2‐line Ferrihydrite Enhance Microbial Synthesis of Plant Biostimulants in Composted Biosolid by Regulating Phyla <i>Pseudomonadota</i> and <i>Actinomycetota</i>

The discovery of plant biostimulants (PBs) in sewage sludge offers a promising avenue for biosolids valorization. Here, the study investigates how two mineral additives, including 2-line ferrihydrite (a disordered iron oxide) and disordered birnessite (a manganese oxide), modulate microbial activity and molecular pathways to enhance PB production during aerobic sludge composting. Application of 2-line ferrihydrite significantly promotes the synthesis of growth-promoting PBs, including arginine, valine, decanoic acid, and indoleacetic acid (IAA), while disordered birnessite primarily enhances resistance-related PBs, such as decanoic acid, L-pyroglutamate, and trans-aconitic acid. In pot trials, composted biosolids amended with 2-line ferrihydrite significantly improve plant biomass and leaf area compared to mineral-free and birnessite treatments. Metagenomic profiling reveals that PB biosynthesis is dominated by members of the phyla Pseudomonadota and Actinomycetota, with temporal niche partitioning across the thermophilic and maturation stages. 2-line ferrihydrite enhances the abundance of critical biosynthetic genes (e.g., trpA/C/D/E/F), particularly within taxa such as Xanthomonadaceae, Sphingomonadaceae, and Streptosporangiaceae. Additionally, genes involved in IAA and indole biosysnthesis (ALDH, DDC, and tnaA) are enriched, supporting enhanced tryptophan-to-IAA conversion. This study provides a mechanistic link between iron oxide-mediated microbial modulation and PB production in composted biosolids, offering a sustainable approach for upgrading waste into high-value agricultural inputs.