This study profiled the nitrifying bacteria and archaea associated with maize rhizosphere and evaluated their diversity across different growth stages. Also, the environmental factors were analyzed and correlated with the nitrifying community. The pH is seen to be moderately acidic (5.93) according to USDA  characterization. This could be as a result of the high level of sulfur noted (336 mg/kg). Sulfur is said to increase the acidity of soil when at a high level . The ratio of Carbon to nitrogen (9:1) is slightly higher than USDA  recommendation (8:1). Also, the NH4 to NO3 ratio (1:1.4) falls short of expectation, Liu, Du and Li  reported a ratio of 1:3 for the soil microorganism. The holistic physical and chemical parameter does not seem to be favorable to the proliferation of nitrifying community with an average of 0.5% relative abundance (Table 3). Kong, Eriksen and Petersen  report a favorable pH of 7.0 to 7.5 for nitrifying bacteria.
Nitrifying bacteria and archaea are ubiquitous and are found in varying environmental conditions. The 9 genera of nitrifying bacteria identified in this study are; Nitrospira, Nitrosospira, unclassified (derived from Nitrosomonadales), unclassified (derived from Nitrosomonadaceae), Nitrobacter, Nitrosovibrio, Nitrosomonas, Nitrosococcus, Nitrococcus. The order Nitrosomonadaceae and Nitrosomonadales still have unclassified and yet to be cultured bacterium species that are likely to be nitrifying bacteria. The only archaea genus discovered was Candidatus Nitrososphaera, which carry out ammonia oxidation , it had also been reported by Melnichuk, Abdurashytov, Andronov, Abdurashytova, Egovtseva, Gongalo, Turin and Pashtetskiy  and Enebe and Babalola  to be associated with crops including maize.
Ammonia oxidizing bacteria noted in this study were Nitrosospira, Nitrosomonas, Nitrosococcus  and Nitrosovibrio . Nitrosomonas was recently discovered in maize rhizosphere soil in low abundance by Wang, Rogers, Ng and He . The nitrite oxidizing bacteria carrying out the second stage of nitrification were the genus Nitrospira, Nitrobacter and Nitrococcus . Nitrospira is known to be well distributed globally and was found to be most abundant. It was recently observed by Sun, Zhao, Fan, Chen, Ruan and Wang  in a maize rhizosphere. Also, Nitrobacter was noted in a maize-soybean rotation system by Meier, Lopez-Guerrero, Guo, Schmer, Herr, Schnable, Alfano and Yang . Unclassified nitrifying microorganisms were seen in the order Nitrosomonadaceae and order Nitrosomonadales. This affirms the possible presence of novel nitrifying bacteria in the studied maize rhizosphere. Stein , mentioned there has been an increasing number of novel nitrifying microorganisms discovered lately. This could be as a result of advanced technologies used in sequencing.
Schlemper, Leite, Lucheta, Shimels, Bouwmeester, van Veen and Kuramae  affirm the existence of variation in bacteria population across different growth stages. The rarefraction curve shows that each of the growth stages had high number of species diversity (Fig. 2.). The PCoA plot showed a distinct diversity and gap across the growth stages (Fig. 6). The phylum Nitrospirae which had the most abundant nitrifying bacteria showed an increase from the BU to the TA and a decrease at the FR (Table 3). Also, Nitrospira genus was most abundant at the TA stage. This could be as a result of increasing demand of nutrient as the plant increase in growth. According to Rocha, Kuramae, Borges, Leite and Rosolem , the abundance of microorganisms associated with nitrification increases with increasing developmental stages. Furthermore, Lu et al. (2018) explain that the increased and prolonged availability of nitrogen in the rhizosphere by nitrifying microorganisms delays flowering.
The heatmap showed that all the nitrifying bacteria genus were unequally distributed across the different growth stages (Fig. 5). A likely trend was observed in the overall microbial community of a study carried out by Fu, Xiao, Liu, Zhang, Wang and Yang  at varying maize growth stages. This would probably be due to the varying composition of nutrients at the different growth stages. Although, the alpha diversity showed no significant difference. However, there was a significant difference (P=0.01) in the beta diversity of the different growth stages. Peiffer et al. (2013), also reported a significant difference between the beta diversity between maize bulk soil and rhizosphere soil. They attributed it to the maize genotype. The result obtained from the correlation affirms there is indeed a direct and indirect interlink within the environmental factors. Also, between them and the nitrifying community, the environmental factors showed both positive and negative correlations with a substantial number of the nitrifying community. This was also observed by Fu, Xiao, Liu, Zhang, Wang and Yang  between microbial community and soil nutrients.