Biomass of lucerne harvested at the budding stage or at the beginning of flowering is a bulky feed with a high content of protein. This plant is less often harvested at the full maturity for seeds. Nitrogen fertilization is not frequently applied in lucerne cultivation, since it lives in symbiosis with nodule bacteria that reduce atmospheric nitrogen into ammonium forms available for plants. This work presents the result of the pot experiment that aimed to determine the effect of the development stage of lucerne on the dynamics of nitrogen uptake from the atmosphere, from the mineral fertilizer and from soil. The studied plant was harvested at the beginning of flowering and at full flowering, and after reaching full maturity. In fertilization of the test plant, ammonium sulphate with ¹⁵N excess was applied and the method of isotopic dilution was used to assess the dynamics of nitrogen uptake. The control plant was spring barley. Systematic increase in the dry matter of lucerne was recorded at successive development stages. At the beginning of flowering and at full flowering of lucerne, the percentage of roots, stems and leaves in the whole plant mass was similar, whereas at the full maturity, the roots had the highest percentage in the dry matter yield (44%), and seeds (4%) and stripped pods (5%) the lowest. The average amount of nitrogen in the whole dry matter of the studied species was similar at successive harvest times. At the beginning of flowering and in full flowering, lucerne accumulated the highest amount of nitrogen in leaves, whereas after reaching the full maturity, in leaves and roots. The main source of nitrogen for the studied plant at successive developmental stages was the atmosphere. Nitrogen biologically reduced by lucerne harvested at the beginning of flowering and at full flowering accounted for 83.8 and 86.1%, respectively, and at the full maturity stage, 94.9% of the total uptake of this macroelement. The other small amount of nitrogen was taken up from the mineral fertilizer and from soil. At the beginning of flowering and at full flowering of lucerne the percentage of nitrogen taken up from the fertilizer did not exceed 10%, and from the soil was maximally 7%. At full maturity to harvest for seeds, the percentage of nitrogen taken up from the fertilizer and soil amounted to 2.4 and 2.7%, respectively.

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