zero crop rotation. Moreover, leaching problem that can be attributed to high sand content (Table1&2) and the resultant light texture of soils also might be the cause of soil organic carbon reduction. The highest mean in the enset land could be due to frequent soil organic matter addition and mulching while the lowest value in the cultivated land is due to continuous cultivation, removal of crop residue, soil erosion and rapid oxidation and/or mineralization.  The result agreed with [22],[26],[8],[3] who stated the organic management, topography, grazing activity and rapid oxidation and farm yard manure (FYM) application influenced the total SOC and TN stocks, mainly in the upper soil layers, as a response to the utilization of organic residues as fertilizer and mulch. [19],[18] also reported that lower levels of soil OM content was observed in cultivated land. With regard to the interaction effect of land use and soil depth, the both organic carbon and total nitrogen were significantly decreased from the upper soil layer of 0-5cm to lower soil depth of 25-30cm (Table 3). Among all land uses and soil depths the lowest mean soil organic carbon (1.37% or 9.14t/ha) in the sub layer of cultivated land and the highest (3.93% or 22.59t/ha) in the upper soil layer of enset farm land (Table3). Soil organic carbon stock and total nitrogen stocks content (t/ha) for 0–5, 5–10, 10-15, 15-20, 20-25 and 25-30cm soil layers under the Enset farm, woody and cultivation land showed significant variationat (P<0.001). It was significantly highest in the upper soil layer (0-5 and 5-10cm) and declined with soil depth (Table 3). The soils of the Enset farm and woody land have the highest soil carbon and nitrogen stock while cultivated land has lowest mean value. An analysis of variation showed that both soil organic carbon and total nitrogen stocks were decreased from upper soil layer to lower soil depths.

Figure 2. TN concentration /stock comparisons in differentl and use types, slope categories and soil depth.

The above figure 2 shows that the average SOC and TN concentration /stocks for all land use types, slope categories and soil depths. Both SOC and TN concentrations follow the order enset farm> woody land> cultivated land. Regarding to slope categories the concentration of SOC and TN of lower slope> middle slope> upper slope position.

3.3. The Relation Between SOC, TN and Soil Texture

Soil organic carbon (SOC), total nitrogen (TN) and soil texture analysis showed a strong relationship (Figure 3). As shown in the figure below, positive relationships were found between SOC & clay fraction, SOC & TN, SOC stock and TN stock (t/ha). However, soil clay and sand fraction showed a negative relationship (Figure 3). Numerous studies also confirmed the relationship between clay content, total nitrogen and SOM in soils from different sites in the tropics (Feller and Beare, 1997, Burke et al.1999, Plante et al.2006, E. Sakin, 2012).

Figure 3. The relationship between SOC, TN stock and soil texture.

Figure 4. The comparison of SOC, TN and selected soil physical properties under different slope positions, land use types and soil depths.

4. Conclusions

Topographic position, land use types and soil depth interaction were significantly influences soil physical and chemical properties in the study area. Lower slope position has improved soil organic carbon and total nitrogen stock. Similarly, enset and wood land has highest clay content, organic carbon and total nitrogen. However, soil bulk density, soil silt and sand contents were lowest in the enset and woody land. This signals the great need for soil nutrient management measures such as integrated nutrient management (application of organic and inorganic fertilizers), growing of N2 fixing crops and application of agro-forestry in to the farming systems. so that, soil organic carbon total nitrogen and soil clay contents will be improved and enabled production of sufficient food for the farm households by minimizing nutrient depletion.


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