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

Abstract 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.