| Summary: | Relative amounts of nutrients, especially nitrogen, the most abundant macro-element, and also the distribution of ammonium ions in relation to nitrate ions, in the soils of different ecosystems, are determined by many factors. The most important are: temperature, pH of substrate, accumulation of organic matter, presence of allelopathic compounds, degree of oxygenation, etc. The ability of plants to adapt to these variations influences their production of bio-mass, the rate of expansion in different habitats, and the impact on ecosystem and biodiversity. This paper analyzes the impact of different forms of nitrogen (NO3 - and NH4 +) on the growth of seedlings of invasive species Acer negundo L. and Ailanthus altissima (Mill.) Swingle. The results show that nitrogen nutrition only in the form of NH4 + ions significantly affects the reduction in shoot dry weight. Compared to the nutrition with both forms of nitrogen together, aerial parts of Ailanthus altissima were reduced by 62.5%, and leaf area by 66.7%, while Acer negundo seedlings had reduction in dry mass of aboveground part by 89.5%, root by 81.2% and leaf area by 85.8%. Nutrition with nitrate form of nitrogen led to a proportionally small, but statistically significant decrease in dry mass of aboveground parts and roots as well as leaf area of Acer negundo, while in Ailanthus altissima seedlings, it was only the mass reduction of aboveground parts that was significantly influenced, so it can be assumed that this species is more resistant to the lack of both forms of nitrogen. The fact that both species produced significantly more biomass when nitrogen was present in both forms may be important in controlling the spread of alien species, or in their potential use in phytoremediation.
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