Effect of Nitrogen Fertilizer on Maize Performance, Nitrogen Use Efficiency and Soil Available Nitrogen Concentrations in Smallholder Farms in Rongo, Kenya
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Reduced crop productivity among the smallholder farms is the main cause of food insecurity in Sub-Sahara Africa (SSA). Maize is a staple food crop in Kenya and is especially grown in the parts of North Rift and Western Kenya. In the Lake Victoria region and particularly in Rongo sub-county, maize potential production is low ranging between 0.8 to 1.6 tons ha-1. The low soil productivity is due to low soil fertility, low soil nitrogen, inadequate use of inputs such as mineral fertilizers, improved maize seeds, and inadequate agricultural practices to boost production. The purpose of this study was to determine the effect of nitrogen fertilizer on maize yields, nitrogen use efficiency (NUE), its effect on soil properties, and distribution in the soil profile and on soil microbial population. The study was carried out at Kambija and Koderobara in Migori County during the long rainy season from March–July, 2018. The experiment was arranged in a Randomized Complete Block Design with three replications. The treatments of N fertilizer (CAN) were at four levels (0, 25, 50 and 75 kg N ha-1). Tri- Superphosphate (TSP) at 10 kg P ha-1 was applied as blanket. Maize variety (Sc Duma 43) was used as a test crop. Data were analyzed with the ANOVA procedure using GenStat statistical software (15th edition) and treatment means separated using least significant difference at (P≤0.05). Results indicated that treatments with nitrogen fertilizer application significantly increased maize yields (p=0.001) at Kambija and (p=0.01) at Koderobara. Applied N fertilizer at 50 kg N ha-1 exhibited the highest maize yields compared to other treatments at both sites (4.34 t ha-1 for Koderobara and 3.41 t ha-1 at Kambija). High rates of N fertilizer increase soil acidity and decrease maize yields. Highest nitrogen use efficiency (NUE) was recorded at 25 kg N ha-1 with 23.84 at Kambija while at Koderobara was recorded at 50 kg N ha-1 with 22.61, in general NUE decreased with increased applied N rates. At Kambija, 75 kg N ha-1 had the highest N content in plant tissues with 2.07% in the heading stage while at Koderobara, the highest N content of 1.90% was recorded at 50 kg N ha-1. Due to rainfall variability, mineral N fractions (N03 − and NH4 +) were higher in sub-layers (10-20 cm and 20-30 cm depth) at seedling stages but at heading stages, N mineral concentrations were higher in top-layers (0-10 cm depth) in both sites. Plots fertilized at 50 kg N ha-1 recorded highest concentration of available N in the soils in both sites of the experiment. Soil microbial population was slightly increased by N fertilizer application and declined with increased soil depth. Based on the results of this study, application of N fertilizer at 50 kg ha-1 can be recommended among the smallholder farms for improvement of maize yield in Rongo-sub County, Migori County, and the catchment of Lake Victoria.