Thanks a lot Michael for your elaborated comments. I need the mass of N fixed until the time when the plant samples were taken for nodule biomass determination, which is normal after 50% flowering.

Hi Getachew,

Nodule mass is weakly correlated with BNF, at best. You need to measure plant biomass and %N to calculate the total amount of N in the plant. Then, depending on the facilities (and money!) available to you, you can determine the % of this N which has been derived from BNF - this is usually denoted "%Ndfa" (%N derived from air). The simplest and cheapest methods to determine %Ndfa are the "N-balance" and "N-difference" techniques, but more expensive and more accurate ones are based on 15N and mass spectrometry. I won't detail any of these simply because there is an excellent and FREE online manual available from here:

Unkovich M, Herridge D, Peoples M, Cadisch G, Boddey R, Giller K,

Alves B, Chalk P. 2008. Measuring plant-associated nitrogen fixation in

agricultural systems. Canberra, Australia: ACIAR.

http://www.aciar.gov.au/publication/MN136

Click on the link and it will lead to a large PDF of the whole manual. It details and describes every method (good and bad!) to measure BNF, and I totally recommend it to any N-fixation researcher.

Good luck!

Euan