Investigating the interactive effect of rhizobacterial inoculation associated with nitrogen fertilization on Mombasa Grass: isotopic approaches (15N), antioxidant profile, N metabolism, hormone quantification and ultra-morphological changes in the root

Abstract

The low efficiency of nitrogen fertilizers is a global problem that hinders the sustainable development in agriculture and can generate risks to the environment and human health.The inoculation or co-inoculation of rhizobacteria that promote plant growth (PGPR) by multiple mechanisms has been considered a promising alternative in increasing the efficiency and recovery of nitrogen, but these associations are still little explored in tropical pastures. The objective of this project is to evaluate the effect of inoculation or co-inoculation with PGPR on Mombasa grass, and how this process can be impacted by N fertilization levels. The first experiment (greenhouse and pots filled with soil), will be conducted in a randomized block design, following a 6×4 factorial scheme, with 8 repetitions. The treatments consisted of a control (non-inoculated), individual inoculations with Azospirillum brasilense (Azo), Pseudomonas fluorescens (Pf), Bacillus subtilis (Bs) and combined with Azo+Pf and Azo+Bs, associated with levels: i) absent (0 mg N kg-1), ii) low (50 mg N kg-1), iii) adequate (150 mg N kg-1) and iv) high (300 mg N kg-1) nitrogen fertilization. Half of the repetitions (4/8) will receive 15N-enriched urea and the rest will receive conventional urea (isotopic control). In this experiment (1st cycle + regrowth), two combined 15N isotopic techniques (natural abundance ´15N 0 and isotopic dilution) will allow quantifying the contribution of N2-fixed by these microorganisms and the recovery of the applied N-fertilizer, also evaluating gas exchange, nutritional measures (nutrient use efficiency), productivity and grouping of the root system in diameter classes (obtaining and imaging roots) in plants of all treatments. The most promising organisms will be selected (Inocprom) especially on the basis of contribution in N-fixed and recovery of applied N-urea. In the second experiment (nutrient solution), Inocprom will be evaluated based on analyses related to N metabolism (activity of the enzymes nitrogenase, nitrate reductase and soluble N fractions), antioxidant enzymes (CAT, SOD, APX) and oxidative stress indicators (H2O2 and MDA), in addition to the quantification of hormones allied to microscopy technique. The treatments (control + Inocprom) will be conducted in a simple factorial scheme in randomized blocks, with 5 repetitions in adequate N supply (low or no N can drastically limit the days of assay in solution). Taking into account the complementarity of both experiments, it is expected that co-inoculation of RPCP will be more effective in promoting mombasa grass growth compared to inoculation alone, stimulating hormone production and root development (which may be accompanied by changes at the root cellular level), increasing N use efficiency (via N-fixed and greater recovery of N-fertilizer and soil-derived N). It is also believed that the effectiveness of inoculation (growth-promoting effect) is strongly attenuated in high N supply. Furthermore, inoculation/co-inoculation may lead to improvements in the antioxidant system, reinforcing the protection of chloroplasts, reflected in greater CO2 assimilation and intensification of N metabolism, promoting the growth of mombasa grass and favoring the accumulation of other nutrients (not only N). (AU)