Rock phosphate fertilization harms Azospirillum brasilense selection by maize

Maize is the most important crop cultivated worldwide. It needs a significant amount of nitrogen and phosphorus fertilization to maintain high yields. However, the high cost of fertilization makes production more expensive and damages the environment. The present study used Azospirillum brasilense and Bacillus subtilis bacteria in an attempt to supply nitrogen and phosphorus fertilization to a maize crop. The experiment was carried out with maize plants under greenhouse conditions with a factorial scheme (4 x 2 x 2), where the first factor corresponded to bacterial inoculation: (a) control (no inoculation); (b) A. brasilense inoculation (AZ); (c) B. subtilis inoculation (BS) and (d) inoculation with a mixture of (AZ+BS), the second factor corresponded to the presence or absence of rock phosphate fertilization and the third factor corresponded to the presence or absence of top-dressed nitrogen fertilizer. Evaluated plant parameters were height, shoot dry matter (SDM), root dry matter (RDM), and soil parameters were total colony forming units of bacteria (CFU), nitrogen, soluble phosphorus and microbial biomass carbon (MBC). Although some parameters were improved with mineral fertilization in general, the findings showed that there were many adverse effects with the use of rock phosphate fertilization and A. brasilense inoculation. When both were applied together in treatments, there was reduction in plant height, microbial biomass carbon and total number of bacteria compared to treatments without rock phosphate fertilization. These results strongly suggest that rock phosphate harms the ability of A. brasilense to promote plant growth and demonstrate the necessity of new studies to verify whether this negative effect occurs under field conditions and could reduce yields in maize crop production.