Inorganic and organic additives differently regulate compost microbiomes in response to heavy metals immobilization

Abstract

Various composting additives, both organic and inorganic, have been used to effectively reduce the bioavailability of heavy metals (HMs) in compost. However, a comprehensive assessment of their comparative contributions and the mechanisms involved is still lacking. In this study, we selected shell powder and biochar as representative amendments to assess their efficacy in immobilizing various HMs. Our results demonstrated that the addition of shell powder and biochar markedly enhanced the synthesis of humic acids (HA), reduced the bioavailability of HMs, and indirectly enhanced microbial carbon metabolism and growth pathways. This enhancement subsequently enriched the abundance of key HA synthesis contributors, including Enterobacter, Thermobispora, Pseudomonas, and Sphingobacterium, creating a positive feedback loop. Additionally, adding biochar altered the abundance of Pseudomonas, Chelatococcus, and Pseudoxanthomonas during the mesophilic phase, thereby reducing the expression of HMs resistance genes (HMRGs). In contrast, shell powder supplementation modulated the response to HMRGs reduction by regulating the abundance of Sphingobacterium, Pseudomonas, and Thermobispora during the thermophilic phase. Ultimately, regional cultivation trials confirmed the potential of these innovative organic fertilizers to significantly reduce the spread of HMs to plants. Collectively, these findings highlighted the similarities and differences between organic and inorganic additives to modulate microbial community responses during HMs immobilization, providing a theoretical framework for diminishing the risks associated with HMs dissemination and improving the quality of organic fertilizers.