Ashe, have faith. As things break down and plants grow, conditions tend to improve.
Maje, as far as fungi go a better question would be "What don't they do?".
Some fungi can fix nitrogen from the atmosphere, many help break down and recycle organic matter. Some have symbiotic relationships with specific types of plants and assist them with nutrient uptake. Oak
trees can graft thier
root tips together to communicate and, with the assistance of mycillium networks, transfer nutirents or moisture from a tree with plenty to one in need.
Some fungi even form cells similar to our own neurons that can process information on a network much larger than our own brain. Whoah, how's that for profound.
The subject is far too vast to think we can control or even really understand it, but if we work towards healthy soil, the rest falls into place naturally. The fungi will be there doing things we don't even realize.