Municipal landfills are hot spots of dynamic bioprocesses facilitated by complex interactions of a multifaceted microbiome, whose functioning in municipal landfills at different maturing stages is poorly understood. This study determined bacterial community composition, interaction conetworks, metabolic functions, and controlling physicochemical properties in two landfills aged 14 and 36 years. High throughput sequencing revealed a similar distribution of bacterial diversity, evenness, and richness in the 14‐ and 36‐year‐old landfills in the 0–90 cm depth. At deeper layers (120–150 cm), the 14‐year‐old landfill had significantly greater bacterial diversity and richness indicating that it is a more active microcosm than the 36‐year‐old landfill, where phylum Epsilonbacteraeota was overwhelmingly dominant. The taxonomic and functional diversity in the 14‐year‐old landfill was further reflected by the abundant presence of indicator genera Pseudomonas,Lutispora,Hydrogenspora, and Sulfurimonas coupled with the presence of biomarker enzymes associated with carbon (C), nitrogen (N), and sulfur (S) metabolism. Furthermore, canonical correspondence analysis revealed that bacteria in the 14‐year‐old landfill were positively correlated with high C, N, S, and phosphorus resulting in positive cooccurrence interactions. In the 36‐year‐old landfill, negative coexclusion interactions populated by members of N fixing Rhizobiales were dominant, with metabolic functions and biomarker enzymes predicting significant N fixation that, as indicated by interaction network, potentially inhibited ammonia‐intolerant bacteria. Overall, our findings show that diverse bacterial community in the 14‐year‐old landfill was dominated by copiotrophs associated with positive conetworks, whereas the 36‐year‐old landfill was dominated by lithotrophs linked to coexclusion interactions that greatly reduced bacterial diversity and richness.

Humans are dependent upon soil which supplies food, fuel, chemicals, medicine, sequesters pollutants, purifies and conveys water, and supports the built environment. In short, we need soil, but it has little or no need of us. Agriculture, mining, urbanization and other human activities result in temporary land-use and once complete, used and degraded land should be rehabilitated and restored to minimize loss of soil carbon. It is generally accepted that the most effective strategy is phyto-remediation. Typically, phytoremediation involves re-invigoration of soil fertility, physicochemical properties, and its microbiome to facilitate establishment of appropriate climax cover vegetation. A myco-phytoremediation technology called Fungcoal was developed in South Africa to achieve these outcomes for land disturbed by coal mining. Here we outline the contemporary and expanded rationale that underpins Fungcoal, which relies on in situ bio-conversion of carbonaceous waste coal or discard, in order to explore the probable origin of humic substances (HS) and soil organic matter (SOM). To achieve this, microbial processing of low-grade coal and discard, including bio-liquefaction and bio-conversion, is examined in some detail. The significance, origin, structure, and mode of action of coal-derived humics are recounted to emphasize the dynamic equilibrium, that is, humification and the derivation of soil organic matter (SOM). The contribution of plant exudate, extracellular vesicles (EV), extra polymeric substances (EPS), and other small molecules as components of the dynamic equilibrium that sustains SOM is highlighted. Arbuscular mycorrhizal fungi (AMF), saprophytic ectomycorrhizal fungi (EMF), and plant growth promoting rhizobacteria (PGPR) are considered essential microbial biocatalysts that provide mutualistic support to sustain plant growth following soil reclamation and restoration. Finally, we posit that de novo synthesis of SOM is by specialized microbial consortia (or ‘humifiers’) which use molecular components from the root metabolome; and, that combinations of functional biocatalyst act to re-establish and maintain the soil dynamic. It is concluded that a bio-scaffold is necessary for functional phytoremediation including maintenance of the SOM dynamic and overall biogeochemistry of organic carbon in the global ecosystem

A mutualistic relationship between grasses, coal-degrading fungi, and arbuscular mycorrhizal fungi was proposed to account for the phyto-biodegradation of coal discard. In this study pot trial experiments were carried out to confirm transformation of the carbonaceous substrate, in the presence of a suite of coal degrading fungi and arbuscular mycorrhizal fungi, into a humic-enriched soil-like material in the Cynodon dactylon/coal rhizosphere. The results show that after 47 weeks of C. dactylon growth on coal discard the concentration of humics increased from (62.9 ± 1.5) to (112.1 ± 5.4) mg/kg. Substrate humic acid-like substance concentration positively correlated (r ² = 0.95) with accumulation of above ground C. dactylon biomass. FTIR spectroscopy of the extracted humic-like substances confirmed both product identity and increased oxidation of the coal discard substrate. Substrate ash content and electrical conductivity declined coincident with an increase in humic acid-like substance concentration, which together reduced the intensity of acidity in the C. dactylon/coal discard rhizosphere. These observations support the proposal that biological oxidative degradation of coal discard leads to increased humic-like substance concentration and formation of a soil-like material. Results have profound implications for use of coal discard as an organic substrate to replace topsoil in phyto-bioremediation strategies for sustainable large-scale rehabilitation of coal discard dumps.