Bacillus-functionalized sewage sludge biochar boosts cabbage growth through improved nitrogen assimilation

Abstract Sewage sludge biochar (SSBC) emerges as a promising microbial inoculant (MI) carrier due to its rich organic carbon, bioavailable nitrogen species, and essential inorganic minerals. However, optimizing SSBC for MI loading and understanding its plant growth-promoting mechanisms remain challenging. Here, we developed a novel SSBC carrier, SSBC37, using a stepwise pyrolysis method: extracting dissolved matter (DM300) from the SSBC produced at 300 ℃, re-pyrolyzing the residual solid at 700 ℃, and reincorporating DM300 to preserve nutrients and create a microbial-friendly structure. DM300 enhanced the growth of Bacillus velezensis ZJ-11 by upregulating metabolic pathway genes, specifically those encoding ABC transporters. The metabolite trans -4-hydroxyproline (T-4-Hyp) produced by ZJ-11 may be involved in upregulating ABC transporter genes, thereby facilitating the uptake of DM300. SSBC37 loaded with ZJ-11 (BCZJ) significantly enhanced cabbage growth ( P < 0.05), increasing aboveground dry weight by 38.9 ± 1.9% and 17.5 ± 1.6% compared to ZJ-11 or SSBC37 alone. The loaded Bacillus increased nitrogen-related bacterial taxa Burkholderiales, through the suppression of unclassified Sordariales fungi. Furthermore, structural equation modeling (SEM) revealed that BCZJ enriched key bacterial taxa ( Ramlibacter ) that enhanced soil urease activity and ammonium nitrogen accumulation, thereby promoting plant nitrogen uptake and growth. This study provides a novel approach for engineering SSBC carriers to enhance MI efficacy, offering promising prospects for developing functional biofertilizer to regulate the rhizosphere microbiome for sustainable agricultural practices. Conceptual diagram illustrating the mechanism by which the growth-promoting effects of bacillus-functionalized sewage sludge biochar on cabbage. The SSBC37 prepared by stepwise method promoted the biomass and colonization of Bacillus . Bacillus -loaded SSBC37 enhance the absolute abundance of Burkholderiales by antagonizing Sordariales fungi. Finally, Bacillus and Burkholderiales collectively promote plant uptake of nitrogen. Graphical Abstract