1 Introduction
Ecological restoration is a complex process that involves improvement of green vegetation coverage, prevention of soil erosion, and reduction of natural disasters. Currently, revegetation is a hot topic of research in ecology, encompassing soil physical and chemical properties and soil and water conservation (Zeng et al., 2014), ecological stoichiometric characteristics of vegetation and soil (Jiao et al., 2013), soil nutrient element preservation (Ma et al., 2014), biodiversity of undergrowth vegetation (Chen and Cao, 2014), and soil microbial communities (Zhang et al., 2010). As the agents of decomposition in the ecosystem, soil microorganisms play a key role in forest succession and nutrient cycling. They are the link between the aboveground and underground components of the ecosystem and represent an important index for evaluation of ecosystem restoration (Wardle, 2004; Lewis et al., 2010; Van der Heijden et al., 2008). Plant community in forest ecosystems by changing the species composition, biomass, litter–earth system components, forest and soil environment factors, such as soil microbial community structure and composition (Okubo et al., 2016; Prescott and Grayston, 2013; Spohn and Widdig, 2017; Ushio et al., 2013), and many relevant studies have reported the species and soil physical and chemical properties of soil microbial community composition and diversity (Ding et al., 2017; Nacke et al., 2016; Pei et al., 2016; Zhalnina et al., 2015).
With the development of soil microbial research methods, high-throughput sequencing technology, also known as either ”next-generation sequencing technology” or deep sequencing, has gradually become a conventional method for analyzing the diversity of soil microorganisms. Currently, this method is used widely to determine the effects of vegetation, crop cultivation and land management, organic pollutants, heavy metals, seasonal changes, climatic conditions, and other factors on soil microbial community structure and biomass (Ren et al., 2018; Zeng et al., 2016; Zhang et al., 2016, 2018).
It is a long-term and arduous task to harness the ecological environment and restore the vegetation in the Loess Plateau. Revegetation is not only the first step in ecosystem reconstruction but also an important measure of ecological environment management and soil and water conservation on the Loess Plateau, and it can effectively reduce soil erosion and improve soil quality (Fierer and Jackson, 2006; Xing et al., 2013). Wuqi County in Shaanxi Province is located in an interlaced area of wind erosion and water erosion on the Loess Plateau. In that area, the soil erosion is serious and the ecological environment is fragile. Since 1998, the government has adopted a development strategy based on “closing hills and returning farmland, afforestation to grass, indoor sheep production, and by planting trees to move out of poverty.” The change in vegetation development and species composition in Wuqi County following the closure reflects the trend of increasing stability of the degraded matrix and enhancement of the community environment (Luo, et al., 2006).
In recent years, the revegetation project on the Loess Plateau has increased the area of vegetation considerably, and an increasing number of studies have examined the revegetation of the returned farmland in this area (Chen et al., 2015; Deng et al., 2015; Feng et al., 2013; Feng et al., 2016; Kou et al., 2016; Nie et al., 2015; Song et al., 2015). In general, these studies have focused on the impacts of revegetation on soil properties, changes of species diversity, the benefits of soil and water conservation, and the allocation mode of revegetation in the process of land restoration. However, there has been no systematic analysis of the interaction between revegetation, soils, and microorganisms in the process of land restoration.
Hippophae rhamnoides , which is a pioneer and associated species of vegetation structure in the Loess Plateau, has clear ecological and economic benefits. Its root system can co-exist with nitrogen (N)-fixing actinomycetes to form N-fixing nodules and fix free N. Furthermore, this root system can decompose insoluble organic matter and minerals, thereby helping to improve the soil (Hu et al., 2014; Hu, 2012; Li et al., 2003).
Therefore, to determine feasible strategies for revegetation in Wuqi County, we used high-throughput sequencing technology to analyze the characteristics of soil microbial carbon (C)and N elements and the relationship between soil microorganisms and the soil environment in the area, taking the pure and mixed H. rhamnoides forests as the research object and bare grassland as a control.