Exploring the potential of Azospirillum brasilense and Bacillus subtilis to improve productivity, quality and nitrogen use efficiency of open field-grown fennel

As the widespread use of nitrogen fertilization is causing serious environmental issues, new eco-friendly cultivation practices are being investigated by researchers involved in agronomic studies. Microbial biostimulants, specifically plant growth promoting bacteria (PGPBs), could represent a cutting-edge methodology to minimize the need for chemical fertilizers while boosting mineral uptake, productivity and quality of crops. We tested two PGPBs ( A. brasilense and B. subtilis ) on open field-grown fennel to assess their activity under both optimal or reduced nitrogen (N) application (0, 45, 90 or 180 kg ha -1 ). Data underlined that longitudinal and transversal bulb diameters, number of swollen leaf bases, total yield, dry matter, nitrogen concentration, total chlorophyll, carotenoids, total sugars, antioxidant activity, nitrogen use efficiency (NUE) and nitrogen utilization efficiency (UE) were higher during the first year compared to the second year. Our study showed that both PGPBs enhanced growth traits and total yield compared to the control, as well as plant qualitative traits. Azospirillum brasilense reduced N concentration in the bulbs (-14.0% compared with the control). Our research also indicated that A. brasilense gave the highest NUE index (+7.7% compared with the control), whereas B. subtilis revealed the highest UE index (+4.7% compared to control). Overall, this study underlined that A. brasilense gave better results than B. subtilis when it was combined with 45 or 90 kg ha -1 of N, while B. subtilis was efficient at the highest N rate (180 kg ha -1 ). The variable responses of plants to PGPB inoculation could be related to nitrogen fixation capacity. Azospirillum brasilense likely acted more effectively when plants were cultivated under low N input, in contrast to the action of Bacillus subtilis which had a greater impact under N optimal availability.