Maize seed inoculated with <i>Azospirillum brasilense</i> as strategy to improve photosynthetic nitrogen use efficiency in the Amazonian periphery

In the Amazonian region of Brazil, the combination of different inoculation forms of Azospirillum brasilense and N rates has not been studied for its effect on photosynthetic N use efficiency and yield in maize plants. The effects of inoculation forms of A. brasilense and N rates on photosynthetic N use efficiency and maize yield were evaluated. Two field trials were conducted, combining four N rates (0 to 180 kg ha−1) with different inoculation forms of A. brasilense (seed, leaf, or control). Photosynthetic N use efficiency (photosynthesis per unit leaf N) was taken at the beginning of tasseling stage, while yield and N use efficiency components were evaluated at physiological maturity. Photosynthesis and specific leaf N increased with increasing N rates, regardless of inoculation forms. Azospirillum-inoculated seeds, along with 114 kg ha−1 of N provided the maximum yield (6.0 Mg ha−1). Without N, seed inoculation increased yield by 33% and photosynthesis by 31% compared to the control. Averaged across N rates, seed inoculation increased photosynthetic N use efficiency by 17%–24% compared to the control. Azospirillum-inoculated seeds improved N use efficiency compared to the control under low N available. The results show that the Azospirillum-inoculated seeds could potentially decrease N rates from 180 to 114 kg ha−1 of N, while increasing yield by 9%. Besides, Azospirillum-inoculated seeds could enhance photosynthetic N use efficiency in a sustainable manner on low-fertility soils. Therefore, the benefits of the A. Brasiliense drive a sustainable agriculture by improving yield and reducing the need for N inputs.