Abstract: We selected enclosed grassland, grazed grassland and shrublands with different planting years (3, 12, 22 years)/densities (intervals of 2, 6, 40 m) to investigate soil N mineralization dynamics in the growing season (April-October) and its influencing factors during the process of desert steppe-degradation-shrub introduction. The results showed that soil moisture at 0-200 cm layer was decreased with increases of shrub age and density, and that the variations of soil moisture at 0-10 cm layer coincided with seasonal change. Compared with grazed grassland and enclosed grassland, the positive effect of shrubs on soil carbon, nitrogen and phosphorus contents first increased and then decreased with the increases of age and density. Moreover, soil N mineralization significantly varied across months and sites. Soil NO₃^- content and microbial biomass carbon and nitrogen were significantly higher from June to August. The proportion of NO₃^- to inorganic nitrogen significantly increased from 30.5% in enclosed grassland to 69.5% in shrublands. NH₄⁺ content was mainly affected by months compared with sites. In the process of steppe-degradation-shrub introduction, the increases of shrub age and density significantly enhanced seasonal differences of soil nitrification and ammonification, but not on the seasonal mineralization of microbial biomass carbon and nitrogen. Soil NH₄⁺ and NO₃^- contents were significantly and positively correlated with total nitrogen, organic carbon and N/P. Soil stoichiometric ratios (C/N and N/P) directly regulated N mineralization process.

Key words: desert steppe, shrub anthropogenic introduction, nitrogen response, in-situ mineralization, seasonal dynamics.