Quantification of Biologically Fixed Nitrogen by White Lupin (Lupins albus L.) and Its Subsequent Uptake by Winter Wheat Using the 15N Isotope Dilution Method

A field experiment was carried out in 2016-2018 in a white lupin (Lupinus albusL.)-winter wheat (Triticum aestivumcv. 'Bogatka') crop rotation. The aim of this study was to determine the amount of nitrogen (N) that was biologically fixed by the white lupin crop in the first year of the rotation and to estimate how much of this N was then taken up from the lupin residues by winter wheat in the second and third years of the rotation. Biologically fixed N was determined by the isotope-dilution method ((IDN)-N-15) by applying 30 kg N ha(-1)of(15)N-labeled fertilizer ((NH4)-N-15)(2)SO4(containing 20.1 at.%N-15) to the white lupin and the reference plant spring wheat. The yields of white lupin seeds and crop residues were 3.92 t ha(-1)and 4.30 t ha(-1), respectively. The total amount of N in the white lupin biomass was 243.2 kg ha(-1), which included 209.3 kg ha(-1)in the seeds and 33.9 kg ha(-1)in the residues. The(15)N-labeled residue of white lupin was cut and ploughed into soil. Our results indicate that 111.2 kg N ha(-1)was fixed from the atmosphere by the lupin plants, with 93.7 kg ha(-1)found in the seeds and 17.5 kg ha(-1)in the residues. In the second and third years of the rotation when winter wheat was cultivated, the plots were divided into two groups of subplots (1) without N-fertilization (control) and (2) with an application of 100 kg N ha(-1). In the first year of winter wheat cultivation, 20.0% and 21.0% of N from the crop residues was taken up by the control and N-fertilization plots, respectively, while in the second year, uptake was lower at 7.12% and 9.27% in the control and N-fertilized plots, respectively.