It is an interesting field of investigation . How rhizobium is able to discriminate betweeen exogeneously applied nitrogen against indigenous source of nitrogen which the rhizobium is able to accumulate inside the plant root nodules formation as a process of symbiotic nitrogen fixation . Please have a look at following intersting PDFs
Ismael, Rabab R., Enas S. Ahmad, M. Abdel- Fattah, M N.A. Omar, Clara R. Azzam and H. H. Zahran (2018). Effect of plant growth-promoting rhizobacteria (pgpr) on growth and symbiotic nitrogen fixation of vicia faba plants under salt stress. PLANT SCIENCE JOURNAL, 7(1):01-19.
Ismael, Rabab R., M.N.A. Omar, Clara R. Azzam, Enas, S. Ahmad, M. Abdel-Fattah and H.H. Zahran (2018). Characterization and evaluation of rhizobium isolates from vicia faba for some plant growth promoting traits. BIOSCIENCE RESEARCH, 15(4):2971-2982.