In-vitro utilization of fungal necromass and plant litter by ectomycorrhizal fungi under contrasting mineral nitrogen availabilities

Ectomycorrhizal (ECM) fungi represent major drivers of soil carbon (C) and nitrogen (N) cycling, as they liberate nutrients by decomposing soil organic matter (OM), especially when labile N is limited. However, in contrast to saprotrophic fungi, knowledge on the decomposition of OM of different origin by ECM fungi remains limited. Here, we investigated  the decomposition of fungal necromass and leaf litter by various ECM fungal species under different availabilities of mineral N, using in-vitro stable isotope tracing. We hypothesised that (I) the narrow C/N ratio of fungal necromass enhances decomposition and fungal growth compared to leaf litter, (II) N limitation increases the share of OM-N over mineral N in the fungal biomass, and (III) N limitation enhances respiration.

We grew four different ECM fungal species (Hebeloma cylindrosporum, Paxillus involutus, Laccaria bicolor, Suillus luteus) in the absence of OM, with Agaricus bisporus fungal necromass (C/N = 8) or with leaf litter of Ulmus laevis or Quercus alba (C/N = 29 and 60, respectively) on nutrient medium containing ¹³C-enriched glucose and two concentrations of ¹⁵N-enriched ammonium. We calculated the utilization of OM-C and OM-N for fungal growth and respiration after a minimum growth period of 45 days.

In accordance with hypothesis I, C from fungal necromass was more effectively utilized by ECM fungi (40% of the necromass-C) than C from leaf litter (around 5%). In contrast, the percentage utilization of OM-N was highest for the Q. alba leaf litter (40%). However, due to the narrow C/N of the necromass, this treatment still resulted in the highest absolute amount of OM-N being incorporated into ECM fungal biomass and consequently increased fungal growth. As expected in hypothesis II, the relative share of OM-N in the fungal biomass was higher under mineral N limitation, even if the absolute uptake of N from leaf litter was decreased. We did not find support for hypothesis III as mineral N limitation did not lead to an increased respiration. However, under N limitation, respiration of ECM fungi growing on leaf litter was increased while growth was reduced compared to the controls without OM, suggesting a shift in C and energy investment from growth to decomposition in the presence of OM. Interestingly, the patterns were surprisingly uniform across the tested species.

Our findings show that OM type and mineral N availability control ECM fungal C and N uptake, growth, and respiration across four tested species and highlight fungal necromass as an important source of organic N and C for ECM fungi.