Effects of foliar-sprayed bio-fertilizer with N-fixing Methylobacterium symbioticum on morpho-physiological traits of maize under varying N fertilization rates

Nitrogen (N) fertilization remains a critical challenge in sustainable agriculture. Plant growth-promoting bacteria offer a promising strategy to enhance nitrogen use efficiency and improve cereal crop productivity while reducing reliance on synthetic inputs. This open-field study evaluated the morpho-physiological effects of foliar application of <i>Methylobacterium symbioticum</i> (MS) on maize in NE Italy. Following a baseline application of liquid digestate (300 kg N ha^-¹) to all plots, four treatments were compared: N300 (digestate only, serving as the control); N300 + MS (digestate with MS); N350 (digestate with 50 kg ha^-¹ chemical N); and N320 + MS (digestate with 20 kg ha^-¹ chemical N and MS). ESEM imaging confirmed colonization of leaf surfaces by <i>M. symbioticum</i>. Its application significantly promoted aboveground growth and delayed leaf senescence by improving chlorophyll retention, increasing seasonal average SPAD from 46.9 in control to 49.4 (+5.3%, N300 + MS) and 48.8 (+4.1%, N320 + MS), likely mediated by the ascertained ACC-deaminase activity of MS. Root electrical capacitance showed treatment-specific effects, with the highest readings under the N320 + MS treatment (+54% vs. control at flowering). Yield responses were non-linear with respect to N dose, with N300 + MS showing a 12% (1,364 g/m²) and N320 + MS a 6% non-significant increase vs. control. MS-treated plants also exhibited a non-significant 5% increase in grain protein content, but significantly higher aboveground N accumulation. It is concluded that, this microbial inoculation strategy can enhance N use efficiency, particularly under reduced synthetic fertilization, presenting an environmentally-friendly and sustainable agricultural strategy for maize cultivation.