In complex growth substrates fungal C:N:P values do not indicate nutrient demands
Shifts in stoichiometric ratios can be useful to predict organismic nutrient demands, as applied for variable leaf N:P ratios in plants (Güsewell 2004). However, the observed shifts in C:N:P ratios only allowed to infer fungal N or P demands in controlled media with the respective element as the only limitation (Camenzind et al.2020). In complex growth substrates, i.e. more “costly” C sources and unknown limitation patterns, stoichiometric ratios were not related to responses in fungal growth or activity to N and P supply. In complex media (co-)limitation by various elements may impede simple causalities (Kaspari & Powers 2016). The parallel shifts in C:N and C:P ratios reported here, which were also observed for other elements (unpublished data) complicate the interpretation of stoichiometric shifts: Wide C:P ratios, for example, are indicative of both, limitations of N and P and potentially also other nutrients. Here, N:P may be explored as a preferential stoichiometric indicator of nutrient limitations (Reich & Oleksyn 2004; Persson et al. 2010). In addition, C limitation in soil appears to primarily control fungal biomass production - as shown in SEA media - most likely due to the investment of C into hyphal wall structures compared to the efficient (re)use of cytoplasmic N and P contents (Paustian & Schnürer 1987b; Camenzind et al. 2018a).