The paper presents results of the research on the content of mineral nitrogen (N-NO3, N-NH4) in soil profile (0–90 cm) in the ecological and conventional systems. The research was conducted on a field trial located at the Experimental Station in Osiny (Lublin province, Poland) on grey brown podzolic soil. Ecological and conventional systems are characterized by different crop rotations and crop production technologies. In the ecological system the following crop rotation was applied: potato - spring barley - red clover with grass grown two years - winter wheat + catch crop, whereas in conventional one: winter rape - winter wheat - spring barley. The content of mineral nitrogen in a soil profile in compared systems was very differentiated and depended on the crop succession, date of manure application, nitrogen rates and weather conditions. In the ecological system in most fields the increase of mineral nitrogen content and a reduction of its losses were observed. In the conventional system due to simplified crop rotation there were favourable conditions for losses of mineral nitrogen. The largest potential danger due to Nmin losses was observed in a site after grass with clover in the ecological, and after rape in the conventional system.