Introduction

The total area of monoculture and mixed-species plantations in China is the largest of any nation globally, covering 69.3 million ha in 2014. Approximately 50% of the country’s tree plantations are coniferous monocultures (State Forestry Administration 2014). However, in comparison with natural forests, the monoculture coniferous plantations affect forest ecological services, such as decreasing above- and below-ground biodiversity, causing soil acidification, and reducing soil carbon (C), available nitrogen (N), and phosphorus (P) levels (Yang et al. 2010, 2013). The introduction of native tree species to coniferous stands may restore forest ecological services. For example, mixed forest stands could improve soil C sequestration and biodiversity (Wardle et al. 2004; Brassard et al. 2011; Xu et al. 2021). Soil microbes are important components of soil that can affect C and nutrient cycling in forest ecosystems (Bohlen et al. 2001). Furthermore, the compositions and activities of soil microbial communities are more sensitive to changes in forest composition than soil C and nutrient alterations (Pei et al. 2016; Gunina et al. 2017). However, only a few studies have focused on changes in microbial communities occurring in mixed plantations in temperate forest ecosystems (Thoms et al. 2010). Therefore, it is essential to understand the differences in the compositions and functions of microbial communities, as well as their relationships with soil properties in monoculture and mixed plantations.

Soil microorganisms are known as essential components of soil function (Harris 2009). Urbanová et al. (2015) reported that microbial community composition was determined by tree species. Tree species compositions affect microbial communities directly through litter and root quality and nutrient uptake, as well as indirectly by altering soil properties (Bach et al. 2010; Huang et al. 2013; Liu et al. 2012). In our study area, the initial levels of soil C and N content may have weakened the effects of tree species on soil microbial communities.

Although no significant change was found in fungal and bacterial abundances (except for bacterial abundances in the litter), microbial community compositions shifted in the mixed plantation. Our hypothesis that a mixed plantation can alter the structures and functions of microbial communities was confirmed. Several researchers have demonstrated that the structures of microbial communities are more sensitive than chemical parameters in response to changes in plant species (Gunina et al. 2017; Pereira et al. 2019). We observed that there are differences in the fungal and bacterial communities of forest litter and soil in monoculture and mixed-species plantations. Previous research has demonstrated that mixtures of Larix litter with broadleaved tree litter exhibited higher rate of litter decomposition than monoculture Larix litter. Specifically, mixing Larix litter with broadleaved litter could change the composition and function of the microbial community and improve the decomposition of hemicelluloses and lignin (Zhang et al. 2019). In contrast, soil that was mainly composed of fungal genera in the mixed plantation was only marginally different from that in the monoculture plantation. In addition, rare bacterial genera significantly contributed to the observed structural community differences. This result indicates that dominant bacterial genera are not sensitive to mixed plantations in temperate forest ecosystems; whereas, in evaluating bacterial community responses, rare bacterial genera were found to be critical. We also found that bacterial and fungal abundances decreased with soil depth, which is consistent with the results of other studies (Hartmann et al. 2009; Eilers et al. 2012).

Conclusions

The mixed-species plantations can significantly change the compositions and functions of microbial communities, but not the C and N contents, in comparison with monoculture plantations. Our study demonstrates that fungal communities are more sensitive than bacterial communities in responding to mixed plantations. Furthermore, compared to monoculture plantations, mixed-species plantations can significantly enhance soil available N and exoglucanase, β-glucosidase, and N-acetyl-β-glucosaminidase activity levels. Given that microorganisms are mediators of several important ecosystem functions, such as C and nutrient cycling, having mixed plantations might affect C and nutrient pools in the long term. In addition, our research suggests that fungal community compositions and enzyme activities are sensitive indicators of soil properties in mixed plantations that are more than 20 years old. In future, all these microbial properties should be considered in the selection of mixed tree species for soil restoration.