The Effect of Slope Gradient on Soil Organic Carbon Concentration in the Jira Watershed, Tigray, Northern Ethiopia

Introduction

Northern Ethiopia, particularly the Tigray region, is described as semi-arid climate, undulating topography, and large population pressure. These influences have generally led to severe land degradation, including deforestation, overgrazing, and widespread soil erosion. Soil organic carbon in such environments is not just a metric of fertility; it is essential for environmental resilience. Soil organic carbon (SOC) is considered to be an important indicator of soil health, environmental productivity and global carbon cycle dynamics [14]. It influences key soil properties, including structure, water-holding capacity, nutrient cycling, and erosion resistance [10]. Minor changes in SOC affect crop productivity and the soil’s role as a carbon sink. Higher SOC levels can improve soil structure, reduce erodibility and increase infiltration, which is vital for water preservation in moisture-stressed areas [17].

The concentration and storage of SOC are not uniform across landscapes but are influenced by a complex interplay of factors, primarily topography (especially slope gradient), climate and land use/land cover (LULC) [13]. Slope influences erosion deposition patterns, soil depth, moisture retention, and microclimate, so it modifies SOC inputs (plant litter and root biomass) and losses (erosion, decomposition [6]. Steep slopes are inherently more susceptible to soil erosion, which can selectively remove SOC-rich topsoil, while different land use types such as forests, grasslands, and croplands affect the balance between organic matter input and decomposition rates [9]. On steep slopes, the force of gravity and water runoff lead to significant soil erosion. This process preferentially removes the fine, light, and carbon-rich soil particles, leading to a net loss of SOC from upper slope positions and its deposition at lower slope positions (foot slopes and valleys) [11]. Consequently, one would expect a clear gradient of decreasing SOC with increasing slope steepness on eroding upper slopes. Slope aspect and gradient also create microclimates (variations in temperature, moisture, and solar radiation) that influence the rate of organic matter decomposition [7].

Multiple field studies from Ethiopia and comparable landscapes show consistent patterns: (1) significant differences in SOC among land-use classes, and (2) consistent effects of slope position or gradient on SOC and related soil properties. For example, analyses across the Upper Blue Nile and several northern Ethiopian watersheds reported that topographic position (upper/mid/lower slopes) and land cover jointly explained much of the spatial variability in SOC and total nitrogen stocks. Studies in Lake Tana, Gumara and other NW/Ethiopian highland watersheds have documented lower SOC on steep cultivated slopes and higher SOC under forest, grassland or conserved areas at lower slope positions. While numerous studies have examined either land use effects on SOC or erosion rates on slopes in Ethiopia, fewer have explicitly and simultaneously quantified the interactive effect of slope gradient on SOC pools.

Northern Ethiopia’s rugged highlands combine steep slopes, intensive cultivation, and a history of land-use change, making slope-mediated SOC dynamics particularly important for soil fertility and watershed health. Local studies emphasize that targeted soil conservation (terracing, exclosures, and agroforestry) and land-use planning that considers slope position can reduce SOC losses, restore degraded soils, and improve carbon sequestration potential. These findings justify focused research that quantifies SOC by slope class across representative land uses, and that links SOC changes to erosion rates and management practices [1].

In the fragile and often degraded ecosystems of Northern Ethiopia. Understanding the effect of slope on SOC is critical for sustainable land management and climate change mitigation. This research focuses on investigating the specific effect of slope gradient on SOC concentration across different land use types in the Jira watershed. It seeks to fill that gap by providing empirical evidence on how slope affects the amount of SOC in different land use types, thereby offering guidance for land-use planning and conservation prioritization. Understanding these interactions is essential for developing targeted soil conservation strategies, enhancing agricultural productivity, and improving carbon sequestration efforts in this vulnerable region.

Methodology

The study area

The study is conducted at Jira watershed, which encompasses a total area of 361.1 hectares. Geographically, it is situated within the Kola Tembien district of the Tigray region in northern Ethiopia. For broader geographical context, the watershed is located approximately 125 kilometers to the west of the regional capital, Mekelle. Its precise coordinate place it between latitudes 13^o39’34” and 13^o41’33” North and longitudes 38^o56’56” and 38^o57’48” East, delineating a clearly defined territorial extent. The topography of the watershed is characterized by significant altitudinal variation, with elevations rising from a lower baseline of 1764 m to peak of 2289 m above sea level, a relief that undoubtedly influences its local climate, hydrology and ecological diversity. The specific location of the study area is presented in Figure (1).

Figure 1: Location of the study area A) in Ethiopia B) in Kola Tembien district and C) Jira watershed with indication of drainage lines.

                             Figure 2: Slope classification map (A) and elevation map (B) of the study area.

Climatically, the study area is classified as a cool semi-arid zone. Meteorological records from the Kola Tembien district for the period 1996-2016 indicate a uni-modal rainfall pattern with an annual average of 680 mm, fluctuating between 500 and 860 mm. The climate is characterized by average temperature of 13.3^oC (minimum) and 29.8^oC (maximum). Rainfall is highly seasonal, with the main rainy season of June-August accounting for more than 80% of the total annual precipitation, peaking in the humid months of July and August. The projected potential evapotranspiration is 137 mm per year (Figure 3).

Figure 3: average annual temperature (T), rainfall (RF) and potential evapotranspiration (PET).

The Jira watershed’s geology is dominated by the Precambrian Tsaliet group (44%) and massive granite (36%), with the remaining area consisting of Mesozoic sedimentary rock (19%) and a small basalt formation known as Alage (1%). The area’s three primary soil types are Cambisols, Arenosols, and Lithosols. In terms of land use, natural vegetation (including shrubs and forest) is the most extensive at 60.4%, followed by cropland (25.1%) and grazing land (13.9%), while settlements occupy a minimal 0.6% of the area [19].

In the Jira watershed, agricultural production is based on a rain-fed, crop-livestock mixed system, making land and livestock the primary economic assets for the local population. The main method of tillage employs a traditional ard plow, locally referred to as a Maresha, which is pulled by oxen. The dominant crops cultivated are Teff (Eragrostis teff), Maize (Zea mays), and Sorghum (sorghum bicolor), alongside a variety of livestock including cattle, goats, sheep, donkeys, and poultry.

The watershed’s primary plant life consists of a mix of native and introduced species. Key indigenous varieties include Acacia, Ziziphus spina-christi, Faidherbia albida, Cordia africana, and Croton macrostachyus, while prominent exotic species are Grevillea robusta, Eucalyptus camaldulensis, and Leucaena leucocephala.

Methods of Data Collection

Soil Sampling Technique

This study employed a stratified sampling approach to assess the influence of slope gradient on soil organic carbon (SOC) concentration. Land uses (cropland, grazing land, forest land, and shrub land) were first classified into upper, middle, and lower parts according to their topographical position within a watershed. A total of 36 composite soil samples were then collected from these categories across three distinct slope gradients (Gentle <5, Medium 10-15%, and steep 20-25%). Sampling was confined to the upper 20 cm soil depth, justified by literature showing it contains the predominant fraction (70-90%) of SOC stocks [2, 3, 20]. Sampling was conducted within 10m x 10m plots, combining soil from five sub-points (four corners and one center) [18]. The samples obtained from the five sampling points were thoroughly mixed together to get composite samples for the determination of SOC concentrations (Figure 4).

Laboratory Analysis

A composite soil sample weighing approximately one kilogram was obtained from a 10 m x 10 m sampling area. Following collection, the sample was prepared by air-drying at room temperature, crushing, homogenizing, and sieving it to a particle size of less than 2 mm. The quantification of Soil Organic Carbon (SOC) concentration was performed at the Shire Soil Laboratory and Research Center in accordance with established soil analysis procedures. The measurement was carried out via the Walkley-Black oxidation technique, which employs a potassium dichromate solution [21].

Methods of Data Analysis

Statistical Analysis

Differences in soil organic carbon (SOC) concentration among various slope gradients were evaluated statistically using SPSS software (version 20), and a one-way analysis of variance (ANOVA) was employed. Upon establishing a significant overall effect (P < 0.05) and verifying data normality, pairwise comparisons between slope types were conducted with Tukey’s Honest Significant Difference (HSD) test for mean separation.

Results and Discussion

The Effect of Slope Gradient on Soil Organic Carbon Concentration

The analysis of variance revealed that slope gradient has significantly influenced soil organic carbon (SOC) content. The results, presented in Table 1), show clear trends in how slope interact to affect SOC concentration.

Table 1. Mean Soil Organic Carbon (%) and Standard Error (SE) across different slope gradients. Different superscript letters (a, b) within a column indicate significant differences between slope categories for a given land use type.

A key finding is the consistent and significant decline in SOC as slope gradient increases from gentle to steep. The R² value of 0.344 indicates a real and ecologically significant weak to moderate tendency for soil organic carbon to decrease as slope steepness increases, primarily driven by erosional processes (Figure 5). This pattern is evident across all four land use types. In Cropland, SOC decreased significantly from 0.72% on gentle slopes to 0.61% and 0.58% on medium and steep slopes, respectively. A similar significant decrease was observed in Grazing land and Shrub/Bush areas. Even in Forest land, which had the highest overall SOC, content still declined significantly from 1.06% on gentle slopes to 0.89% on steep slopes.

Figure 5: Relationship between slope gradient and soil organic carbon (SOC) concentration.

This inverse relationship can be attributed to increased soil erosion on steeper slopes. In steep or upper-slope positions, higher surface runoff and erosion tend to remove fine, SOC-rich top soil, producing lower SOC stocks compared with lower-slope or positions where eroded material accumulates and SOC concentrates [4]. Furthermore, steeper slopes often have shallower soils and poorer soil development, which limits the accumulation of organic carbon over time. Conversely, lower slopes and flat areas often have deeper soils, higher moisture and more vegetation cover, which favor SOC accumulation. In addition to this, the carbon lost from the steep upper and mid-slopes is often deposited at the foot slope or in valley bottoms. In Gentler Slopes, water has more time to infiltrate the soil, reducing erosion. Organic matter (like leaf litter and dead plants) can accumulate and incorporate into the soil more effectively. However, the magnitude and direction of these slope effects depend strongly on land-use type (e.g., cropland, grazing, forest/woodland, shrub land, plantations) and management practices: intensive cultivation or overgrazing can deplete SOC across all slope positions, while restoration (e.g., exclosures, afforestation, conversion to grassland) can increase SOC especially where erosion is reduced [22].

Studies conducted by [8] and [15] in the highlands of Amhara (e.g., Anjeni watershed) and Oromia have consistently shown that cultivated land on steep slopes (e.g., >15%) can have 30% to 60% lower SOC stocks compared to gentle slopes (<5%) under similar land use. The soil and carbon that are eroded are often deposited in foot slopes and valleys, creating a spatial redistribution. Steeper slopes are often less managed for agriculture due to their difficulty, but when they are cultivated, they tend to have lower biomass productivity and yield. This results in less plant residue (litter, roots) being returned to the soil to decompose into organic carbon. For example, a research conducted by [16] in the Gilgel Gibe catchment found that besides erosion, the lower input of organic matter on steep, cultivated slopes directly contributed to the reduced SOC levels compared to gently sloping areas with better crop growth.

Conclusion

In undisturbed ecosystems, steeper slopes might be less accessible and therefore remain under dense natural vegetation (e.g., shrub land, forest) for longer, while gentler slopes are converted to agriculture first. In a landscape-scale study, one might find a positive correlation between slope and SOC simply because the steeper land has been preserved as forest, which has high carbon stocks, while the gentle slopes are degraded croplands. This is a correlation driven by land use history, not a direct effect of slope.

In summary, the results demonstrate that slope gradient exerts a significant modifying effect on SOC, but within a context defined by land use. While steeper slopes universally lead to lower carbon stocks due to erosional processes, the inherent capacity of a land cover type to build and protect organic matter is even more critical. Forest ecosystems are the most effective carbon sinks, even on less ideal steep terrain, whereas croplands are the most vulnerable. This underscores the importance of implementing soil conservation practices, such as contour plowing, terracing, and agroforestry, particularly on sloping agricultural land, to mitigate SOC loss and preserve soil health.

The government of Ethiopia and local communities have invested heavily in soil and water conservation (SWC) measures over recent decades, such as the construction of stone bunds, terraces, and check dams. These structures are specifically designed to reduce the erosive impact of slope, trap sediment, and create conditions favorable for SOC accumulation. Therefore, any study on slope and SOC in Northern Ethiopia must also consider the presence or absence of these interventions, as they can decouple the natural negative relationship between slope and SOC.

This finding highlights the importance of implementing soil conservation practices (e.g., contour plowing, terracing, and maintaining vegetation cover) on steeper slopes to prevent the loss of this vital soil resource. It informs decisions that less fertile, erosion-prone steep land might be better suited for forestry or permanent pasture than for intensive crop production.

Acknowledgement

This study was made possible through the generous financial support of the Tigray Agricultural Research Institute (TARI). The author also wishes to extend his sincere gratitude to the staff of the Abergelle Agricultural Research Center for their invaluable technical assistance and expertise throughout the research process.

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