I'll add a few points of fact, opinion and some documentation. Like wine and many things mulch gets better over time. I started using mulch in the early 1980s and all I had was guinea grass, a tall panicum found all over the tropics. During the 90's I farmed in Arkansas and found a derelict sawmill site which had a 20 year old mountain of sawdust/bark which had almost turned to something like chocolate cake. I worked on that for years till somebody playing around set the newer stuff on fire and it burned down deep underground.
So most recently 1-1/2 years ago I managed to get 2000 cubic yards of Hurricane Irma debris delivered. here is a video showing it:
https://www.youtube.com/watch?v=XnaLWHgib5AThere is a big difference between mulches, usually folks speak of carbon/nitrogen ratio comparing grasses, vegetable matter, leaves and wood chips but really it comes down to the amount of carbon it contains. No organic matter except charcoal contains more carbon per volume than wood.
So far I've covered about 2 acres with it at the rate of about 1000 yards/acre. The decomposition in that time is close to 75% such that one foot turns into 3-4 inches. What I really want to show is how that happens so today I took a few pictures. In my case, in southwest Florida with 50 inches rain per year mostly in the summer and a very poor sandy soil with low organic matter the best agent of decompostion are millipedes both the long and short pill-bugs but mostly the former. These arthropods are primary decomposers of woody material worldwide but especially in tropical conditions where earthworms seem a little soft and maybe vulnerable the arthropods dominate.
Here are photos of the mulch pile once about 10 feet tall in the video above and after 1-1/2 years now about 5 feet. The millipede shown is one of thousands, millions or hundreds of millions across the property which have turned a foot of the mulch into 3-4 inches of feces technically called 'frass'.
Close-up showing the black granular frass.
These guys work day and night for me, all they want is food.
Usually the most rich ecosystems form at the interface edges between radically different systems. Examples would be estuaries where fresh meets salt water, field edges, stream edges, rocks and structures in water. All of these particular edges serve as habitats for species which can co-exist in either system but find the edge to be the best. Hunters and fishermen know this and it's true on the micro as well and the macro level.
When you put mulch on soil you create a brand new ecosystem directly at the interface of soil and air. It is cool enough, dark enough and moist enough for organisms dwelling both in and outside the soil to coexist. It is where fungi can send out their fruiting bodies called mushrooms and breed.
Here are some photos of what you can see happening at the surface of well mulched soil:
millipede working on dead banana leaves:
Millipede working on fallen avocado fruitlets:
Mushrooms on mulched seedling Pitomba tree on top of decomposing papaya tree trunks:
Mango tree prunings skeletonized by millipedes:
Mushrooms on logs which have been covered by mulch under avocado tree dripline:
So when millipedes eat the mulch they are consuming lignins and cellulose but derive most benefit from the bacteria and fungi already on that material. They are being predators on the primary micro-saprophytes which are already there. Within the millipede gut there is a population of lignin-digesting microorganisms which further digest the matter and thus as it is expelled as frass the material contains innoculum of bacteri, fungi and higher order creatures.
The frass has actually entered national commerce from south Florida, with the trade name "milli-poo". The sale price is $60 USD per cubic foot with the intended use as an ingredient in compost tea. An acre 3 inches deep with frass is 10,000 cubic feet so I'm almost a millionaire.
https://www.boogiebrew.net/millipoo/Yes there's probably some hype there but they did get the stuff analyzed and found it highly enriched in fungal and less in bacteria.
https://drive.google.com/file/d/1Kpnx1WsGFi0gzmNbA17LTPGcQv3FkP_g/viewhttps://drive.google.com/file/d/1dFDltJXoZXDi6x7bZ1MmJEF1b2DniQUU/view?usp=sharingIf you like journal papers this one took dried leaves in vitro with millipedes and found they do a good job generating nitrogen and calcium and can modify soil ph.
https://www.fs.fed.us/global/iitf/pubs/bc_iitf_2012_Gonzalez001.pdfLast night we hit the lottery. A 1/4 inch rain fell across our 2 acres, it equalled 13,000 gallons of milli-poo tea!