Maize growth benefited by biochar and Azospirillum brasilense inoculation in subtropical soil

Use of biochars as a nutrient source and inoculation of plants with plant growth-promoting bacteria (PGPB) have been studied, but the interaction between different biochars and these bacteria remains poorly explored, especially in subtropical soils with low natural fertility. This study evaluates whether swine digestate biochar (BD) and poultry litter biochar (BC), alone or combined with A. brasilense inoculation, can partially substitute liming and soluble NPK fertilization under short-term greenhouse conditions in maize cultivation in a low-fertility Ultisol. The study was conducted in 8 L pots, in a greenhouse, using samples of a Ultisol, in a randomized block design with a 4 × 2 factorial scheme. Treatments consisted of nutrient sources: 1-BD; 2-BC; 3-liming and NPK; and 4-control; and the presence or absence of A. brasilense inoculation on the heirloom maize seeds. Biomass, accumulation of C, N, and S in biomass, SPAD index, stem diameter, chlorophyll, and carotenoids were evaluated in maize plants. After cultivation, the soil was evaluated for pH, P, K, Al, Ca, Mg, N, C, and S. Biochars produced responses comparable to the liming + NPK treatment for most of the variables analyzed in this pot expriment. Synergistic responses were observed when inoculation was combined with biochars, although inoculation effects were treatment-dependent; for stem diameter, responses varied with nutrient source and sampling date, with the clearest endpoint increase observed in BC. In the BC and BD treatments, inoculation was associated with marked reductions in exchangeable Al and increases in cation exchange capacity and base saturation. Additionally, inoculation increased the availability of K in BC and P in BD, indicating treatment-specific shifts in nutrient availability associated with inoculation. In contrast, in the NPK and Control treatments, the predominant effect of inoculation was the mobilization of Al, raising exchangeable Al and Al saturation to potentially toxic levels (above 34%). Overall, our results suggest that inoculation with A. brasilense in acidic soils may benefit from the concurrent use of an amendment that mitigates Al toxicity (e.g., biochar), thereby reducing the risk of Al mobilization observed in unamended treatments. We conclude that these manure-derived biochars—especially when combined with A. brasilense inoculation—represent a promising strategy to reduce reliance on liming and highly soluble fertilizers in the short term under controlled conditions, warranting field-scale and longer-term validation before agronomic recommendations can be made.