The aim of the study was to work out a method for quantitative assessment of soil nitrogen available for grassland sward (net mineralization), released in alluvial soils in Żuławy. Quantitative assessment of the pool of released nitrogen enables improvement of N management in fertilization of grassland sward and may contribute to the limitation of its dispersal in the natural environment. Studies were carried out in Żuławy Elbląskie in heavy, shallow humic alluvial soil containing about 350 t of soil organic matter and from 13 to 20 t of total nitrogen (TN) per hectare. The study object was permanent meadow sward of different intensity of utilisation and inorganic fertilisation. The efficiency of net mineralization of soil N was determined with the use of indirect balance method. Łaukajtys’s mini lysimetres installed on each experimental plot were used to estimate nitrogen losses in leachates. Depending on the frequency of mowing and different NPK fertilization, meadow sward took up from 80 to 170 kg N released due to mineralization of soil organic matter, which made up about 25 to 50% of nutrient demands of meadow sward. It was also shown that meadow sward uses from 58% to 78% of the total amount of introduced N for growth increment, depending on the intensity of utilization and fertilization.