James Pinner

Well Mycorrhizal fungi host n-fixing bacteria. So it is the bacteria that fixes nitrogen, but you still need the fungi to host the bacteria. I apologize for not being clear.

80% of the earths atmosphere is Nitrogen (N2) gas

Bacteria and fungi make atmospheric nitrogen available to the plants they associate with.

N-fixing fungi are called Mycorrhizal fungi
N-fixing bacteria also are found in soil.

Mycorrhizal fungi associate with roots. The fungi form nodules on the roots providing nitrogen to the plant to make proteins. The plant in turn feeds the Mycorrhizal fungi with sugars produced through photosynthesis.

This is cool!!!
Plants up to 80% of the energy is used to feed soil organisms. These organisms work as extensions of the roots. Fungi create networks underground between all the plants. If one plant is in need the fungi will feed it from the energy from the nearby plants.

Not all plants have n-fixing associations... So where do they get the nitrogen the need to make proteins???

All plants contain nitrogen in the form of proteins, but N-fixing plants have much higher concentrations. When they die or leaves fall to the ground. Soil organisms break them down (bacteria and fungi). Nematodes, protozoa, bugs, worms, etc. in turn eat the Bacteria and fungi. They then turn it in to plant available nitrogen.