## 1 Introduction  
In recent years, organic farming methods have become an increasing competitor to conventional agriculture. Organic farming is as attractive as traditional farming in many European regions and countries [1; 2]. Organic agriculture uses organic fertilisers, and diseases and pests are combated with biological preparations, positively affecting the environment, human health, and biodiversity protection [3; 4; 5]. In the case of conventional methods, it is the pesticides and other chemicals that maximise yields and accelerate plant growth. According to the available literature, organic food is characterised by a higher nutritional value and lower levels of nitrates, nitrites, pesticide residues, growth regulators, antibiotics, or synthetic food additives [6; 7; 8; 9; 10; 11; 12].  
Excessive accumulation of nitrates in vegetables has been the subject of intensive research for many years. The amount of nitrogen in plants is mainly influenced by the cultivation method and environmental factors. Moreover, the content of nitrates is genetically controlled and may constitute a characteristic feature of a species or even a variety [13]. Therefore, the nitrate level in nine plant species from different cultivation systems was determined. Additionally, an attempt was made to understand and compare nitrogen metabolism based on the measurement of chlorophyll content and the activity of nitrate reductase (NR) and glutamine synthetase (GS).  
Readily available nitrogen from mineral fertilisers can boost plant growth and the synthesis of nitrogen-containing compounds, but it may also limit the synthesis of carbon-based compounds, such as secondary metabolites [14]. Conversely, the absence of synthetic pesticides in organic farming may increase plant sensitivity to pests and pathogen attacks, thereby reinforcing phenolic compound (Phe) syntheses [15; 16]. Their increased content is essential for plant defence systems [17; 18; 19; 20]. The concentration of phenolic compounds can be affected by environmental conditions such as light intensity, carbon dioxide levels, temperature, fertilisation, and biotic and abiotic factors [21].  
Phenolic compounds remarkably affect human health [22; 23; 24; 25]. They are of great interest because of their potential role in preventing cancer and heart disease. This role may be related to their antioxidant activity, which was shown to be higher than ascorbic acid (AsA) against reactive oxygen species [26]. Usually, plants with higher phenolic compound quantities prove their increased antioxidant capacity. According to the literature, the results regarding the antioxidant capacity of vegetables from conventional and organic farming vary according to plant type and the bioactive compound measured [27; 28; 29; 30; 31].  
This study aimed to determine the level of selected indicators of nitrogen metabolism in vegetables from organic (organic food store) and conventional (supermarket and local market) crops and their antioxidant capacity. Research is being conducted worldwide to compare the quality of raw materials and products obtained from different cultivation systems. Constant food monitoring is crucial, regardless of the place of cultivation. Therefore, the issues discussed in this work seem to be fully justified.