We know that truffles are a mycorrhizal fungus and lives associated with tree roots in symbiosis. However, for years to debate at scientific level if at the time of formation of truffles, ie during the formation of the fruit of the fungus, truffles keep associated with the tree and they keep on feeding from the tree or, on the other hand, truffles detached from the roots and they are able to absorb nutrients directly from the ground and even degrade dead organic matter.
Scientific papers published in 1991 and 1992 show us how the truffle mycelium in pure culture in the laboratory can use as nutrients cellulose, lignin (they are on wood), chitin (found in the skeleton of insects) and tannins. However since 1999 when the book of Callot  came out, the theory that the truffle once formed is detached from the tree, ever had more adherents. In this book, from which the two images in this post come, explains how from external peridium (truffle skin), hyphae and mycelia come out and colonize roots and dead cells of other tissues, even insect excrements, such as the image below:
3 years ago a French research , however, shows us through the carbon and nitrogen isotopes as they move from tree leaves to the roots and later the truffle, showing that at all times there is a link between the truffle and the tree.
What remains unclear is how much of the nutrients will come from the tree and how much truffle uses this “saprophytic” ability to degrade soil organic matter. What seems clear is that truffle depends at least somehow on the tree throughout the whole fruiting cycle and it is likely that at no time during the cycle completely “disconect” from the tree.
 Callot G., Bye P., Raymond M., Fernandez D., Pargney J.C., Parguey-
Leduc A., Janex-Favre M.C., Moussa R., and Pages L., 1999. La
truffe, la terre, la vie. Éditions INRA, Paris, 210 p.
 Zeller, B.; Bréchet, C.; Maurice, J.-P.; Le Tacon, F. 2008. Saprotrophic versus symbiotic strategy during truffle ascocarp development under holm oak. A response based on 13C and15N natural abundance. Annals of forest science 65 (6) 607. DOI: 10.1051/forest:2008037