Spatial Variability of soil along Toposequance in the Workaria Watershed, South Wollo Zone, Ethiopia

Technical Summary: Spatial Variability of Soil along Toposequence in the Workaria Watershed, South Wollo Zone, Ethiopia

Problem Statement
The Workaria Watershed in the South Wollo Zone of Ethiopia faces challenges related to soil degradation, erosion, and declining fertility driven by rugged terrain, deforestation, overgrazing, and flooding. While soil is a dynamic and heterogeneous resource essential for sustainable development, there is limited integrated information regarding the spatial variability of soil nutrients and their relationship with topographic positions in this specific region. Understanding these variations is critical for achieving Sustainable Development Goals (SDG 2: Zero Hunger and SDG 15: Life on Land), yet effective land-use planning requires site-specific data that is currently lacking.

Methodology
The study employed a systematic approach to characterize soil properties and map their spatial distribution across the 375-hectare Workaria Watershed.

• Study Design: The watershed was classified into three distinct topographic positions: upper slope (30% gradient), middle slope (8–30%), and lower slope (0–8%).
• Sampling Strategy: A grid-based auger sampling approach was utilized. Thirty-two auger points were pre-defined using ArcGIS with a grid size of 316 m × 316 m. At each grid point, four sub-samples were collected from a 0–20 cm depth to create composite samples.
• Pedon Description: Three representative pedons were selected corresponding to the upper, middle, and lower landscape positions. These were described in situ to a depth of 200 cm (or until bedrock), with samples collected from all identified genetic horizons.
• Laboratory Analysis: Soil samples underwent analysis for morphological properties, particle size distribution (hydrometer method), bulk density, pH, electrical conductivity (EC), organic carbon (Walkley-Black), total nitrogen (Kjeldahl), available phosphorus (Olsen), and exchangeable cations (Ca, Mg, K, Na). Cation Exchange Capacity (CEC) and base saturation were also calculated.
• Classification and Mapping: Soils were classified according to the World Reference Base (WRB) system. Spatial variability was analyzed using geostatistical techniques (Kriging interpolation) in ArcGIS 10.4, following normality tests (Shapiro-Wilk). A weighted overlay approach was used to generate site-specific management maps.

Key Results

• Soil Classification and Distribution: Three dominant soil types were identified, strongly correlated with topographic position:
  • Cambisols: Covered 55.2% (206.9 ha) of the watershed, dominating the upper slope positions.
  • Luvisols: Occupied 32.4% (121.4 ha), found primarily on middle slopes.
  • Vertisols: Accounted for 12.3% (46.2 ha), located in the lower, nearly flat areas.
• Physical Properties: The soil texture across all positions was predominantly clay to heavy clay. Bulk density ranged from 0.97 to 1.78 g/cm³.
• Chemical Properties:
  • pH: Ranged from 5.42 to 7.53, with surface layers specifically ranging from 5.61 to 7.53. The soil reaction was moderately acidic in upper and middle slopes but neutral in lower slopes, generally increasing with depth and decreasing slope gradient.
  • Organic Carbon (OC) and Nitrogen (TN): OC levels were generally low to very low (1.95–3.41%), while TN ranged from medium to high (0.17–0.29%). A strong correlation was observed between OC and TN.
  • Available Phosphorus (Av. P): Ranged from 4.03 to 9.04 mg/kg, classified as very low to low across the watershed.
  • Exchangeable Bases: Exchangeable Potassium (K) showed high variability (CV = 61.49%), with low levels of both exchangeable K and Total Nitrogen found in small areas (26.7%) in the northern part of the watershed. Calcium (Ca) and Magnesium (Mg) were generally medium to high.
  • CEC and Base Saturation: CEC was very high (52.2–62.22 cmol(+)/kg), attributed to the clay-rich parent material. However, Percent Base Saturation (PBS) was low (21.14–28.25%), indicating significant leaching of basic cations.
• Spatial Variability: Geostatistical analysis revealed that pH, Ca, Mg, CEC, and Base Saturation were least variable, while K was the most variable. Available P and OC showed moderate spatial dependence.

Significance and Claims
The paper claims that toposequence-driven soil redistribution processes are the primary controllers of soil properties and fertility status in the Workaria Watershed. The study provides critical baseline data demonstrating widespread deficiencies in soil organic carbon and available phosphorus, which are the most limiting factors for crop production in the area.

The authors assert that the generated spatial distribution maps and site-specific management recommendations offer a scientific foundation for:

1. Targeted Interventions: Enabling farmers and extension services to apply fertilizers (specifically P, N, and K) and organic amendments based on specific nutrient deficits rather than uniform application.
2. Sustainable Land Management: Identifying vulnerable sites, such as upper slopes prone to erosion, to implement conservation practices.
3. SDG Alignment: Contributing directly to SDG 2 (Zero Hunger) by optimizing agricultural productivity through evidence-based nutrient management, and SDG 15 (Life on Land) by promoting the sustainable management of terrestrial ecosystems and soil restoration.

The study concludes that while soil texture is relatively uniform, fertility status varies significantly with landscape position, necessitating differentiated management strategies to ensure food security and environmental sustainability in the region.