The effects of grass growth and N deposition on the leaching of nutrients from forest soil were studied in a lysimeter experiment performed in the Moravian-Silesian Beskydy Mts. (the Czech Republic). It was assumed that the grass sward formed on sites deforested due to forest decline would improve the soil environment. Lysimeters with growing acidophilous grasses (Calamagrostis arundinaceaandC. villosa), common on clear-cut areas, and with unplanted bare forest soil were installed in the deforested area affected by air pollution. Wet bulk deposition of sulphur in SOcorresponded to 21.6-40.1 kg ha-1 and nitrogen in NHand NOto 8.9-17.4 kg N ha-1, with a rain water pH of 4.39-4.59 and conductivity of 18.6-36.4 µS cm-1 during the growing seasons 1997-1999. In addition, the lysimeters were treated with 50 kg N ha-1 yr-1 as ammonium nitrate during the 3 years of the experiment. Rapid growth of planted grasses resulted in a very fast formation of both above- and below-ground biomass and a large accumulation of nitrogen in the tissue of growing grasses. The greatest differences in N accumulation in aboveground biomass were observed at the end of the third growing season; inC. villosaandC. arundinacea, respectively, 2.66 and 3.44 g N m-2 after addition of nitrogen and 1.34 and 2.39 g N m-2 in control. Greater amounts of nitrogen were assessed in below-ground plant parts (9.93-12.97 g N m-2 inC. villosaand 4.29-4.39 g N m-2 inC. arundinacea). During the second and third year of experiment, the following effects were the most pronounced: the presence of growing grasses resulted in a decrease of both the acidity and conductivity of lysimetric water and in a lower amount of leached nitrogen, especially of nitrates. Leaching of base cations (Ca2+ and Mg2+) was two to three times lower than from bare soil without grasses. An excess of labile Al3+ was substantially eliminated in treatments with grasses. Enhanced N input increased significantly the acidity and losses of nutrients only in unplanted lysimeters. The leaching of N from treatments with grasses (3.9-5.6 kg N ha-1) was 31-46% of the amount of N in wet deposition. However, the amount of leached N (4.2-6.0 kg N ha-1) after N application was only 7.1-8.9% of total N input. After a short three year period, the features of soil with planted grasses indicated a slight improvement: higher pH values and Ca2+ and Mg2+ contents. The ability of these grass stands to reduce the excess nitrogen in soil is the principal mechanism modifying the negative impact on sites deforested by acid depositions. Thus it is suggested that grass sward formation partly eliminates negative processes associated with soil acidification and has a positive effect on the reduction of nutrient losses from the soil. [ABSTRACT FROM AUTHOR]