Status of soil organic carbon and nitrogen stocks in Koga Watershed Area, Northwest Ethiopia

Abstract Background Soil organic carbon and nitrogen are key indicators to evaluate farmland management. Many studies of soil organic carbon (SOC) and total nitrogen (TN) in Ethiopia had focused on either rainfed or irrigated farming. A comparative study was meager and less understood on the variation of SOC and TN between farming systems. Thus, the objective of the study was to carry out comparative analysis on the status of SOC and TN stocks between farming systems. Thirty-six composited and cumulative soil samples had been collected from 25 m2 plot designed at discrete intervals as 0–15 and 15–30 cm in both farming systems. Soil organic carbon and nitrogen were analyzed using Walkley and Black, and Kjeldahl method, respectively, and performed in Environmental Protection Authority (EPA) in Addis Ababa. Results The result showed that mean SOC stock increased by 2.85 t C ha−1 (3.44%) and total nitrogen stock by 0.12 t N ha−1 (1.99%) in irrigated compared to rainfed farming up to 30 cm soil layer. Similarly, soils of irrigation farming had sequestered at the rate of 0.41 t CO2 ha−1 year−1. One-way analysis of variance (ANOVA) revealed that SOC and nitrogen stocks did not show the significant difference between farming systems (p < 0.05) because of slow turnover of organic matter. However, SOC and TN stocks had shown significant variation along depth (p < 0.05). Problems of soil acidity were found in both farming systems, but 9.3% mean pH value of soils of irrigation showed lower acidity than rainfed farming. Conclusion The present study revealed that farming systems and soil depth had shown the variation in the spatial and vertical distribution of organic carbon and nitrogen stocks. Soils in irrigation farming system sequestered higher carbon and nitrogen and promise climate mitigation than the rainfed farming system.