Abstract
Lacking of systematic evaluations in soil quality and microbial
community recovery after different amendments addition limits
optimization of amendments combination in coal mine-soils. We performed
a short-term incubation experiment with a varying temperature over 12
weeks to assess the effects of three amendments (biochar: C; nitrogen
fertilizer at three levels: N-N1~N3; microbial agent at
two levels: M-M1~M2) based on C/N ratio (regulated by
biochar and N level: 35:1, 25:1, 12.5:1) on soil quality and microbial
community in the Qilian Mountains, China. Over the incubation period,
soil pH and MBC/MBN were significantly lower than unamended treatment in
N addition and C+M+N treatments, respectively. Soil organic carbon
(SOC), total nitrogen (TN), available nitrogen (AN), available
phosphorus (AP), available potassium (AK), microbial biomass carbon
(MBC) and microbial biomass nitrogen (MBN) contents had a significant
increase in all amended treatments (P <0.001). Higher
AP, AK, MBC, MBN and lower MBC/MBN were observed in N2-treated
soil(corresponding to C/N ratio of 25:1). Meanwhile, N2-treated soil
significantly increased species richness and diversity of soil bacterial
community (P <0.05). Principal coordinate analysis
further showed that soil bacterial community compositions were
significantly separated by N level. C-M-N treatments (especially at N2
and N1 levels) significantly increased the relative abundance
(>1%) of the bacterial phyla Bacteroidetes and Firmicutes,
and decreased the relative abundance of fungal phyla Chytridiomycota
(P <0.05). Redundancy analysis illustrated the
importance of soil nutrients in explaining variability in bacteria
community composition (74.73%) than fungal (35.0%). Our results
indicated that N and M addition based on biochar can improve soil
quality by neutralizing soil pH and increasing soil nutrient contents in
short-term, and the appropriate C/N ratio (25:1: biochar+N2-treated
soil) can better promote mass, richness and diversity of soil bacterial
community. Our study provided a new insight for achieving restoration of
damaged habitats by changing microbial structure, diversity and mass by
regulating C/N ratio of amendments.
Keywords: Mine soil, Biochar, Microbial agent, N fertilizer,
C/N ratio, Soil quality, Microbial community
Introduction
Mining activities in mountainous areas alters soil properties, nutrient
availability, and microbial activity, posing environmental threats
associated with land degradation, water and soil erosion, and loss of
biodiversity (Józefowska et al ., 2017; Ahirwal and Maiti, 2018;
Garbin et al ., 2018). Effective
soil reclamation processes become
urgent and arduous tasks aimed at recovery of the destroyed environment
to a self-sustaining state in opencast mining areas. An environmentally
sustainable method for achieving soil reclamation in mining areas is the
use of soil amendments (Asensio et al ., 2013; Zornoza et al.,
2013).
The success of amendments in soil reclamation can be evaluated mainly on
two aspects: efficient increase in soil nutrients to support vegetation
demand, and growth of soil microbial community (Senesi et al .,
2007; Zornoza et al., 2016). However, applications of various amendments
lack systematic evaluations of their effectiveness in restoring mining
ecosystems, limiting the selection of materials and amendments for soil
reclamation and constraining critical improvements in soil quality and
the growth of soil microbial biomass in mining areas.
Biochar amendments have been
recently widely and successfully used in mine reclamation (Lehmannet al ., 2011; Moreno-Barriga et al ., 2017). Previous
studies have reported positive effects on soil quality and health of
biochar created through the pyrolysis of organic residues (Lehmannet al ., 2011; Marchetti et al ., 2012). The addition of
biochar materials to mine soils can efficiently contribute to the
formation of soil organic matter, retention of nutrients, and
sequestration of heavy metals; meanwhile, biochar additions tend to
alter some soil microbial communities (Grossman et al ., 2010; Zhuet al ., 2017) and stimulate the growth of other microbial
communities (Moreno-Barriga et al ., 2017; Li et al .,
2018). These benefits of biochar indicate that biochar can be used in
combination with other amendments to enhance positive effects on mining
soils. Soil microorganisms play key roles in ecological functioning of
ecosystems, including regulating organic matter decomposition and carbon
stabilization, and mediating nutrient cycling (Sun et al ., 2016;
Pan et al ., 2018). However, extreme soil conditions caused by
severe mining disturbance usually have a negative influence on the
recovery of soil microbial community diversity and mass (de Quadroset al ., 2016). Previous studies of reclaimed mine soils indicated
that microbial biomass and diversity may take 5 to 14 years or longer to
recover to undisturbed soil levels (Mummey et al ., 2002; Dangiet al ., 2012). Thus, given the importance of soil microbial
community to damaged mining habitat, we try to add microbial agents on
basis of biochar amendments in order to activate microbial activity. It
is important to verify whether the addition of microbial agents combined
with biochar can activate soil microbial activity, and may give new
insights on how to promote soil microbial recovery in damaged habitats.
Biochar additions to soils can
also absorb mineral nitrogen, which can reduce nitrate-nitrogen
(NO3−-N) leaching, increase
ammonium-nitrogen (NH4+-N) retention,
and improve the use efficiency of nitrogen fertilizer (Clough et
al ., 2013; Ameloot et al ., 2015). Studies have shown that a
combined application of biochar and nitrogen fertilizer had significant
effects on soil nutrient content, microbial biomass carbon, nitrogen,
and crop yields in agricultural lands (Zheng et al ., 2012; Zhuet al ., 2014). However, there are few reports on the combined
application of biochar and nitrogen fertilizer in mine soils. In mine
soils, the effects of amendments on soil physicochemical properties,
microbial biomass and diversity may depend on
the adjustment of the C/N ratio
(Lucas et al ., 2014). In general, low C/N ratio of amendments
could have inhibitory effects on the activity of microorganisms,
including decreasing microbial biomass and metabolites (Treseder, 2008).
However, the effects of combined
applications of different levels of nitrogen fertilizer and biochar
(adjusting C/N ratio in soil) on soil microbial biomass and diversity
are still unclear in soil reclamation of mining areas. Therefore, to
explore the most favorable ratio of combined application of nitrogen
fertilizer and biochar for soil microbial biomass and diversity, which
can provide scientific basis for the sustainable recovery of soil in
mining area.
Qilian Mountains are important ecological security barrier in the
western part of China (Du et al ., 2015), and contain abundant
hydropower and mineral resources (i.e. iron ore, copper ore, tungsten
ore, coal mine). However, the local environment of the Qilian Mountains
has been severely damaged due to long-term illegal mining (i.e.
unlicensed mining, mining of protected minerals) for economic benefits.
The restoration and reconstruction of the damaged ecosystem in Qilian
Mountains becomes an important task of environmental protection.
However, there are still many difficulties in ecosystem restoration at
field level in this area due to high-altitude, complex topography, large
soil heterogeneity, cold and changeable climate. Therefore, a short-term
laboratory soil incubation experiment close to the local varying
temperature can overcome the above difficulties and provide reference
and basis for application at different topographical locations with
damaged habitats in mining areas.
To relieve soil nutrient impoverishment and restore soil microbial
diversity and mass caused by opencast mining in high-altitude areas,
soil reclamation was carried out with the addition of different
amendments. We aimed to add microbial agents on basis of biochar
amendments in order to activate microbial activity, and determine the
most favorable C/N ratio (combined application of nitrogen fertilizer
and biochar) for promoting soil nutrients, microbial diversity and mass,
and select the most effective combination of amendments to mining soil
by analyzing shifts in soil physicochemical and biological properties.
We conducted a short-term laboratory soil incubation experiment with a
varying temperature for 12 weeks with 13 combined treatments by three
amendments(biochar, nitrogen fertilizer, microbial agent). Our
objectives were to: i) Determine dynamics of soil physicochemical
properties (pH, EC, soil organic carbon, total nitrogen, available
nitrogen, available phosphorus, available potassium) over incubation
time, ii) Determine the effects of different amendments on microbial
biomass (carbon and nitrogen), and composition and diversity of
bacterial and fungal community; and iii) Verify which C/N ratio adjusted
by the combination of biochar and N level is more suitable for microbial
growth.
Materials and methods