I'm not the expert here as I've only recently been turned on to a thick mulch layer. This said, I'm very happy with last falls leafs and MJ plant trimmings. They leafs do contain fungi and a host of critters including several varieties of soil mites, which feed on decaying plant material and each other and pot worms. Other stuff that people suggested to me are weeds from outside that have not gone to seed - in particular nettle, dandelion, yarrow, and chamomile. I believe it is good to have a diverse materials in the mulch. Apparently if you do this all correctly and recycle soil you can build a completely self sustaining system whereby you don't need to fertilize; beneficial bugs feed on mulch and each other and feed microbes which intern feed the plants and fungus works to break down stuff that bugs and bacteria aren't interested in yet.
Pine
It's pretty cool to read up on this stuff,....
I've got 1 critter in my room,... SpringTails. No mulch, but there they are..... I'm debating on letting them go, they do crawl on the stalks and get stuck in the resin making the lowest buds basically worthless.
Unless I'm wrong on what little critter I've got?
FYI, the ones that get stuck in the resin are grey/black but look the same physically as the white ones that are in the soil.
Mine look exactly like the top illustration,... even the grey ones on the stems. I'd post pics, but I don't have a USB microscope