Nitrate Leaching under Farmers’ Fertilizer and Irrigation Water Use in the Central Rift Valley of Ethiopia

In conventional furrow irrigated agriculture, excess water often causes leaching of nitrate-nitrogen. This experiment was aimed at quantifying nitrate leaching under farmers’ fertilizer and irrigation water use practices. It involved factorial combination of four rates of nitrogen fertilizer (0, 92, 184, and 368 kg N ha-1) and two levels of irrigation (100% crop water requirement =1.00 CWR and farmers’ practice =1.25 CWR). It was conducted in onion (Allium cepa L.) planted drainage lysimeter at Melkassa Agricultural Research Center in 2015/16 cropping season. Nitrogen loss for the production season was determined from leachate collected every 10±3 days interval. Nitrogen uptake, dry matter (DM) accumulation, bulb yield, and bulb storability were measured. The result indicated low nitrate leaching for the season (< 22.46 kg NO3- ha-1) however the losses were three-fold higher in high fertilizer rates or high irrigation water level. Independent t-test showed no significant difference (P ≥ 0.05) between irrigation levels on total N uptake, DM accumulation, and marketable bulb yield. Significant differences (P<0.05) were recorded between any different sub groups of N fertilizer rates on total N uptake except between 92 and 184 kg N ha-1. The significant difference between 184 and 368 kg N ha-1 on N uptake was not reflected in the DM accumulation and marketable onion bulb yield indicating low N use efficiency. The result showed about 48.5 to 69.4% surplus N accumulation in soil due to use of high rates of N fertilizer. Higher storage loss was depicted for application of 184 and 368 kg N ha-1, and 1.25 CWR. Irrigating at 1.00 CWR and N fertilizer not exceeding about 184 kg N ha-1 can reduce the problem without yield reduction. However, further field study is suggested within soils that have been under irrigated vegetable production for the last many years to reach at comprehensive conclusion and management recommendations.