1 Introduction
China is a large country in coal resources and the largest coal consumer
in the world. Underground mining, which has been the most common coal
mining technique in the world, accounts for the most coal outputs in
China (Hu et al., 2019). Large-scale underground coal mining in China
has frequently resulted in soil collapse, damaging land resources
irreversibly, and affecting agricultural production and the ecological
integrity of the environment. At present, it is estimated that for each
1 × 104 t of coal that was mined, about 0.1 to 0.3
hm2 of land was sinking in China (Hu and Luo, 2006).
Moreover, in mining areas where water tables are near the ground
surface, surface subsidence has led to surface water accumulation,
provoking shifts from terrestrial to aquatic ecosystems with dramatic
changes in the original ecological equilibrium. The latter issue has
also led to the destruction of large farmlands, threatening food
security, and triggering conflicts between rural people who once worked
and lived from the land. To circumvent these issues, many subsided areas
are currently being reclaimed into agricultural land.
Agricultural land provides essential resources related to food security
worldwide, in general, and in China, in particular. As a result, land
reclamation has become an important approach to increase the surface of
agricultural land after the subsidence of the land overlying coal mines
in China (Bian et al., 2018). It is estimated that about 32.6% of
agricultural land has been reclaimed from coal-grain composite areas in
China (Qu et al., 2018). The coal-grain composite areas in the eastern
regions of China are vital to support food security in this area, but
land subsidence as a result of coal mining activities are jeopardizing
the integrity of its agroecosystems (Hu, 2019).
Land reclamation in subsided mining areas involves refilling the receded
area with topsoil followed by compaction operations (Qu et al., 2017;
Hu, 2019). As a result, the natural structure of the soil is severely
altered (Bai et al., 2018). Moreover, key soil attributes are also
modified after land reclamation. Reclaimed soils may present less soil
organic matter, altered soil texture, lower soil water retention
capacity, less soil nutrients, and lower native soil microbial diversity
than the original soil (Ahirwal and Maiti, 2016; Luo, et al., 2020). All
these changes may limit crop yield and land development following land
reclamation (Zhao et al., 2015). Natural succession of reclaimed soils
is slow and ecological equilibrium is normally reached after a long time
following reclamation (Bai et al., 2018). During this time, the
reclaimed land may also be exposed to wind and water erosion, as well as
to contamination (Hu, 2019). Therefore, monitoring the status of key
soil attributes after reclamation and over time is essential for gaining
insights into soil development and land restoration following
reclamation (Ngugi et al., 2018; Shrestha and Lal 2011). However, only
few studies have addressed this issue (Qu et al., 2018, Min et al.,
2017) and even fewer have focused on the existing ecological networks
between soil microbial communities (SMCs) after land reclamation in
mining areas (Hu, 2019; Luo et al., 2020; Ma et al., 2020).
Soil microorganisms are an essential part of the soil ecosystem. SMCs
play a key role in the processes of decomposing soil organic matter,
nutrient cycling and utilization of nutrients by plants (Novianti et
al., 2018; Yuan et al., 2018). After land reclamation, changes in the
soil physical and chemical properties may provoke changes in key soil
attributes (e.g. pH value) but also in the SMCs and their related
functions in the soil ecosystem (Harris, 2009; Delgado-Baquerizo, et
al., 2019). The reclamation of mining soil into farmland mostly focuses
on restoring the integrity of the agroecosystem aboveground whilst the
restoration of the integrity and function of the belowground
compartments is largely overlooked (Liu et al., 2019). Detailed
assessment of SMCs is essential to evaluate the success of soil
reclamation and development (Dangi et al., 2012). In this regard,
healthy soils will normally present diverse SMCs (Tu et al., 2020),
indicating an adequate functioning of the soil ecosystem
(Delgado-Baquerizo, et al., 2016) and an adequate provision of ecosystem
services by the soil compartment (e.g. organic matter decomposition,
nutrient cycling) (Griffiths and Philippot, 2013). However, little is
known about the diversity of and the function provided by the SMCs found
in soils subjected to reclamation after coal mining activities.
In recent years, high-throughput sequencing technology has become a
common method to provide information related to SMCs. Nonetheless, the
translation of the outputs from sequencing into new practical knowledge
is challenging due to the size of the data generated throughout and the
difficulty to correlate these with other environmental variables such as
key soil attributes (Wang and Brose, 2018; Delgado-Baquerizo, et al.,
2019). Molecular ecological networks (MENs), using the mathematical and
bioinformatics methods to construct ecological association networks, and
provide a conceptual framework to identify microbial interactions and
key populations, and relate with key attributes and functions occurring
in the soil compartment (Deng et al., 2016). Moreover, MENs can be
useful to identify keystone taxa of soil microorganisms (Kitano, 2004).
Keystone taxa play important roles in the microbiome, such as driving
community composition and functionality, and their variation might cause
changes in microbiome composition and functioning (Delgado-Baquerizo et
al., 2018). While previous studies have reported keystone taxa in the
soil under multiple environmental conditions, information on how
keystone taxa may change over time following land reclamation in mining
areas is severely lacking (Berry and Widder, 2014; Fierer, 2017; Ramirez
et al., 2018; Banerjee et al., 2018 and 2019).
The aim of this study is to assess the restoration success of disturbed
mine soil following land reclamation into arable land. To do so, we
investigate changes in key soil attributes and in the composition of
SMCs along a reclamation chronosequence of 17 years following land
restoration. We also evaluate changes in the complexity of the
ecological networks established between the SMCs and their interactions
as an indicator of soil development after land reclamation. The results
from our investigation will provide novel and practical insights into
the ecological restoration of disturbed mine soils worldwide.