The examples that I am aware of always involve symbiosis of fungi with nitrogen fixing bacteria. A notable example is a fungi that produces nodular structures known as tuberculate mycorrhizae on the roots of their host plants. A notable example, Suillus tomentosus produces these structures with its plant host lodgepole pine (Pinus contorta var. latifolia). Mycorrhizae structures host nitrogen fixing bacteria which contribute a significant amount of bioavailable soil nitrogen to colonize nutrient-poor sites. Phosphorous is intricate to this process as well. In my experience, fungi require a symbiont organism of some kind to grow and thrive in nutrient-poor soil.

I suggset possibly using a variation of Kjedahl nitrogen analysis:

Ammonia electrode analysis of nitrogen in Microkjeldahl digests of forest vegetation January 1981 · Communications in Soil Science and Plant Analysis 12(1):19-30 DOI · 10.1080/00103628109367123 Projects · Pacific Southwest Forest and Range Experiment Station Forest Nutrition Project F. Powers · Daniel Van Gent· Richard F. Townsend

indeed, I isolated from the soil a strain of mold that can grow on a Nitrogene free medium. the litterature does not describe atmospheric nitrogen fixing fungi. something that intrigues me !!

is there a method to suggest to me to confirm this observation?

If you have access to a mass spectrometer and you are certain you have an absolutely pure culture of the mold, you could consider spiking the isolate or exposing it to enriched gaseous N-15 stable isotope of nitrogen, then determine if it is being incorporated into the vegetative portion of the mold by digesting it in a Microkjeldahl procedure. Nitrogen-15 abundance in nature is only 0.37% relative to Nitrogen-14 at 99.63% abundant. Do you happen to know if the mold is able to produce nitrite or nitrate ionic forms of nitrogen? These moieties can be steam distilled out of the isolate and tested for nitrogen-15 relative abundance.

thank you sir for your detailed answer