A field experiment was conducted in a newly reclaimed soil at El-Saff region, El-Giza Governorate, Egypt to study the effects of different rates of nitrogen (N: 62 to 248？kg？ha？1) with or without soil inoculation of sulfur- (S-) oxidizing bacteria (SoxB) and combined inoculation of SoxB and N-fixing bacteria (NFxB) on yield, quality and nutritional status of onion (Allium cepa L., “Giza 20”). Elemental S at 620？kg？ha？1 was applied to all treatments. Application of N at 62, 124, and 248？kg？ha？1 rates increased onion yield, plant height, and N uptake by 28 to 76%, 32 to 53%, and 61 to 145%, as compared to those of the plants that received no N. Inoculation of SoxB at various N rates increased onion yields by 47 to 69% and N uptake by 76 to 93%, as compared to those of the plants which received the respective rates of N but no SoxB inoculation. Inoculation with SoxB and NFxB increased onion yield by 221%, plant height by 62%, and N uptake by 629%, as compared to those of the plants grown without inoculation and no N applied. 1. Introduction Recommendation of optimal nitrogen (N) fertilization strategy and improvement of N management efficiency heavily rely on precise evaluation of N status in plant-soil system. For most annual crops, N is generally applied as a base dressing before planting. Prolonged lag time between N application to the soil and its maximum crop uptake, following seedling emergence and rapid growth, may result in N leaching below the rootzone, hence not available for plant uptake. To maximize the benefits of N nutrition and to ensure adequate N availability throughout the growing season, additional N application later during the growing season may be required. Therefore, it is important to follow an optimized N fertilizer application as per recommended or accepted procedures for high yields and quality of onion production [1]. Sulfur (S) is needed by plants and microorganisms, and its speciation in soil is dependent on the chemical state of the soil, including (1) redox potential, that is, tendency of the soil solution to either gain or lose electrons and thereby suggests the aerobic or anaerobic status of the soil and (2) soil acidity [2]. Plant availability of S in a given soil is dependent on the S speciation in soils, influenced by pedogenetic processes and physicochemical factors, that is, water logging. The oxidation of S to in soil is a biological process and is carried out by several kinds of microorganisms, that is, Thiobacillus thiooxidans, T. ferrooxidans, T. thioparus, T. denitrificans, and T. novellus. The rate at which this
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