Interactions of saprotrophic and root symbiotic fungi control the transformation of humic substances and phosphorus in Norway spruce needle litter

The interactions of fungal guilds have recently been proposed as drivers of organic matter transformation in forest soils. We conducted a pot experiment with Norway spruce seedlings planted in spruce needle litter inoculated with several fungal strains belonging to different ecological guilds (saprotrophic, mycorrhizal, and root endophytic) to assess how the fungi and their interactions affect the transformation of humic substances (HS) and phosphorus (P) in the litter. Several methods for the characterization of P forms and HS were employed, including 31P NMR, UV–Vis and FTIR spectroscopy. Our results show that fungal interactions influence not only the flow of P in decaying (plant) litter but also the transformation of the soil organic matter itself. Pots with saprotrophic Gymnopus androsaceus generally retained more P and prevented the accumulation of phosphonates caused by mycorrhizal Hyaloscypha finlandica, highlighting the strong competitive ability of the former species. The increased mineralization of P caused by G. androsaceus was not observed in the combined treatments, suggesting that other present fungi took up part of the inorganic P. The tested fungi did not affect the amount of HS produced but changed the characteristics of the HS. Mycorrhizal H. finlandica and root endophytic Phialocephala fortinii increased the relative proportion of carboxylic moieties in the HS regardless of the presence or absence of G. androsaceus, probably via the production and incorporation of melanins. The UV–Vis absorbance characteristics of the HS were significantly influenced by fungal interactions. Mycorrhizal H. finlandica and Hebeloma bryogenes retarded humification, as determined by the A4/6 ratio. We attribute the similar shift observed in Serpula himantoides to the partial oxidative degradation of HS. Our study shows that fungal root endophytes can significantly contribute to litter transformation along with mycorrhizal and saprotrophic fungi. The extent and patterns of the transformation seem to be species-dependent in all studied fungal groups.