Szili-Kovacs, T., T. Tóth, M. Halassy and K. Torok. 2000. Restoration of sandy grasslands through the immobilization of soil nitrogen. 1. Laboratory incubation experiments. (Hung.) Agrokémia és Talajtan. 49:479-490. It is a presumption of many ecologists that the vegetation recovery of disturbed lands highly depends on the soil N-availability. High available-N inhibits expansion of natural vegetation while favourable for early-seral r- strategists.Experiments were design to test the applicability of nitrogen immobilization as means of accelerating the recovery of endemic open sand grassland community (Festucetum vaginatae danubiale) on old-fields in the Great Hungarian Plain. Sucrose and oak sawdust as carbon sources were selected for field application to increase microbial N-immobilization. The quantity of C- sources applied in the field was 700 kg C/ha/year. The sawdust was broadcasted only once (300 kg/ha) in early spring while sucrose 4-times (1300kg/ha) during the vegetation season.Three experimental sites were located along an elevation gradient parallel with an aridity and productivity gradient (R="meadow"; Bk="depression"; Bt="hummock"). The first one is the most wet, productive and organic-rich. Six replicate plots (10 by 10 metres) both for control and treated in one site have been assigned for field observations.Soil organic matter, microbial biomass-C, cellulose decomposition and inorganic-N were measured during the vegetation season in 1998. Soil organic matter, microbial biomass-C and decomposition rate changed from site to site in accordance with the elevation gradient. We found significant increase in microbial biomass-C and decrease in soil available-N on the treated plots in two sites (Bk and Bt). The third site having the highest organic-C (R) did not shown significant differences due to the carbon source addition. Cellulose decomposition rate was significantly lower in treated plots only in Bk site which can be probably attributed to the reduced soil mineral-N caused by microbial N-removal. The average rate of cellulose decomposition was 27.1; 20.9; and 7.0 mg cellulose per g cellulose per day for site R; Bk and Bt respectively during the vegetation season. The plant communities did not shown any alteration due to the treatment in the first year, however they markedly differed in the sites.