Productivity formation of winter rapeseed as affected by nitrogen fertilization level in the western forest-steppe of Ukraine

The article presents the results of field and laboratory studies on the peculiarities of vertical and seasonal migration of mineral forms of nitrogen (NO)₃⁻-N and NH₄⁺-N) in the soil profile for growing winter rape (Brassica napus L.) hybrids of DSV selection (Temptation, Dominator, Duke) in the conditions of the Western Forest Steppe of Ukraine. The study was conducted in the farm «Victoria-92» of Ternopil district on typical medium loam chernozem with the use of various forms of nitrogen fertilizers (ammonium nitrate, urea, ammonium sulfate) with single and fractional application in doses of 60–175 kg N/ha. The relevance of the work is due to the intensive use of nitrogen fertilizers in the cultivation technologies of modern rapeseed hybrids and the associated environmental risks — by nitrate leaching and denitrification, which leads to soil fertility degradation and environmental pollution. The purpose of the research was to establish the patterns of migration of mineral forms of nitrogen depending on the phases of crop development and to evaluate the effectiveness of various forms of nitrogen fertilizers. Samples were taken from layers 0–20, 20–40 and 40–60 cm in the key phases of organogenesis of DSV (Temptation) breeding hybrids. No content₃⁻-N and NH₄⁺-N was determined by standard agrochemical methods. The total nitrogen dose was 175 kg/ha (ammonium nitrate — 250 kg/ha, ammonium sulfate — 100 kg/ha, urea — 150 kg/ha), applied once at the end of February – at the beginning of March. It was established that the maximum concentration of nitrate nitrogen is formed in the layer of 0–20 cm in the spring during the period of intensive plant growth, while in the late phases of vegetation, with sufficient moisture, it migrates to deeper horizons (20–60 cm), which increases the risk of leaching. The ammonium form is characterized by less mobility and is mainly accumulated in the upper layer of the soil. It has been proven that fractional application of nitrogen fertilizers ensures more uniform saturation of the soil with mineral nitrogen, increases the coefficient of its use by plants and reduces losses. The highest yield was obtained by using the ammonium-sulfate form of nitrogen. The obtained results justify the expediency of optimizing the norms, forms and terms of applying nitrogen fertilizers in order to increase the productivity of rapeseed and minimize the environmental risks of ground and undergroundwater pollution with nitrates. DSV (in particular Temptation) selection hybrids demonstrated high efficiency of mineral nitrogen absorption, which contributed to reducing its losses under optimized fertilization rates.