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Summary: During the last few decades, a plethora of sequencing studies provided insight into fungal community composition under various environmental conditions. Still, the mechanisms of species assembly and fungal spread in soil remain largely unknown. While mycelial growth patterns are studied extensively, the abundant formation of asexual spores is often overlooked, though representing a substantial part of the fungal life cycle relevant for survival and dispersal. Here, we explore asexual sporulation (spore abundance, size and shape) in 32 co‐occurring soil fungal isolates under varying resource conditions, to answer the question whether resource limitation triggers or inhibits fungal investment into reproduction. We further hypothesized that trade‐offs exist in fungal investment towards growth, spore production and size. The results revealed overall increased fungal investment into spore production under resource limitations; however, effect sizes and response types varied strongly among fungal isolates. Such isolate‐specific effects were apparent in all measured traits, resulting in unique trait spaces of individual isolates. This comprehensive dataset also elucidated variability in sporulation strategies and trade‐offs with fungal growth and reproduction under resource scarcity, as only predicted by theoretical models before. The observed isolate‐specific strategies likely underpin mechanisms of co‐existence in this diverse group of saprobic soil fungi. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Summary: The dependency of microbial activity on nutrient availability in soil is only partly understood, but highly relevant for nutrient cycling dynamics. In order to achieve more insight on microbial adaptations to nutrient limiting conditions, precise physiological knowledge is needed. Therefore, we developed an experimental system assessing traits of 16 saprobic fungal isolates in nitrogen (N) limited conditions. We tested the hypotheses that (1) fungal traits are negatively affected by N deficiency to a similar extent and (2) fungal isolates respond in a phylogenetically conserved fashion. Indeed, mycelial density, spore production and fungal activity (respiration and enzymatic activity) responded similarly to limiting conditions by an overall linear decrease. By contrast, mycelial extension and hyphal elongation peaked at lowest N supply (C:N 200), causing maximal biomass production at intermediate N contents. Optimal N supply rates differed among isolates, but only the extent of growth reduction was phylogenetically conserved. In conclusion, growth responses appeared as a switch from explorative growth in low nutrient conditions to exploitative growth in nutrient‐rich patches, as also supported by responses to phosphorus and carbon limitations. This detailed trait‐based pattern will not only improve fungal growth models, but also may facilitate interpretations of microbial responses observed in field studies. [ABSTRACT FROM AUTHOR]

Ecological stoichiometry represents a powerful tool to understand and predict biochemical processes at different scales. Differences among soil, plant and microbial element ratios are used to predict nutrient limitations, element cycling and ultimately the fate of carbon (C) in soils. These seminal applications are based on fundamental assumptions about soil microorganisms, which have rarely been tested at the level of individual microbial species. This is especially the case for saprotrophic fungi, which substantially contribute to nutrient recycling in soils. Thus, we tested hypotheses derived from those theoretical assumptions that (1) unlike autotrophs soil fungi show strong homeostasis especially regarding C:N ratios and (2) nitrogen (N) and phosphorus (P) demands in different environments can be predicted from organismic C:N:P ratios. We used a well characterized fungal collection of 16 species isolated from a temperate grassland. Fungi were exposed to an experimental N and P gradient in defined inorganic growth media, and their molar C:N:P ratios were determined as well as a range of fungal traits. Additionally, fungal C:N:P ratios were analyzed in more complex cellulose and soil-extract medium (SEA) and related to responses of glucose, cellulose, N and P additions, respectively. Interestingly, the fungal C:nutrient ratio is extremely plastic and may reach unexpectedly high values, especially in low nutrient media. By contrast, N:P ratios are more homeostatic and phylogenetically conserved. In growth environments with easily available glucose as a C source, C:N and C:P ratios represent good predictors of N and P demands, fungal growth rates and activity. However, in case of more complex C sources in natural environments, high fungal C:nutrient ratios rather indicate C demands, supporting the general assumptions of predominant C limitation of microbes in soil. These data suggest that the special physiology of mycelial growth causes very distinct stoichiometric patterns in fungi, characterized by highly efficient re-translocation of N and P within the mycelium, in contrast to high demands for C as a structural component. [ABSTRACT FROM AUTHOR]