if you are working in the field, you can plant a non fixing plant like maize close to the legume you are interested in, then harvest leave samples of both the legume and non fixing plants and analyze them for 15N isotopes. This is the natural abundance method. you can also use the N dilution method in pots. It all depends whether you need absolute or relative amounts of N fixed

Maureen's natural abundance for 15N is definitely one way to do it, but will only work if the natural abundance of 15N in the soil is different enough from the atmosphere, which varies widely around the world. The way I have done this is with labeled 15N soil to greatly increase the 15N ratio in the soil. Then in that labeled soil you would grow your legume plant in question, a non-nitrogen fixer and a known nitrogen fixer (clover works well), and examine the 15N ratio in the grown plants. The non-nitrogen fixer will roughly give you the 15N ratio of your soil and that can be compared to the 15N ratio of your legume....all the difference in ratio can be assumed to be from atmospheric nitrogen fixation. 15N labeled urea is really quite cheap. If you have lots of money and want a really accurate method you can do the opposite and grow the plant in an enclosed tank with 15N labeled gas...but that is absurdly expensive.

Juan is right in that you can do ARA, and incubate the nodules in acetylene and examine the conversion to ethylene, but this is a very noisy method, will only give the the fixation that is occurring at that moment in time, and generally is not accepted anymore unless you also do a conversion of ARA to 15N gas incubation to get the ratio of ARA to actual nitrogen fixation, since it has been show to be highly variable.

EMBRAPA, the Brazilian department of ag, has done a lot of this with sugarcane. See Boddey et al 1988, 1991, 1995 for different 15N techniques and problems they have had. Crew et al 2000 and Vitousek 2011 have good ARA techniques.

Hi Noa, I think what you just described is the 15N dilution method I mentioned?

Hi Maureen...yes you are right...I only noticed the natural abundance portion of your post.

Thank you Juan, Maureen and NOA. I followed your contributions with gratitude, and will go for the references. We are interested in wild fallow leguminous weeds that are making their input to re-invigorate soil in term of atmospheric nitrogen fixation. I will keep in touch and will explore possibility of collaboration.

One point that Rasheed doesn´t mention is where he is interested in conducting the field research. If it is on a tropical region (which seems likely due to his University :)), and he is planning on the N15 route (which is indeed the best if economically feasible), one of the main concerns is in finding a REALLY non-fixing plant species with similar nitrogen absorption rates.

There are strong indications that endophytic nitrogen fixing isn´t particularly rare in the tropics, and since he mentions native legumes, there is a potential for biologically fixed nitrogen to be already available on the soil.

I would advise going with a large mix of species believed to be non-fixers...