This study aimed at estimating the amount of nitrogen derived by yellow lupine from the atmosphere, mineral fertilizer and soil at different nitrogen fertilization rates and developmental phases of this test plant. Yellow lupine was cultivated in three fertilization variants: without nitrogen fertilization and after application of 30 and 150 kg N·ha^-1. Harvest was performed in flowering and full maturity. In the study ¹⁵N nitrogen isotope was introduced into soil in the form of ammonium sulfate, and the method of isotopic dilution was used. The biomass yield of yellow lupine and the nitrogen content in its individual parts were not significantly dependent on the nitrogen rate. The amount of yellow lupine biomass harvested in full maturity was more than two times larger than in flowering. Nitrogen content in lupine harvested in flowering was higher than in full maturity. Differentiated fertilization with nitrogen, irrespective of the harvest time of lupine, did not have a significant effect on the total amount of taken up nitrogen, which was about two times higher in full maturity than in flowering. The amount of nitrogen derived from biological reduction in the biomass of yellow lupine harvested in flowering was smaller after application of the higher rate of this element, whereas in full maturity this relationship was opposite. The amount of nitrogen derived from the fertilizer increased along with increasing rate. Percentage of nitrogen derived from biological reduction in the biomass of yellow lupine was similar in flowering (53.4 %) and full maturity (51.6 %). Proportion of nitrogen derived from the fertilizer was larger in the 1st time of lupine harvest than in the 2nd time, whereas in the case of nitrogen derived from soil this relationship was opposite. Differentiated fertilization with nitrogen did not significantly affect the percentage of nitrogen derived from biological reduction in yellow lupine. Yellow lupine fertilized with the higher rate of nitrogen contained a higher percentage of this element derived from the fertilizer and lower from soil reserves than after application of the lower rate.

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