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

## TL;DR

Adding 2-line ferrihydrite to compost boosts plant growth by enhancing microbial production of biostimulants.

## Contribution

2-line ferrihydrite specifically enhances biosynthesis genes in Pseudomonadota and Actinomycetota, increasing plant biostimulant production.

## Key findings

- 2-line ferrihydrite promotes growth-promoting biostimulants like arginine and IAA.
- Compost with 2-line ferrihydrite improves plant biomass and leaf area in pot trials.
- Metagenomic analysis shows enrichment of biosynthetic genes like trpA/C/D/E and tnaA.

## 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.

This study explores how iron and manganese oxides transform sewage sludge into plant biostimulants during composting. Non‐targeted identification reveals the main species of plant biostimulants. Metagenomic analysis reveals that 2‐line ferrihydrite specifically enriches microbial genes for biosynthesis, boosting plant‐growth promoters. Compost amended with this iron oxide enhances crop growth, providing a sustainable strategy for upgrading waste into high‐value agricultural inputs.