Iron-rich waste activated sludge-derived vivianite for boosting pak choi growth: fertilizer effectiveness and mechanism elucidation

This study investigated the fertilizer effectiveness and mechanisms of vivianite, recovered from iron-rich waste activated sludge (WAS) via anaerobic fermentation, for enhancing pak choi growth through hydroponic cultivation. The vivianite-added groups achieved comparable effects to the commercial nutrient solution (NS) group in promoting chlorophyll synthesis, plant height growth, and biomass accumulation and significantly outperformed the control groups. The vivianite could serve as a slow-release fertilizer, continuously supplying nutrients to enhance the growth of pak choi. This enhancement was facilitated by a synergistic collaboration among root-associated microorganisms, including Burkholderia-Caballeronia-Paraburkholderia, Mucilaginibacter, Caulobacter, Sphingomonas , and the primary phosphorus-solubilization contributors, namely, Novosphingobium, Brevundimonas , and Pseudomonas. Moreover, the vivianite improved root-associated microbial diversity and upregulated the activity of a key iron-acquisition protein, the outer membrane receptor. These effects collectively contributed to pak choi growth. Such iron-rich WAS-derived vivianite could represent a promising fertilizer candidate for hydroponic systems, offering a sustainable pathway to repurpose iron-enriched WAS for agricultural use.