In the last decades, Poland has become the leader in mushroom Agaricus bisporus production in Europe. Mushroom production generates large amounts of spent substrate, which as a waste material must be recycled. Significant in this macronutrient content, inducing nitrogen, encourages the use of this organic waste material to fertilize, especially light soils. The aim of the study was to assess the effect of spent mushroom substrate fertilization on the properties of A horizon of the Stagnic Luvisol, especially for the variation in the nitrogen fraction, in the two-year cultivation period. The following laboratory analyses were performed: soil pH, organic carbon content, total content of nitrogen and sorption properties. The sequential extraction of nitrogen compounds was performed with a 0.25 M KCl solution (for the extraction of mineral nitrogen forms and the most labile organic nitrogen compounds) and with 0.25 and 2.5 M H2SO4 (hot hydrolysis for sequencing of organic nitrogen compounds that are easily hydrolysing and difficult to hydrolyse). The application of spent mushroom substrate to the A horizons of the experimental objects had an effect on differentiation of soil properties and increased their value (pH, Corg, N-tot, Hh, CEC) in comparison to the control object to a greater degree after the first, then after the second year of cultivation. As the content of nitrogen mineral forms was differentiated, more nitrogen was found in an ammonium form (N-NH4) than in a nitrate form (N-NOx). The organic nitrogen content formed the following series of increasing amounts: soluble organic nitrogen NOS < NOEH < NODH < NONH.
