The tree and the truffle form a symbiotic relationship. The tree uses the mycelial web of the fungus as a fishing net that allows it to absorb water and nutrients that by itself can not assimilate or would do it in much smaller amount. The truffle obtains sugars from the tree, which it produces through photosynthesis. This exchange takes place in the roots of the trees through some organs called mycorrhizae, the symbiotic structure. At a cellular level, the fungus absorbs basically inorganic phosphorus (P) from the soil, in the form of polyphosphates and nitrogen mineral (N) in the form of nitrates. On the other hand, the starch stored in the tree is transformed into sucrose that breaks down into glucose and fructose, forms that the fungus can assimilate.
Nitrogen is one of the essential compounds of living organisms, necessary for the synthesis of proteins. Tuber melanosporum contains on average 5% N. This can come from the soil or atmospheric fixed by the bacteria that inhabit the same truffle. Each gram of truffle contains up to 100 million microorganisms!
The fruiting bodies, the truffles, can not directly absorb the nitrates from the soil, but these come from the mycorrhizae at all times.
In mycorrhizae, nitrate can be reduced to ammonium, which is then transferred to the truffle.
Barbieri et al. 2010. New evidence for nitrogen fixation within the Italian white truffle Tuber magnatum.