Does a decrease in microbial biomass alter mycorrhizal attributes and soil quality indicators in coal mining areas under revegetation process?

Losses of microbial diversity in degraded ecosystems still have obscure consequences, especially when considering the interaction between arbuscular mycorrhizal fungi (AMF) and soil bacteria. This study investigates the effect of decreasing microbial biomass on mycorrhizal attributes and soil quality indicators. The dilution-to-extinction approach was applied in microcosms to search for associations among bacterial diversity, mycorrhizal attributes, and soil quality indicators. The experiment was conducted with four soil treatments (undiluted control 10⁰ = D0, 10⁻³ = D3, 10⁻⁶ = D6, and 10⁻⁹ = D9) from a short-term (two years = 2Y) and a long-term (15 years = 15Y) coal mine revegetation area. Microcosms were inoculated with 300 spores of Acaulospora colombiana, Gigaspora albida, and Claroideoglomus etunicatum with millet as the host plant. Results included the total number of AMF spores, mycorrhizal colonization, soil aggregation, glomalin, fluorescein diacetate hydrolysis (FDA), basal soil respiration, microbial biomass, and soil bacterial microbiome. Larger differences were observed between areas than between dilution treatments within the sampling area. Attributes that presented differences in the dilutions compared to D0 2Y samples were mycorrhizal colonization (D0 = 85% and D9 = 43.3%), FDA (D0 = 77.2% and D9 = 55.5%), extractable glomalin-related soil protein (D0 = 0.09 and D9 = 0.11) and bacterial diversity (D0 = 7.3 and D6 = 5.3). D0 15Y samples presented differences in microbial biomass nitrogen (D0: 232.0) and bacterial diversity (D0: 7.9, D9: 5.6) compared to the dilutions. Bacterial microbiome present in the D0 samples formed distinct clusters as to other samples and correlated with soil aggregation and basal respiration attributes. Results suggest that AMF inoculation and dilution-to-extinction did not affect soil quality indicators preeminently, but the bacterial community is affected and can influence the process of environmental revegetation. A long-term revegetation period is substantial to improve quality indicators and establish the diversity of microorganisms and consequently revegetation in areas impacted by coal mining.