Biochar and biomass organic amendments shaped different dominance of lithoautotrophs and organoheterotrophs in microbial communities colonizing neutral copper(Cu)-molybdenum(Mo)-gold(Au) tailings

The present study aimed to characterize effects of organic amendments (i.e., biochar (BC) and sugarcane mulch (SM), respectively) on microbial properties in fresh and neutral Cu-Mo-Au tailings, for the purpose to eco-engineer tailings into technosols in glasshouse microcosms with native plant cover. Organic amendments modified key physicochemical conditions, including water holding capacity, cation exchange capacity, and levels of labile organic carbon and nitrogen. SM elevated available C and N, and microbial diversity, respiration rate and enzymatic activities in the amended tailings, regardless of plant species established. Specifically, SM amendment elevated the abundance of Bacteroidetes, especially the organic matter decomposers (e.g., Algoriphagus, Chitinophagaceae). Although BC addition reduced the Cu availability and uptake by plants, it had little impacts on levels of available nutrients, microbial biomass and enzymatic activities in the treated tailings. The BC amendment enhanced the abundance of Actinobacteria and Gammaproteobacteria, among which Fe- and S-oxidizing bacteria (Acidithiobacillus) appeared to be the most abundant. The improved water-soluble organic carbon and nitrogen status in SM amended tailings were the strongest driving factors in the shift of bacterial communities towards organoheterotrophic dominance. The plant biomass-based option (e.g., SM) together with tolerant pioneer plants as hosts would be preferred for eco-engineering the neutral Cu-Mo-Au tailings into functional technosol.