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.