Litter-inhabiting fungi show high level of specialization towards biopolymers composing plant and fungal biomass

Fungi are recognized as efficient decomposers of biopolymers contained in soil or litter, but not all saprotrophic taxa are equally efficient in accessing various C sources. While many fungi may be considered generalists that are able to utilize complex biomass of plant, bacterial, and fungal origin, it is less clear which of the individual biopolymers that compose these substrates they utilize. Here we analysed fungal communities in forest topsoil enriched in bags with polymers composing plant (cellulose, xylan, glucomannan, pectin, lignin) and fungal (chitin, beta-1,3-glucan, and beta-1,3-1,6-glucan) biomass along with fungal abundance and the activity of enzymes. There was a high degree of specialization among saprotrophs, each biopolymer being preferred by distinct taxa. White-rot fungi and general saprotrophs were most common on cellulose and xylan, while pectin and lignin-associated communities were dominated by moulds, and animal pathogens were found almost exclusively on chitin. Although several enzymes were produced on all biopolymers, the composition of enzyme pools was significantly different among substrates and different from litter. Activity of endocellulase, beta-galactosidase, beta-mannosidase, and beta-glucosidase significantly correlated with the fungal to bacterial biomass ratio indicating the important role of fungi in their production. The results indicate the high level of specialization among litter-inhabiting fungi and differences in the substrate preference across nutritional guilds of saprotrophic fungi. While most of the litter-inhabiting fungi utilize plant biopolymers, fungal biopolymers are also frequently targeted.