Fungal growth and stoichiometric analyses
At the end of experiments, agar media were melted in a microwave and mycelium collected on a 20 µm mesh. To remove superficial element traces, mycelia were washed with 1 L of >90 °C demineralized H2O, and cleaned with 0.1 M HCl for one minute. This washing method was inevitable to ensure complete removal of agar medium from the mycelium (Maynard et al. 2017), but we showed in a separate experiment that it did not alter element concentrations or treatment effects (Fig. S1). Fungal material was freeze-dried and kept at -20 °C. Fungal dry biomass was determined, as well as fungal density [mg cm-2 mycelium] based on the actual size of the mycelium, since mycelial density as a trait proved to be a good estimate of fungal fitness and indicator of nutrient limitations (Camenzind et al. 2020). Fungal enzymatic activity was assessed as an additional important response trait (for details see Supporting Information S1). For element analyses, fungi were milled and C and N contents determined with an Elemental Analyzer (EuroEA, HekaTech, Germany). P content was analyzed after aqua regia digestion (1:4 HCl:HNO3) by ICP-OES analyses (Optima 2100 DV, Perkin Elmer, Germany). In cellulose media, P contents were only analyzed for four isolates (RLCS16, RLCS17, RLCS27 and RLCS28) in media with C:N of 5 and 200, respectively. Likewise, in SEA media P contents were only analyzed for four isolates (RLCS12, RLCS22, RLCS27 and RLCS28).