Multi-species assessment of South African coal mine reclamation soils for ecosystem recovery
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At present, there is no comprehensive soil quality assessment practice for the coal mining industry in South Africa. Studies have shown that belowground soil biota are crucial to soil ecosystem functioning and are sensitive bioindicators for soil quality monitoring. Due to the limitations of some previously developed methods for analysing microbial community structure, a more robust approach involving high-throughput culture-independent molecular techniques was utilised to assess post-coal-mining reclamation soils for potential ecosystem recovery, support function and potential microbial bioindicators. Also, the potential impact of soil physicochemical properties in shaping soil biotic communities were assessed. The study was conducted in three stages. Firstly, the potential contribution of soil stockpiles to post-mining reclamation success was assessed by analysing arbuscular mycorrhizal fungal (AMF) spore density and viability as well as microbial community diversity and structure. Overall, results suggest that AMF spore density in stockpiles do not differ significantly (P <0.05) from those of an unmined soil but spore viability may be affected by stockpile height. Also, variations in the microbial community structure of soil stockpiles were site-specific, but when compared to the unmined site, the microbial community structure and diversity observed in soil stockpiles were impaired. Thus, the impairment in soil microbial diversity and structure suggest post-mining reclamation success may be affected. Secondly, soil samples were collected from reclamation areas in three coal mining sites, as well as from reclamation areas of different ages (ranging from 3 years to 24 years) within a single coal mine. The samples were analysed using a combination of methods that includes community-level physiological profiling (CLPP), enzyme assays, and high-throughput sequencing of the bacterial 16S rRNA gene, fungal ITS2 and a Glomeromycotan-specific partial small subunit. The results provide evidence to support the hypothesis that indeed the microbial communities of post-coal mining soils are significantly (PERMANOVA, P < 0.05) differentiated along a temporal scale of years since reclamation as well as between unmined areas and reclamation areas. When compared to the unmined area, bacterial community richness and diversity data support that restoration is a function of time, and occurs between 15- and 19-years post-reclamation. Furthermore, relative stability in fungal community diversity over years of reclamation compared to bacterial community diversity suggests that bacterial communities are more likely to serve as bioindicators of ecosystem restoration in the post-mining soil environments. Of all the methods, CLPP could not detect significant (P > 0.05) differences in microbial community richness and diversity amongst samples while enzyme activities were highly variable within-sites. The assemblages of the obligate plant symbiont, AMF, were less differentiated when compared to other microbial groups suggesting that AMF assemblages could be less suitable bioindicators of ecosystem recovery. Some genera with soil quality indicator potential such as Acidothermus, Bryobacter and Halingium, as well as plant-growth promotion potentials such as Mesorhizobium, Bradyrhizobium and Microvirga were relatively more abundant across soils, whereas a vast majority of other microbial species and their functions in reclamation soils are still largely unknown. Lastly, the capability of the soil to serve as a habitat to support soil biota association and functions was assessed using an ecotoxicological approach by utilising earthworms as bioindicators. Endpoints such as avoidance behaviour, growth, reproduction and mortality of earthworms were assessed. There was no evidence to suggest that the ecosystem habitat function of stockpile and reclamation soils is significantly limited compared to the Organisation of Economic Cooperation and Development’s artificial control soil. Nevertheless, support functions were highest in unmined soils as determined by the earthworm avoidance behaviour test. Data generated in this study strongly supports that microbial species richness and diversity levels are restored over the years since reclamation, though community composition and structure still differ from the pre-disturbance community. Furthermore, microbial communities of reclamation soil environments are predominantly shaped by pH, phosphorus and nitrogen sources. Overall, bacterial communities are the most responsive and indicative of ecological changes during soil ecosystem restoration. In conclusion, as molecular methods are not without limitations, and because the soil ecosystem environment is governed by an interplay of factors, a comprehensive soil monitoring programme for post-mining reclamation soils in South Africa must comprise a combination of physicochemical properties and microbial community diversity indices as part of a minimum dataset. Furthermore a responsible stockpiling procedure which entails proper excavation and storage of topsoil, as well as the inclusion of microbial inoculants during post-mining reclamation operations, is strongly recommended. Such an approach will help improve coal-mining disturbed soil quality as well as facilitate a quicker ecosystem recovery period.