Synergistic effects of NPK bacteria, arbuscular mycorrhizal fungi and biochar on tomato seedling growth and organic carbon in substrate

Abstract

The combined application of NPK bacteria + AMF + Biochar significantly enhanced tomato seedling growth and soil health indicators. The highest shoot and root lengths were recorded under this treatment, attributed to the complementary roles of NPK in nutrient availability, AMF in phosphorus uptake and root vigor, and biochar in improving substrate structure and nutrient retention. Stem diameter, biovolume index, fresh and dry weights also improved notably with this combination, reflecting increased biomass accumulation and plant vigor. Microbial parameters supported these trends. The highest rhizosphere bacterial population was observed in the NPK bacteria + AMF + Biochar treatment, indicating that biochar offered a stable microhabitat that, along with nutrient input and AMF-induced root exudates, enhanced bacterial proliferation in the rhizosphere. Similarly, mycorrhizal colonization peaked in this treatment, reflecting favorable conditions created by biochar and nutrient synergy for AMF establishment. Importantly, NPK (%) and organic carbon (OC%) content was highest in the shoot of the NPK bacteria + AMF + Biochar treatment, followed by AMF + Biochar and NPK bacteria + Biochar. This increase suggests enhanced soil organic matter retention due to biochar’s inherent carbon content and AMF-stimulated root biomass, contributing to improved soil fertility and structure. NPK bacteria alone contributed significantly to nutrient availability and early plant growth by supplying essential macronutrients (N, P, and K) in readily available forms. However, its efficiency was substantially enhanced when combined with AMF and/or biochar, which improved nutrient uptake dynamics, Mycorrhizal root colonization, and microbial interactions in the substrate. 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