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