Understory vegetation plays a vital role in the flow of materials and nutrient cycling in plantation ecosystems. Introducing functional plants (one species or a group of plants that share similar characteristics and can play a similar role in an ecological environment) can quickly improve the environment of the soil of a plantation with a single-stand structure suffering from soil degradation. Five stands composed of Chinese fir plants of different ages (young, immature, near-mature, mature, and over-mature stand forests) were supplemented with leguminous plants to determine the effects on soil nutrients and microbial communities. We supplemented the five stands with five different combinations of four non-native plant species, Dalbergia balansae, Taxus chinensis, Spatholobus suberectus, and Kaempferia galangal, as treatments. After one year, plant growth was estimated, and soil samples were collected for laboratory experiments and high-throughput sequencing. Our results show that supplementing the stands with plants increased the nutrient content of the soil and promoted the growth and diversity of soil microbial communities in Chinese fir plantations. Furthermore, the effects of plant supplementation varied according to the age of the stand in the plantation; thus, the positive effects were stronger for young, immature, and near-mature stand forests than they were for mature and over-mature stand forests. Measurements of the microbial diversity in the soil revealed that supplementation increased diversity in the fungal community more than that in the bacterial community. A principal component analysis (PCA) of the five treatments and controls under different forest stands ages demonstrated that microbial communities differed significantly between treatments and controls and that supplementing Chinese fir plantations with leguminous plants had a greater influence on microbial communities than other plants did. Our study suggests that certain leguminous plants can increase soil nutrients and the diversity of soil microbial communities in one year.