Interactive Effects of Bacterial Consortia and Basal Nitrogen Fertilization on Initial Maize Growth: an Investigation Based on Physiological Parameters and 15N Isotopic Analysis

Strategies that integrate multiple mechanisms to enhance nitrogen use efficiency (NUE), promote plant growth, and contribute to sustainable agriculture should guide modern agricultural practices. We investigated the interactive effects of seed inoculation with rhizospheric diazotrophic bacteria (PGPR-Azospirillum brasilense, and Bacillus subtilis, applied individually or in combination) with or without basal N fertilization (0 or 20 kg N ha(-1) as urea source) in maize plants grown under greenhouse conditions. Biometric, nutritional and physiological evaluations were complemented with isotopic techniques (N-15 isotope dilution and natural abundance-delta N-15 parts per thousand) to elucidate the fate and acquisition of N by maize on initial growth stages (from three up to six fully developed leaves-side-dress timing for N application). Our primary objective was to determine whether inoculation and basal N fertilization could enhance the maize plant's ability to recover and utilize nitrogen (N) from initial fertilization more efficiently, ultimately leading to improved NUE. Additionally, we aimed at investigating the potential benefits of biological nitrogen fixation (BNF) resulting from the inoculations. The study was conducted under controlled greenhouse conditions using an experimental design of completely randomized blocks with eight repetitions, arranged in a factorial scheme of 4 x 2. This factorial scheme consisted of four seed inoculations: (1) control (without inoculation), (2) A. brasilense, (3) B. subtilis, (4) A. brasilense + B. subtilis, each either with or without basal N fertilization of 20 kg N ha(-1). As previously mentioned, data collection was carried out at the V3 and V6 growth stages of maize. The results showed that despite the low initial recovery from basal N fertilization, the effect on early maize growth provided by this application up to V6 is prominent and should be recommended, at least in tropical soils with low organic matter and available N. The co-inoculation with A. brasilense and B. subtilis provided greater root growth, enhanced N uptake, recovery and water use efficiency, improved nitrogen balance index, leading to a greater chlorophyll concentration and net photosynthetic rate, while with a relevant contribution from BNF (20-29% of the N accumulated in maize plants at V3 stage). This multi-approach mechanism was found to benefit carboxylation efficiency, improved maize initial growth and development. Inoculation with A. brasilense is more effective at atmospheric N fixation than B. subtilis. Nonetheless, the BNF contribution of B. subtilis is not negligible (varying between 17 and 19%). Lastly, our results showed that co-inoculation in the absence of N basal fertilization out performs the application of 20 kg N ha(-1) without inoculation up to the V6 stage. Our study proposes new insights for future research based on the utilization of N-15 isotopic and co-inoculation techniques under field grown conditions, which hold significant potential for practical application.