Mercy Kangai Rugendo2025-11-112025-11-112025-09http://repository.embuni.ac.ke/handle/123456789/4492ThesisDeclining soil fertility resulting from nutrient losses due to erosion is a major constraint to agricultural productivity and food security among smallholder farmers in Sub-Saharan Africa (SSA). Managing soil organic carbon (SOC) levels is a promising strategy to mitigate erosion since SOC directly influences soil structure, water retention, infiltration, and nutrient cycling. This study aimed to evaluate the effect of different SOC levels under varying simulated rainfall intensities on water retention, infiltration rates, runoff volume, sediment yield, and nutrient losses in Nitisols and Ferralsols soils found in Tharaka-Nithi County, Kenya. The research was conducted in farmers’ fields located in Chuka and Tharaka-South sub-counties, representing the Nitisols and Ferralsols, respectively. Soil samples were analyzed and categorized into low (1.0 to 1.5), moderate (1.6 to 2.5) and adequate SOC (above 2.5%) content. The experiment was laid out in a split plot design arranged in randomized complete block design, where SOC levels were main plots and rainfall intensities were the sub-plots. Rainfall intensities of 80, 100, and 120 mm/hr were simulated on 1 m² experimental plots fenced with corrugated iron sheets to accurately collect runoff and sediments. Key parameters measured included runoff volume, sediment yield, soil bulk density, hydraulic conductivity, and water retention. Runoff and sediment samples were analyzed for primary and secondary macronutrient concentrations using standardized laboratory methods. The collected data were subjected to analysis of variance (ANOVA) followed by Tukey’s Honest Significant Difference (HSD) test. Pearson correlation analysis was performed to examine the relationship between the different variables of interest. The findings revealed that higher SOC levels significantly reduced runoff volumes and sediment losses across both soil types and all rainfall intensities. Specifically, soils with high SOC exhibited a 40–55% decrease in runoff and a 35–50% reduction in sediment yield compared to low SOC soils. Runoff and sediment volumes were also significantly higher in Ferralsols than in Nitisols. Water retention and infiltration rates improved markedly with increased SOC, indicating enhanced soil physical properties. Nutrient losses, particularly nitrogen and phosphorus, were lower in plots with higher SOC, demonstrating improved nutrient retention capacity. Pearson correlation analysis confirmed a strong positive (r = 0.85) relationship between runoff volume and sediment loss, underscoring the importance of reducing runoff by improving infiltration. The study concluded that enhancing SOC levels is essential in improving soil resilience against erosion, enhancing water availability, and conserving nutrients in Nitisols and Ferralsols. Sustainable agricultural practices such as organic amendments, that build SOC should be promoted to improve soil health and agricultural productivity. These results make a clear practical case: prioritize SOC-building practices (residue retention, organic amendments, cover crops, reduced tillage) to reduce runoff, sediment, and nutrient losses from Nitisols and Ferralsols.en-USThesis