Integrating compost, biochar and Bacillus siamensis enhances plant growth and modifies rhizosphere bacterial communities

Background Composting converts organic residues into a stable amendment rich in nutrients and beneficial microorganisms, improving soil structure and fertility. Bacillus species are common plant-associated microbes that mobilise nutrients, produce phytohormone and protect plant against pathogens. Compost is vital to sustainable agriculture, but are often slow-acting and insufficiently effective under intensive farming conditions. Its partial substitution with mineral fertilisers and reduce practices to maintain soil organic matter, drives soil degradation and biodiversity loss. Integrating compost with biochar and plant growth-promoting rhizobacteria (PGPR) can improve its performance. This study tested the combined effects of compost, biochar and Bacillus siamensis MTA1-3 on plant growth and rhizosphere microbial communities at three doses (1.0%, 1.5%, 3.0%) in a ryegrass (Lolium multiflorum) microcosm experiment to identify the most effective formulation and evaluate its impacts on soil microbial structure and potential functions. Results The triple combination at 1.5% significantly increased plant growth compared to compost + biochar and control treatments. In the first cycle, fresh biomass was 14.83 ± 1.30 g vs. 12.45 ± 0.84 g (compost + biochar) and 7.93 ± 0.58 g (control); dry biomass was 1.89 ± 0.18 g vs. 1.57 ± 0.15 g and 0.97 ± 0.09 g, respectively. In the second cycle, fresh biomass was 12.68 ± 0.70 g vs. 12.03 ± 0.86 g (compost + biochar) and 6.70 ± 0.95 g (control); dry biomass was 1.55 ± 0.13 g vs. 0.89 ± 0.13 g. In addition, Bacillus abundance increased in the rhizosphere under the 1.5% treatment (8.79%) compared with control (1.70%), whereas in the bulk soil the change was minor (2.73%). Predictive functional profiling showed higher relative abundance of groups involve in nitrogen cycle under the 1.5% treatment. Conclusions The triple treatment induces beneficial, yet non-permanent, changes in the rhizosphere microbiome, improving the potential functionality of the soil without altering the bulk microbial community. This approach enhances both plant growth and soil biodiversity, providing a promising sustainable strategy to improve nutrient availability and promote soil biodiversity in intensive agricultural systems. Graphical abstract