GNFMM media is only for screening. If the bacteria can grow well on that media, you can say that these bacteria can fix nitrogen roughly. If you want to be sure, you can measure quantitatively.

Hi Rabiul

You read this paper 1

https://www.sciencedirect.com/science/article/pii/S1517838216310802

2

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0111313

3

https://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-90162013000600004

4

https://onlinelibrary.wiley.com/doi/10.1111/nph.12536/pdf

5

https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2004.02329.x/pdf

6

http://ousar.lib.okayama-u.ac.jp/files/public/1/12834/20160527192202548508/13_097_101.pdf

7

https://file.scirp.org/pdf/AiM_2016051315445064.pdf