| Summary: | The aim of our study was to determine the effect of various nitrogen sources (NH<sub>4</sub>NO<sub>3</sub> (N, 34%), Ca(NO<sub>3</sub>)<sub>2</sub> (N, 15.5%; Ca, 18%), Mg(NO<sub>3</sub>)<sub>2</sub> (N, 11%; Mg, 12%), NaNO<sub>3</sub> (N, 15%; Na, 25%) and urea (N, 46%)) and increasing the intensity of N nutrition with these fertilisers (50, 70, and 90 mg N·dm<sup>−3</sup>) on the yield and quality of flowering Chinese cabbage (<i>Brassica campestris</i> L. ssp. <i>chinensis var. utilis</i> Tsen et Lee). The plants were grown in two different soilless systems, namely pot cultivation (substrate system—mixture of peat and sand) and hydroponic cultivation. The quality of plants was expressed as macro- and microelement contents, pigment contents, antioxidant activity and phenolic content. It was observed that the yield of flowering Chinese cabbage was about 43–70% higher in the hydroponic system than in the substrate. The N source and N nutrition affected the yield volume. The highest mean yield was observed in pot cultivation after fertilisation with Mg(NO<sub>3</sub>)<sub>2</sub> and in hydroponics with Ca(NO<sub>3</sub>)<sub>2</sub>. We found a rather high tolerance of flowering cabbage to sodium and an excessive content of ammonium in the nutrient solution. The nitrogen source and N doses modified plant contents of macro- and microelements (N, P, K, Ca, Mg, Na, Fe, Mn, Zn and Cu) and other quality parameters of plants. In pot cultivation, the highest element contents as well as the highest antioxidant activity were obtained after fertilisation with Mg(NO<sub>3</sub>)<sub>2</sub> at N-70 and N-90. The highest pigment contents (chlorophylls and carotenoids) were obtained in the samples treated with urea at the N-90 dose. Those samples were also characterised by a high Mn content. Generally, the pigment content in the pot system positively correlated with the Mn content in leaves, the microelement which is involved in the process of photosynthesis, but it did not correlate with colour coordinates. In the hydroponic system, the highest pigment contents were observed in the samples treated with Mg(NO<sub>3</sub>)<sub>2</sub> at the N-70 dose. Generally, in hydroponics, chlorophyll levels positively correlated with Ca levels in the aboveground parts of the plants. Additionally, the content of Chl b inversely correlated with L* and b* values. In hydroponic systems, the highest DPPH (2,2-diphenyl-1-picrylhydrazyl) activity was observed after treatment with NH<sub>4</sub>NO<sub>3</sub> at the N-70 and N-90 doses and it did not correlate with phenolic content but rather with pigment content. In conclusion, both the intensity of N nutrition and the fertiliser applied can significantly modify the yield of plants and their quality parameters. For pot cultivation, the most effective fertiliser was Mg(NO<sub>3</sub>)<sub>2</sub> at the N-70/N-90 doses, while for hydroponic cultivation, it is difficult to indicate the most effective fertiliser as the responses varied depending on the method of fertilisation.
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