Abstract: Four types alpine meadow, fenced grassland (FG), lightly degraded grassland (LD), moderately degraded grassland (MD) and heavily degraded grassland (HD), were selected to study the effects of grassland degradation on soil inorganic nitrogen characteristics and their seasonal dynamics in the eastern Qilian Mountain area. Samples of the 0-10 cm, 10-20 cm and 20-30 cm soil layers were collected from the four grassland types in spring, summer and winter. Soil total inorganic nitrogen, ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N) contents and urease activities were measured. The results showed that grassland degradation has more influence on total inorganic nitrogen contents in the surface soils than deeper layers, but these effects varied by season. Compared with FG, the total inorganic nitrogen content of LD, MD and HD soils increased by 39.9%, 28.9% and 68.4% in spring and increased by 19.1%, 28.7% and 33.6% in summer. In winter, the total inorganic nitrogen content of these three kinds of grassland decreased by 20.4%, 27.2% and 47.4% respectively. The results also showed that grassland degradation has varying effects on the forms of soil inorganic nitrogen. In spring, grassland degradation significantly increased NH4+-N content in surface soils and there was little effect on NO3--N. In summer, NO3--N content increased significantly in surface soils while there was little effect on NH4+-N content. Degradation levels were also associated with seasonal differences in soil urease activity. Compared with FG, in spring the urease activities of LD and MD soils increased while HD decreased significantly. Soil urease activity increased gradually with increased degradation levels in summer and winter, which indicates that degradation increased soil inorganic nitrogen supplication and improved the soil nitrogen conversion process.

Key words: grassland degradation, soil inorganic nitrogen, soil ammonium nitrogen (NH4+-N), soil nitrate nitrogen (NO3--N), soil urease activity