The Effect of Rainfall Variability, Soil Erosion, Drainage Improvement, and Nutrient Management on the Vertisols in Thcentral Highlands of Ethiopia
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Date
2024-06
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Addis Ababa University
Abstract
Vertisols, characterized by high clay content and distinct shrink-swell properties in response to changes in moisture content, are the major soil types in the central highlands of Ethiopia. On this soil, 80% of the population lives and 75% of the nation’s economy relies. In traditional farming practices, soils often face management problems due to waterlogging, stickiness, and forming puddles when wet. Additionally, when the soils dry out, they form hard clods and develop deep, wide cracks throughout the soil profile. The sticky nature of the soil further complicates farming operations as it hinders tillage activities and seedbed preparation. These issues can significantly impact the planting process, leading to delays in planting activities in Vertisols due to their waterlogged and sticky nature. This can lead to reduced crop yields and overall productivity. Therefore, this study was initiated with the objectives of studying rainfall variability and its effect on runoff and soil loss, and drainage improvement and crop production. Moreover,, legume residue decomposition and nitrogen (N) mineralization for legume residue, and the transfer of these N to subsequent wheat crops was studied. To address the environmental as well as agricultural production challenges associated with Vertisols, such as rainfall variability, soil erosion, and reduced soil fertility, five interrelated, but separate experiments were conducted at Ginchi and Holeta Agricultural Research Centers. To study rainfall variability, rainfall data of 39 years was used. The Markov Chain with initial and conditional probability and the Gamma distribution probability models were employed. The objective was to analyze rainfall patterns, predict rainfall levels, and interpret climatic data for water resource management, agricultural planning, and cultural practices.Runoff and soil loss study was conducted using conventional plot -used runoff plots to investigate the impacts of three sowing dates and two tillage practices on runoff and soil loss. Soil fertility study was conducted using the cropping system approach of planting legumes, employing drainage systems, and studying the legumes' biomass production and N yield during the main cropping season under an improved drainage system. At maturity, the legumes were harvested. Subsequently, the biomass of these legumes was incorporated into the soil, and the decomposition. Mineralization of legume residue N were studied through in-situ field monitoring using the Raison method. Lastly, the study determined the fertilizer replacement values of mineralized N from legume residues and explored the various nitrogen use efficiencies and the transfer of legume residue to subsequent wheat crops. The results indicated that two years out of five, Ginchi experienced rainfall anomalies, showing the need for drainage improvements and adjusting cultural practices and crop planting dates. Dependable rainfall at 75% probability and moisture availability indices greater than 0.5 start spanning from the 22nd to the 39th week (May 8th to September 30th). On average, soil loss on BBF was 3.8 t ha-1 per year, while on a flat seedbed, it was 2.1 t ha-1 per year. Runoff measured for BBF was higher than the flat seed bed. Legume biomass production, incorporation into the soil, and the transfer of incorporated legume residue N demonstrated that for low-input agriculture like Ethiopia, by using legumes in cropping systems, up to 186 kg ha-1 N per year was accumulated. However, the full potential of N accumulated and mineralized from legume residues was not available to subsequent crop wheat. Only 30 kg ha-1 N in terms of urea fertilizer remained available in the soil at planting wheat in June. This indicates that a huge amount of legume residue N might be lost due to leaching,
denitrification, and immobilization. The combined effects of rainfall, runoff soil erosion, and nutrient leaching highlight the need for appropriate strategies to enhance soil conservation measures, improve drainage systems, and promote effective nutrient management practices. These measures are crucial for ensuring the long-term and sustainable utilization of Vertisols in the Ethiopian highlands.
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Rainfall Variability, Soil Erosion, Nitrogen Fixation, Nitrogen Balance, Residue Incorporation, Fertilizer Replacement Value, Nitrogen Transfer, Crop Production, Vertisols