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Soil samples were collected in June and October from areas with three land-use types, i.e., Robinia pseudoacacia L. (RP), Caragana korshinskii Kom. (CK), and abandoned land (AL), of which the former two were afforested areas, whereas the latter was not. These areas were converted from similar farmlands 40 years prior. Illumina sequencing of 16S rRNA gene and fungal ITS gene was used to analyze soil bacterial and fungal diversity. Additionally, plant communities, soil properties, fine root biomass, and C, N, and P levels in fine root and microbial biomass were estimated. Compared to AL, the C:N:P stoichiometry in fine root and microbial biomass in the afforested lands was synchronously changed, especially the N:P ratio. Soil microbial diversities were affected by afforestation and were more related to N:P ratio than C:P and C:N ratios. Moreover, Alpha-proteobacteria, Gamma-proteobacteria, and Bacteroidetes were significantly more abundant in afforested soils than in the AL soil, and the abundances of Actinobacteria, Chloroflexi, Cyanobacteria, and Nitrospirae ranked as AL > RP or CK. For fungal taxa, Ascomycota abundance responded positively to afforestation, whereas Basidiomycota abundance responded negatively. Changes of soil microbial taxa were significantly correlated with the N:P ratio in fine root and microbial biomass, which explained 54.1 and 55% of the total variation in bacterial and fungal taxa, respectively. Thus, our results provide evidence that compositions of soil microbial communities are linked to the N:P ratio in the plant-soil system. [ABSTRACT FROM AUTHOR]