Isotopic evidence of significant assimilation of atmospheric-derived nitrogen fixed by Azospirillum brasilense co-inoculated with phosphate-solubilising Pantoea dispersa in pepper seedling

The contribution of plant growth-promoting bacteria (PGPB) of the genus Azospirillum to the plant N budget through biological nitrogen fixation (BNF) is still controversial. The aim of this study was to determine the contribution of BNF by Azospirillum brasilense on pepper grown at different N levels, attained using the 15N natural abundance method. To this end, pepper plants were grown in a growth chamber and treated with A. brasilense combined with Pantoea dispersa and then irrigated at four different N levels (0, 1, 3 and 7 mM NO3−). The assimilation of fixed N was clear from the lower δ15N values observed in bacteria-treated plants compared with those of non-bacteria treated plants. The percentage of BNF-derived N decreased with decreasing NO3− levels in the growth medium. BNF contribution to the total nitrogen content of plants was found to be as high as 46%. The results suggest that the bacteria have a potential to supply a considerable amount of N to pepper seedlings, as well as to stimulate plant growth and N uptake when Azospirillum–Pantoea treatment is combined with low NO3− levels.

Research highlights▶ Does Azospirillum contribute to the plant N budget through nitrogen fixation (BNF)? ▶ 15N natural abundance method allows determining BNF contribution. ▶ BNF contribution to the plant N budget was significant. ▶ The percentage of BNF-derived N decreased with decreasing NO3− supply.