Efficiency of corncob-derived biochar and plant growth-promoting rhizobacteria as growth promoters for Coffea arabica L. seedlings.

Corncob-derived biochar and plant growth-promoting rhizobacteria (PGPR) can improve soil fertility and plant performance, but their co-application in Coffea arabica L. nurseries remains poorly characterised. This study evaluated corncob-derived biochar combined with Bacillus megaterium S2-4a1 under well-watered greenhouse conditions. A completely randomised design with six replications was maintained for 180 days and included five treatments: control (T1), S2-4a1 alone (T2), and S2-4a1 combined with 1, 2, or 3% (w/w) biochar (T3-T5). Soil physicochemical properties, culturable microbial populations, soil microbial respiration (SMR), dissolved organic carbon (DOC), microbial biomass carbon (MBC), plant growth, nutrient status, and leaf physiological traits were assessed. T3 and T4 produced the highest soil pH, whereas cation exchange capacity increased with biochar rate and was highest in T5. T5 also had the highest organic matter, total carbon, exchangeable K, and culturable microbial population. T3 showed the highest SMR on day 1 and a secondary increase on day 18, together with the highest DOC and MBC. Plant height was not significantly affected, but T4 produced the greatest stem dry biomass and the highest relative water content, while co-application treatments generally lowered leaf H2O2 and membrane damage relative to the control. Under these non-stressed conditions, the results indicate that biochar rate altered the balance between soil microbial responses and coffee seedling performance. Rates of 2-3% with S2-4a1 were most consistently associated with improved soil chemical properties and seedling physiological status, supporting further evaluation in larger containers and field nurseries.