Humans continue to transform the global nitrogen cycle at a record pace, through an increased combustion of fossil fuels, growing demand for nitrogen in agriculture and industry, and pervasive inefficiencies in its use, all of which has large impact on the health and processes in terrestrial and aquatic ecosystems worldwide. Studies on the effect of nitrogen addition on plant community showed that as the nitrogen input increases the biodiversity losses. Loss of biodiversity and increase in nitrogen inputs are two of the most crucial anthropogenic factors driving ecosystem processes. Both of them have received considerable attention in previous studies; however, information about their interactive effects on ecosystem function has been scarce. In particular, knowledge of how they interactively influence soil microbial communities and functions has been incomplete.Soil microbial communities can be affected directly by variations in the type, complexity, and amount of organic matter input to soils or indirectly via changes in the soil environment (e.g., soil moisture, temperature, and pH). Changes in any of these factors can influence physical and metabolic niche diversity in the soil, and therefore, may affect microbial diversity or composition. In recent years, the growing research has shown that genotypic diversity of dominant species has similar ecological effects as that of interspecific diversity in smaller species and relatively fragile ecosystems. Different genotypes vary in a multitude of traits including, but not limited to, growth rates, secondary metabolism, and physiological processes. Moreover, such variations have been shown to influence associated species (such as other plants, herbivores, soil microorganisms). Although only a few studies have tested the effects of genetic diversity on soil microbial communities, a few studies have shown that gene diversity in Populus may affect soil microbial communities and soil processes in ways similar to species diversity.Not only the number of species but also the density of the same species or genotypes decreased because of severe degradation in some areas of typical steppes of northern China. Atmospheric nitrogen deposition played an important role on plant diversity and soil microbial communities in the process of grassland degradation. Nitrogen addition has been proved to lead to a large reduction in species richness and loss of perennial grasses in mature communities of Inner Mongolia grasslands, and it was found that it reduces microbial diversity (e.g., functional diversity) in a semi-arid temperate steppe. In the present study, the effects of genotype number of Leymus chinensis, nitrogen addition, and their interactions on the content and community structure of soil microbial communities were tested. The results are as follows. (1) Nitrogen addition had significant effects (P < 0.05) on bacterial phospholipid fatty acid (PLFA) content, Shannon-Wiener diversity index, and Simpson dominance index. (2) The number of Leymus chinensis genotypes had no significant effect on observed variables (P > 0.05), but the interaction between the genotype number and nitrogen addition had a significant effect on bacterial PLFA content and fungal to bacterial ratios (P < 0.05). These results provided the scientific data for the effects of nitrogen deposition and decrease in population size of important species on soil microbial community, and the exploration of community dynamics in a typical steppe of northern China in the context of global change.