Mechanistic impacts of cyanobacterial-derived plant growth elicitor and biochar application strategies on soil physicochemical properties and microbial communities

To expand cyanobacterial resource utilization pathways and explore agricultural application potential, this study investigated cyanobacterial growth elicitor (CGE) and cyanobacterial biochar (CB) through pot experiments. Five treatments were established: CK (control), BR (root-applied CB), BF (soil-applied CB), LBR (root-applied CB with CGE), and LBF (soil-applied CB with CGE). The effects of different application methods on soil physical and chemical properties, microbial communities, soybean quality, and environmental risks were systematically studied. Results demonstrated that CGE contains abundant nitrogen, phosphorus, potassium, organic matter, and plant hormones, with minimal allelochemical content, indicating significant agricultural potential. Cyanobacteria biochar can effectively immobilize nitrogen-containing organic matter, polysaccharides, polypeptides in CGE. After being applied to soil, compared with the BR, BF, and LBF treatments, the LBR treatment significantly increased rhizosphere organic carbon, humus content, and cation exchange capacity (CEC). Root application enriched chryseobacterium (organic matter-decomposing bacteria) in BR and LBR treatments, whereas soil application maintained bradyrhizobium (nitrogen-fixing bacteria) dominance in BF and LBF treatments. However, increased abundance of Pseudomonas in soil-applied treatments may contribute to nitrogen loss. Environmental risk assessment showed that the soil heavy metals and health risks in all treatments were at a safe level. After adopting different application strategies, compared with the CK treatment, the soybean biomass, plant nitrogen accumulation, and grain protein content in the LBR treatment increased by 4.45 g, 5.34 g/kg, and 13.43 g/kg, respectively, showing the best effect. This study provides a theoretical basis and technical support for the efficient resource utilization of cyanobacteria and their agricultural application strategies.