Microscopic cells called "mycelium" -- the fruit of which are mushrooms -- recycle carbon, nitrogen, and other essential elements as they break down plant and animal debris in the creation of rich new soil. "We can capitalize on mycelium's digestive power and target it to decompose toxic wastes and pollutants (mycoremediation), catch and reduce silt from streambeds and pathogens from agricultural watersheds (mycofiltration), control insect populations (mycopesticides), and generally enhance the health of our forests and gardens (mycoforestry and mycogardening)."
Some of those nutrient deficiencies are kind of pretty! Although I imagine they won't help with fruit and vegetable production!
I think mycorrhiza have a big role in phosphate transport into plants, and that in general we've overfertilized with phosphates, causing depletion of those fungi.
You make a great point about soil building. I read that a good mycelial population holds soil in place, makes it drain better as well as hold moisture better, due to increased sponginess. A good cellulosic/lignin mulch - wood chips, chopped straw, chopped leaves - might promote mycelium. Also, somewhere I read the mycelial substances put a lot of carbon into the soil, so it's not in the atmosphere, which is a good thing too.