Ideally we want a diverse range of mulch C:N ratios such that we get a diverse range of biology. I’d wondered what that range might be so did some digging, or should that be mulching? Here’s one paper.
Effects of carbon concentration and carbon to nitrogen (C:N) ratio on six biocontrol fungal strains are reported in this paper. All fungal strains had extensive growth on the media supplemented with 6–12 g l−1 carbon and C:N ratios from 10:1 to 80:1, and differed in nutrient requirements for sporulation. Except for the two strains of Paecilomyces lilacinus, all selected fungi attained the highest spore yields at a C:N ratio of 160:1
Seems to confirm that for fungi to reproduce and sporulate, that they need a constant supply of carbon. Note that the study only went to 160:1, and more carbon could be more desirable.
If you look at this chart I made, the curve is steepest at the peak between about 18:1 and 50:1, this range seems likely to be the sweet spot for fungi and for soil carbon priming, however to reproduce fungi, material with a higher C:N is also desirable.