Soil fungal mycelia have unexpectedly flexible stoichiometric C:N and C:P ratios

Soil ecological stoichiometry provides powerful theories to integrate the complex interplay of element cycling and microbial communities into biogeochemical models. One essential assumption is that microbes maintain stable C:N:P (carbon:nitrogen:phosphorus) ratios independent of resource supply, although such homeostatic regulations have rarely been assessed in individual microorganisms. Here, we report an unexpected high flexibility in C:N and C:P values of saprobic fungi along nutrient supply gradients, overall ranging between 7‐126 and 20‐1488, respectively, questioning microbial homeostasis. Fungal N:P varied comparatively less due to simultaneous reductions in mycelial N and P contents. As a mechanism, internal recycling processes during mycelial growth and an overall reduced N and P uptake appear more relevant than element storage. The relationships among fungal stoichiometry and growth disappeared in more complex media. These findings affect our interpretation of stoichiometric imbalances among microbes and soils and are highly relevant for developing microbial soil organic carbon and nitrogen models. Contrary to the common assumption of stoichiometric homeostasis in heterotrophic soil microbes, our results demonstrate high flexibility in C:P and also C:N ratios in saprobic fungi. Detailed element analyses and correlations with growth trait responses also highlight potential mechanisms and the ecological significance of shifts in C:N:P ratios. These findings are relevant for soil stoichiometric models and provide new insights on fungal nutrient use efficiency and carbon sequestration